E. LEVIN
E. LEVIN is deputy director and chief architect of the Timber Research and Development Association (TRADA), Hughenden Valley, High Wycombe, Bucks., England. The paper was prepared in consultation with A.V. Bassili of the United Nations Industrial Development Organization (UNIDO), Vienna, Austria, and is based largely on the background papers which are listed al the end. Grateful acknowledgement is made to their authors.
SINCE THE HOUSING problem in all countries, both the industrially developed and those on the road to development, is largely a problem of providing low-cost housing. it is clear that the subject matter of this section lies at the very core of the Consultation. Although the general decline in the use of wood per dwelling unit can be attributed to many factors, there is little doubt that in the final analysis only the most rational applications of wood in building-having regard both to technical suitability and to competitiveness in cost-will make worthwhile contributions to the solution of the problem.
A good deal of the material presented in other sections already covers some of this ground. Section 3 in particular contains detailed descriptions of common house construction methods in use in North America, as well as of recent innovations. Section 4 deals with many factors which affect design and construction. It is not proposed in this paper to repeat information already provided elsewhere, nor to summarize the many background papers or special papers presented in this section. References will be made where appropriate to all these valuable contributions.
Broadly, this section is concerned with efficiency and economy in design and construction, and will seek to bring into perspective the following questions:
What are the various aspects of design and how do they affect the possible use of wood and economy in house-building?What are the methods available for structural design, the underlying principles of structural systems and their limitations?
How do structural systems relate to methods of production and erection?
What are the options available in production methods and how do they relate to the building industries and to the market conditions in various countries?
How do wooden houses compare in cost with alternative materials and methods of construction?
In the light of the above, what policies should be adopted to increase the rate of wood utilization, improve design efficiency and reduce costs?
This section deals not only with wooden houses but, as the title already implies, with wood-based components as well. Its main preoccupation is with structural elements (walls, floors, roofs) and their separate components, but nonstructural uses (joinery, cabinet making, cladding and other finishes) and ancillary uses (form-work, scaffolding, etc.) are also considered.
DEFINITION
Design is a general term which has many connotations. This paper is concerned primarily with constructional design, signifying the conception of the house, element or component in order to meet the relevant functional requirements for the particular end use (strength and stability, durability, fire endurance, resistance to wear, etc.). Design in the sense of dwelling types and the planning of their accommodation, or of site layouts and other town planning implications, will only be considered briefly in relation to their effect on building construction generally and on the use of wood in particular.
URBAN DEVELOPMENT AND HOUSING DENSITIES
The impact of urban planning considerations, and in particular of residential densities and site coverage allowances, on the use of wood construction for low-cost housing can be far-reaching. In many parts of the world, where forest products are abundant, timber housing has been and remains today a living tradition. It was developed initially as a craft-based method of construction for rural housing. Its subsequent development for urban housing is still carried out in the main by conventional techniques, and even the most industrialized wooden house systems cater mostly for detached single family houses and to a limited extent for low-rise multi-dwelling houses.
New housing requirements in the industrialized countries, and in growing measure in the developing countries as well, are concentrated in the conurbations. If unchecked by urban and regional planning and adequate controls, this pressure almost invariably results in excessive residential densities and in high-rise developments in the cities, which are often ringed by zones of densely packed insalubrious makeshift dwellings. The social consequences of these unplanned growths have been amply discussed in many national and international studies (see Section l and its references). The point that needs to be emphasized here is that with skilful planning and layout reasonably high urban densities and site coverages can be achieved, with most of the dwelling units accommodated in buildings not exceeding three to four storeys in height and moderate densities in two-storey houses arranged in terraces, clusters or other attached arrays. These can be provided in wooden stuctures by well-established building techniques.
Studies in the United Kingdom on the economics of urban development (15, 21) tended to show the diminishing returns (in terms of increased density of accommodation on the site) by increasing building heights, if proper amenities, space about buildings, etc., were to be preserved. The most worthwhile increase (about 40 percent) resulted from building two-storey houses instead of one-storey, but going from two to three storeys increased the net residential density by only 15 to 30 percent (according to different calculations). Above four storeys the increase in density was insignificant.
The relative building costs of dwellings provided in the various types of structure (by conventional methods of construction) were given in the study of the Ministry of Housing and Local Government as follows:
TABLE 1. - RELATIVE BUILDING COSTS PER ROOM ACCORDING TO TYPE OF STRUCTURE
|
Type of structure |
Cost factor |
|
Houses |
|
|
Semidetached |
1.0 |
|
Terrace 2-storey |
1.0 |
|
Terrace 3-storey |
1.1 |
|
Flats |
|
|
3-storey |
1.2 |
|
5-6 srorey, with load-bearing walls |
11.5 |
|
6-10 storey, with framed structure |
21.7 |
SOURCE: United Kingdom. Ministry of Housing and Local Government. The density of residential areas. London, HMSO, 1952.
¹The sharp rise in costs at the 5 6 storey level may be largely attributed to the need for lifts Ä always expensive and often a social nuisance in low-cost housing _ 2 Where amenities such as laundries nurseries, etc., were included the cost factor could rise as high as 2.5.
The conclusions reached were that "cost per room decreases irrespective of land cost, with increase in density up to the maximum possible with two-storey housing. Above this level, when flats become necessary, the increased cost of structure causes the cost per room to rise sharply. At still higher densities it falls again, but unless land is very expensive it does not drop to the first minimum even if density is increased to 200 rooms per acre (say 500 per hectare). In brief, high densities only pay (in the narrow sense) when land is very expensive."¹
(¹ According to Stone (21) the land cost in the United Kingdom in 1959 would have had to exceed £24 000 per acre (£60 000 per hectare) to justify economically building higher than two-storey houses.)
HEIGHT OF RESIDENTIAL BUILDING AND ITS EFFECT ON THE use OF WOOD IN ELEMENTS OF STRUCTURE
The above considerations relate to housing of any type of construction. As far as the use of wood for structural elements is concerned, the implications are quite clear. Extensive use of wood with safety and economy dictates low- and medium-rise housing. Building codes, with fire risk in mind, vary in the severity of their restrictions on the use of wood for residential construction (see Section 4, Part II). Almost invariably, however, limitations increase with height of buildings. Other functional considerations and long experience also tend to favour low construction. The classification of building types in Table 2 indicates the extent to which wood is used or can be used in elements of structure.
It is clear from the table that high-rise construction is at present incompatible with an intensive use of wood for structural purposes, both on grounds of technical experience and of compliance with building codes. Town planning measures to prevent excessive densities are therefore a prerequisite, not only for economical. healthy and socially acceptable residential developments, but to ensure that timber resources can be utilized satisfactorily for structural purposes.
A further point in relation to house type which needs to be borne in mind is the inflexibility of multifamily housing with regard to future extensions. Single-family houses are capable of expansion by an additional storey or rear extension, even if houses are attached in rows or in cluster arrangements. To prevent architectural chaos, such extensions must be planned in advance and rigid controls exercised as to their execution. With the prevailing large families in developing countries, combined with small incomes, such flexibility will inevitably commend itself to planners.² Wood construction is ideally suited to permit future extensions at little or no extra initial cost. In medium-density development, with row or cluster housing, separating walls of nonwood materials or preferably, of wood-wool cement slabs, probably provide the simplest solution.
(² There is a conflict of views on this point between H.J. Burgess, Experience with the promotion of wood in housing in the tropics (38), and the promoters of the recent housing scheme in Peru (reported in Section 1). or, preferably, of wood-wool cement slabs, probably provide the simplest solution.)
HOUSE PLANS AND PLANNING PRINCIPLES
The planning of dwellings, both as to general configuration and the internal arrangement of rooms and services, has an important bearing on economy of building, whatever materials are used. Climate and mode of living are the prime factors, but type of structure and the characteristics and limitations imposed by available building materials also play their part. There is little factual information about the comparative economy of different house plans offering similar accommodation. Nevertheless, it is possible to indicate a few guidelines which are of particular relevance to the economic use of timber.
1. As a general rule, rectangular plans are the most economical, particularly for the use of wood and wood-based panels, which are generally rectilinear and rectangular. Such plans lend themselves best to standardization of elements and components, and the dimensional coordination of plans with available materials can do much to avoid unnecessary labour (in cutting and jointing) and waste. Modular coordination should be adopted in planning when materials and components of buildings are manufactured to modular standards.³ An early adoption of modular standards by national standards institutes will avoid costly changes in industry at a later stage when production is well established. Encouragement of the adoption of standards in general and modular standards in particular can be given by their enforcement in the public sector.2. In detached houses, plans nearest to the square (in overall shape) provide most space for minimum enclosure, but this consideration is secondary to that of using available standard sections and lengths and the plan shapes dictated by them.
3. In row houses (terraces), narrow frontages are usually more economical in general development costs (site services) and in building costs (e.g., reduction of spans for wood members in floors and roofs when spanning between party walls).
4. In general, projections from the plain rectangle add to cost and should be avoided unless dictated by climatic or similar considerations. L-shape plans may be justified in attached houses arranged in patio formation, but the additional hips and valleys (in the case of pitched roofs) add to cost.
5. Internally, wide spans should be avoided wherever possible by making partitions load-bearing (at little or no extra cost). In two-storey houses partitions should be superimposed wherever possible to avoid high bending moments on the intermediate floor joists.
[³ The term module is used here in the more common sense of a defined unit of increment for dimensional coordination of components. For the benefits of such coordination see G.F. Prange, Uses of sawn lumber in housing (45).]
TABLE 2. - POTENTIAL USE OF WOOD ACCORDING TO BUILDING TYPES
|
House type |
Extent of wood use in element of structure |
|
A. SINGLE-FAMILY |
|
|
1. Detached, 1-2 storey |
(i) All-wood construction possible and common, no inherent and few imposed constraints (e.g. adequate distance to boundary where wood cladding or roofing is used). |
|
2. Semidetached, row |
(i) All-wood construction is possible, as some building codes houses, clusters, etc., 1-3 storey permit timber party walls, provided adequate fire resistance and sound insulation are assured. |
|
B. MULTIDWELLING |
|
|
1. Low- or medium-rise (2-4 storey flats or maisonettes)¹ |
(i) All-wood construction possible, but generally more restricted by regulations. Wood-based separating floors with adequate sound insulation are also more difficult to construct than separating walls. |
|
2. High-rise (5-storey and over) |
(i) All-wood construction is generally prohibited by building codes. |
¹Two-storey dwellings with internal stairs, within multistorey structures.
In many countries, government ministries or agencies concerned with housing have not only laid down minimum standards of accommodation and equipment for low-cost housing, but have issued many standard plans for adoption by builders, housing associations and individuals. In some countries, notably in North America and in Scandinavia, forest research organizations or timber development institutions sponsored by the timber trade and industry have also issued design manuals and plans for wooden houses. 4 Many of the plans offered (often at very little cost) are excellent, but others are not aimed at or not suited to the low-cost housing sector. Some plans also express the individual ideas of designers which are not always in harmony with popular modes of living or economy in building. The best solutions in each country or region are likely to be found in the adaptation of traditional house plans to suit the requirements of modern materials and services. Studies to this effect could well be carried out (and have been carried out in some developing countries) by mixed teams of students of architecture and sociology, under expert guidance. Prototype plans should be carefully prepared with an eye to the limitations imposed by available forest products and production techniques. The involvement of experts from the forest and building industries at an early stage is therefore essential.
(4 Some are listed in the references at the end of this paper.)
FUNCTIONAL REQUIREMENTS AND THEIR EFFECT ON WOOD USE
Dwellings have to satisfy many diverse requirements according to climate, living habits, and standards which can be afforded. Some of these requirements have little or no bearing on methods of construction and materials, but others affect them profoundly. 5 In Table 3, the main attributes are listed, and the extent to which they affect the use of wood or wood-based materials in the structural elements of houses.
(5 This is not to say that functional requirements are in fact always consciously considered. In low-cost housing, especially, compromises between the functionally desirable and the economically attainable often lead to neglect of elementary comfort considerations. Moreover, ignorance and prejudice play their part, particularly where wood is concerned, so that both economy and comfort may be sacrificed for illusory gains in strength and solidity or for prestige reasons.)
It will be seen from Table 3 that fire hazards are the most limiting factor in design and the extent to which wood can be used efficiently in multidwelling construction. Durability and maintenance considerations are next in order of importance, and affect especially the choice and treatment of wood materials. Other functional requirements can be met with ease in wood construction, often by the addition of other materials which can be readily tied or fixed to the structure.
DESIGN PROCEDURE AND METHOD
With due regard to the multiple functions to be satisfied in housing design, there has been an increase in the complexity of multimaterial elements and their detailing.
This has led to two dangers:
1. Incompatibility of the properties of different materials juxtaposed (e.g., large differentials in thermal or moisture movements).2. Disregard of the load-sharing properties of many materials added for nonstructural purposes, resulting in less economical solutions. 6 To ensure adequate performance, a systematic design procedure is required. 7
(6 One way of combining economy with simplicity in construction is to develop dual or multifunction materials and components (see Section 3).)
(7 A typical designer's guide for timber structures is given in a TRADA publication (23). The design procedure is related to performance requirements, standards and authorities in the United Kingdom, but the general methodology and check lists are applicable to other countries as well.)
Where an extensive use of wood in a housing project is desired, the designer and investor should consider the choice of house types and arrangement of site layout with an eye to possible inherent or statutory limitations (see above), and modify the concepts or obtain necessary relaxations. The preliminary choice of structural forms will be followed by the systematic review of functional requirements, and the way to deal with them before proceeding to detailed structural design.
As to the method of structural design, there are three recognized procedures, and some codes will accept any of them as valid:
the conventional (traditional or rule-of-thumb);
structural analysis;
prototype testing.
As a rule they are progressively more economical in materials in the order listed. They each have their advantages and limitations. The first method, based on past experience and intuition, takes little account of technological advances and improvements in quality of materials. Its systems often contain structurally redundant members. Members may be oversized, due to inefficient jointing, or ignorance of the strength properties of the materials used. Cutting down on member sizes or traditionally accepted minima in jointing methods (e.g., by speculative builders) may result in inadequate structures.
The second method is based on the application to design of the fundamental laws of mechanics and on empirical knowledge of the behaviour of materials and structures under diverse loading conditions. The resistance of the structure and the applied loadings are then correlated to provide safe buildings. A prerequisite of the analytical method is a knowledge of the strength properties of the materials to be used and of the loadings to be assumed. Neither one nor the other, however. are easily evaluated constants, but are variables over wide ranges. Safety cannot be regarded as a simple factor by which all stresses are reduced or all loadings increased, but must take into account the type of risk involved and an acceptable probability of failure which is statistically determinable. Design codes generally make inadequate allowance for load-sharing between members8 and tend to be conservative in some other important respects as well. There is great need for an international pooling of experience and resources to obtain more economical design criteria everywhere.
(8G.F. Prange, Uses of sawn lumber in housing, p. 10-11 (45), reports on recent improvements in the United Stabs codes following a series of tests on house elements at Virginia Polytechnic Institute.)
TABLE 3. - EFFECT OF VARIOUS FUNCTIONAL REQUIREMENTS ON WOOD WOOD IN HOUSING
|
Functional requirements |
Main parts affected |
Limitations |
|
1. Strength and stability |
All load-bearing elements and, for stability, the structure as a whole |
Generally no limitations. Wood structures able to cope with most severe conditions.¹ |
|
2. Fire safety² |
|
|
|
Noncombustibility |
Generally, load-bearing elements in multistorey houses |
Wood prohibited where the requirement applies. |
|
Structural fire resistance |
Separating walls and floors, load-bearing elements in multistorey construction |
Heavy timber construction, or light construction with protective coverings and infilling, can provide adequate performance.³ |
|
Internal spread of flame |
Wall and ceiling surfaces |
Treatment with fire retardants required, usually in multi dwelling houses; or composite wallboards with noncombustible base or surface. Compartmentation in large but/dings, including fire doors, fire stopping in cavity construction of elements. |
|
External hazard (radiation, flying brands) |
External walls roofing |
Wood cladding requires increased distance to boundaries or adjacent buildings, and may be limited or prohibited on high-rise buildings. Wood framing behind noncombustible cladding may also affect distance. Wood shingles and similar roofing also require prescribed distance to boundary and are often impracticable in densely built-up areas. |
|
Sources of fire |
Electric wiring, heating and cooking installations |
Need for good insulating sheathing or conduit for electric wiring. Distance of heat source or flues to wood materials should be prescribed to avoid excessive temperature rise. |
|
Means of escape |
Public corridors, halls, stairs, lift shafts |
Noncombustible linings, and often structure, may be imposed in multidwelling houses. |
|
3. Weather exclusion (rain, sun, wind) |
Roofs, external walls |
No limitation on wood use, but wide overhanging eaves desirable. Where driving rain prevails tight jointing of walls required or water- and wind-proof membrane behind cladding. |
|
4. Resistance to ground moisture |
Foundations, ground floors, bottom parts of walls |
Wood in contact with ground or porous materials on ground generally requires preservative treatment, unless of highly durable species. Damp course to protect ground floor from rising damp required on most soils, unless considerably open under floor space.4 |
|
5. Thermal comfort (heat insulation, ventilation) |
Roofs, external walls and openings and in cold climates heating installations |
Thermal properties of wood and ease of fixing other insulating materials render it ideal in all zones, especially humid tropics and northern hemisphere. |
|
6. Prevention of condensation(internal surfaces and interstitial) |
Mainly roofs and external walls, particularly in climates with large indoor/outdoor temperature and humidity differentials |
None; but good design requires adequate ventilation and correct positioning of vapour and moisture barriers |
|
7. Sound insulation |
Mainly separating walls and floors between dwellings (both airborne and impact sound); also external walls and, to a lesser extent, partitions and floors within dwellings |
Absence of mass in light wood-based construction necessitates addition of weight (e.g., pugging between joists) or absorbent quilts or insulating fillings (e.g., in partitions or " floating" floors). Space separation (double partitions) extremely effective.5 |
|
8. Daylighting |
External walls (adequate fenestration required) |
Easy to provide large openings in wood-frame construction. |
|
9. Water supply and sanitary services |
Mainly floor coverings in bathrooms, lavatories and, to a lesser extent, kitchens |
Wood flooring may be affected by dampness due to leakage etc.; impervious covering or suitable preservative treatment desirable for nondurable species. |
|
10 Avoidance of vermin infestation (rodents, and insects such as bed bugs, cockroaches, etc.) |
Practically all elements and their junctions |
Where rodent or insect hazard severe, cavity construction to be avoided unless all joints are carefully sealed; with modern insecticides, which can be introduced in construction, problem is now diminished. |
|
11. Durability and low maintenance (protection from wood-destroying fungi and insects, and effects of weather on exposed surfaces) |
All parts of structure, particularly those most subject to humid conditions and lack of ventilation; external cladding |
Wood requires precautions; nondurable timbers generally require effective preservative treatment, depending on severity of exposure to dampness-specially in hot-humid tropics Ä but also elsewhere except possibly very dry climates; design measures promote ventilation and exclude termites;6 preservative treatments and nonfilm-forming exterior finishes reduce costs of external cladding.7 |
|
12. Aesthetic satisfaction |
Houses in their setting; interior and exterior materials and detailing |
Wooden buildings admirable if well designed and finished, but choice entirely subjective; harmony with surroundings desirable. |
¹ See L.R.O. Anderson, The wood frame house resists nature's furies (30).-² See R.G. Silversides, Fire hazard in timber structures (46). - ³ The Building Regulations (1965) for England and Wales list many composite constructions for nonload-bearing walls and partitions with various linings and claddings and with fire resistance rating up to four hours, However, the maximum required in elements of superstructure of residential buildings never exceeds one and a half hours and that only for buildings exceeding 90 feet in height. Fire resistance ratings for various wood-based elements are also given in North American and other codes. See also U.S. Department of Housing and Urban Development and U.S. Department of Agriculture, Manual on wood construction for prefabricated houses, p. 97-102, Washington, D.C. 1947, reprinted 1967 (4). - 4 See D.H. Percival, Present and potential applications of treated pole and post construction for houses (44). -5 Excellent light structures for walls and floors of relatively low cost, and giving Rood sound insulation and high fire resistance, have been developed by TRADA in the United Kingdom for medium-rise construction (23, 24, 25). Figure 1 shows some typical constructions. - 6 See G. Becker, The hazard of fungus and insect attack for wood and wood-based material in houses in various regions and means of alleviating it (31). - 7 See J,M. Black, Finishes construction factors, and design to compensate for effects of weather on wood (33).
Prototype testing of components or elements also assumes a knowledge of loading conditions which have to be simulated with adequate safety factors. A consistency in quality of materials and workmanship in repetitions of the prototypes tested is also assumed. Repeat tests and tests on a number of samples are also required in some codes. Whole house testing was developed in the United Kingdom in the early postwar years (19) and has been applied to detached or semidetached houses, which are not readily amenable to analytical design. When used as a design tool, this method produced extremely economical timber-frame houses, whose structural timber content for the entire house did not exceed that used in conventional brick constructions for the roof and intermediate floor alone.
The first method, although generally extravagant in material, is useful for the " one-off" custom-built house, even where the strength properties of the timbers are known. Design costs in such cases could well exceed any savings in material. Tables indicating appropriate sections for given spans for common components in housing are incorporated in many building codes and, where regularly revised to incorporate new knowledge, will help to diminish the gap between conventional and analytical methods. The latter method is suitable for unusual houses, or where a sufficient number of similar houses justifies the employment of a structural engineer. Prototype testing, whether for components (e.g., roof trusses) or whole houses, is the most costly, and is to be recommended for industrial production in large batches since it almost invariably produces the most economical designs.
FACTORS AFFECTING STRUCTURAL EFFICIENCY
Three main considerations govern the efficiency of wooden structures:
the strength properties of the materials;
the means of connexion between members and parts;
the forms selected (for members, components or complete building structures).
During the past few decades considerable progress has been made in many countries in establishing standards of sizes and qualities for sawnwood and various wood-based panel products,9 often including stress grades based on the limitation of strength-reducing characteristics (such as knots, wane, slope of grain, etc.). Such standards are a prerequisite both for analytical design and for prototype testing. Grade stresses have been laid down not only for practically all available softwood species but for many hardwoods as well. In some countries wood species of similar strength characteristics are assigned group working stresses in order to simplify analytical design procedure. This method would appear to have particular merit in the possible exploitation of mixed secondary species from tropical forests, whose strength classification into groups could be most readily determined by density considerations. The assignment of strength and elasticity values based on a limited range of preferred numbers 10 has been widely advocated and is already applied in Malaysia and east Africa (see H.J. Burgess, 37).
(9 See Section 3, background papers, and references to this section.)
(10 Based on the Renard series of geometric progressions, already widely used in industry for ranges of sizes of various products.)
Grade stresses assigned to visually graded softwoods generally err on the side of caution by the nature of the methodology employed in establishing them.11 Electromechanical grading, already introduced on a limited scale in several countries, including Australia where it has been used for grading hardwoods, is likely to increase the strength rating of timbers of all species to which it is applied.
(11 For a detailed description of the methods, partly statistical and partly arbitrary, used in arriving at working stresses in the United Kingdom, see the commentary by LG. sooth and P.O. Reece on the British Code of Practice for the structural use of timber (2). The methods were similar to those used in North America.)
Whatever grading methods are adopted, there appears to be little doubt that a great many, possibly most, species in tropical forests not at present exploited commercially have ample strength properties for structural uses in housing (38, 41). P.A. Campbell (39) recommends a simple performance specification which divides sawnwoods into three grades: structural, constructional (but nonload-bearing) and joinery quality. By lowering the standard of requirement, especially for the two former grades, he estimates that a saving of 15 percent in cost has been achieved in east African countries where such a specification has been adopted. The simplicity of these grading rules contrasts with the greater sophistication and complexity of the standards which now obtain in highly industrialized countries, in particular in North America (42).
The development of efficient means of connexion has been a major factor in the advance of timber engineering in the past few decades, and has permitted progressive reductions in member sizes of structures. J. D. Boyd (36) gives a comprehensive review of available methods and the factors affecting them and influencing their choice. Particular attention is given to the problems of jointing relatively dense hardwoods and means of overcoming them. Nailing is the most common solution for jointing timbers in housing in the developing countries-as it is in the industrialized regions. This is also the conclusion reached by L.G. Booth (35), who in a series of tables links methods of connexion, and their structural and economic characteristics, with the types of structure for which they are suitable.
STRUCTURAL FORMS
The geometrical form of member sections, elements or the entire structure of a building determines the manner of load distribution through them to the supports. the kind of stresses generated in the material in response to the loads and their efficiency in terms of economy of structural material. Booth's tables show that there is a wide variety of shapes for structural elements which have been developed successfully in timber engineering. Many of these are particularly suitable for medium- and large-span structures and only a limited number are commonly used in the construction of houses, which are generally small cellular structures with planar elements for walls and floors and often for roofs as well. A wide range of design aids such as span tables and computer programmes to simplify design procedures (since they apply to a variety of structural forms, loading conditions and timbers of different strength properties) is now available.12
(12 The universality of such design aids and programmes as developed by the Timber Research and Development Association of the United Kingdom is described by H.J. Burgess in Design aids including computer programs for universal application (37).)
Three generic types of structure can be distinguished by the way in which imposed loads are distributed through them:
1. Solid structures (log cabin walls, plank floors) in which loads are transferred through the body of the element.2. Skeletal structures (post and beam construction, roof trusses, portal frames), in which loads are transferred longitudinally along the members or elements.
3. Surface structures (shell roofs, stressed-skin panel floors), in which longitudinal and transverse loads are transferred through the thin walls of the elements to edge members and thence to supports.
These types of structure are progressively more economical in materials in the order listed, but also require progressively greater sophistication in design and in methods of production.
Skeletal or framed structures lend themselves most readily to composite construction with other materials, and are the predominant types of structure used today in housing with wood elements and components. In practice, however, there are many hybrid housing structures combining elements of different types (e.g., post and beam frameworks with plank floors and roofs). But economy in design and construction depends on the relative availability and cost of the different materials required, on available skills, means of connexion and facilities for production. The following brief notes describe the systems of construction and methods of design for the various structural elements, available for each of the generic structural types, and their main characteristics.
SOLID STRUCTURES
Walls
(a) Horizontal construction, with members consisting of logs or halved logs, often with squared edges to provide good meeting surfaces, laid over each other and generally housed at corners. Economical in conversion labour but extravagant material since vertical loads are transferred transversely through the members in the direction of least strength. Also subject to considerable shrinkage in the vertical direction. Still used on a fair scale for holiday homes and mountain chalets in forest regions but considerable accuracy and skill are required in the preparation of joints. Probably the oldest form of prefabrication of individual members (precut ready for site assembly).(b) Vertical planks, tongued and grooved or spline jointed. Planks grooved at both edges can be assembled in staggered formation to increase resistance to buckling. Planks are usually 2 to 3 inches thick and need to be accurately machined and straight to enable easy jointing Fairly heavy in material usage, but efficient where the single thickness of sawnwood will comply with all the functional requirements. Solid plank party walls can provide a fairly high degree of fire resistance but the joints are vulnerable and double tongues and grooves or grooves and splines will ensure better performance.
Floor and roof slabs
These are usually made of similar material to plank walls, but are designed to span between walls, beams, purlins or arches. They may be used in single or continuous spans or in random lengths (with splined ends). Design limits are usually set by deflection of end spans. This form is at its most efficient when spans are continuous over three spans at least, so it is generally inefficient in small housing units. The combined use of plank elements with pole frames is described by D.H. Percival (44).13
(13 The relative -popularity of this form of construction in North America is based on the availability of large dimension planks, in dressed and grooved form, as standard products.)
SKELETON STRUCTURES
Skeletal frameworks are the most common type of wooden structure used for the various elements in house construction. A distinction may be drawn between widely spaced framing members-as in column and beam construction, of which the pole houses described by D.H. Percival are a classical example, and those with closely spaced members-studs in wall construction, joists in floors and ceilings and rafters in sloping roofs. The former are generally designed as systems of individual load-bearing members and the latter as load-sharing systems when the spacing of members does not exceed 2 feet.14. Portal frames are another common type of skeletal structure, although less used in house construction than for other building They provide structural support for both wall and roof panels, and may also be regarded as load-sharing if closely spaced. (See A.W. Kempthorne, 43.)
(14The 24-inch spacing is a rather arbitrary limitation, since effective load-sharing depends on venous factors, including the stiffness of connecting members or membranes such as floor boards or wall sheathing. Design codes are generally conservative in their allowance for load-sharing (see G.F. Prange [45] for recent amendments to the American Model Codes following tests which demonstrated the load-sharing capacity of conventional frame housing).)
Prerequisites of the pole house are suitable poles and preservation plant, and relatively heavy beam sections of sawnwood and substantial bolts or similar means of connexion. Alternative posts and beams may be built up of small sections, and even short lengths into box or I-form members. 15 Built-up members of such sections have increased radii of gyration and moments of inertia for the net cross-sectional area, hence greater stiffness and resistance to buckling. They have reportedly been used successfully in some developing countries.
Frame structures for walls with fairly widely spaced members are still traditionally used in some countries, with masonry or plaster and lath infill panels. Because of the only moderate racking resistance afforded by the panels, diagonal bracing is commonly required. Many such buildings, centuries old, still exist all over the world. They are heavy on material requirements and the demand for skilled labour. In the thin rendered panel version they are still probably one of the most popular forms of house-building in Japan.
STUD WALLS
In stud wall designs, it is assumed that the studs are efficiently supported laterally, either by sheathing or cladding materials or by nogging or diagonal bracing. They are therefore regarded as columns in a load-sharing system (when spaced at maximum 24 inches), and their strength is calculated about the plane parallel to the wall. For one- or two-storey construction softwood or hardwood studs of 3 × 2 inches or 4 × 1½ inches are generally adequate. 16 Walls in three- and four-storey structures can also be built of such light members, with the members in the lower storeys either doubled up or spaced more closely. 17 Stud framed wall panels may be of single or two-storey height (platform or balloon framing), and a number of variants of these basic framing systems have also been developed to suit various production and erection techniques. 18
(15 Some timber engineering manuals (9, 22) give the percentage of such built-up columns as between 65 and 82 percent of comparable sections of one-piece columns, the reduction depending on the length depth ratio of the column.)
(16 In Malayan experience, according to H.J. Burgess (38), 13/4 X13/4-inch studs at 2 feet centres were found adequate for bungalows where medium-density hardwoods were used.)
(17 Four-storey multidwelling houses with such light wall structures have been built recently in the United Kingdom (19).)
(18 See R.F. Blomquist, Timber framing for on-site construction (34) and TRADA'S design guide (24). The basic framing systems are also described in many other timber construction manuals (see references))
Stud framing for wall construction has three great advantages:
1. The framing members required are of small dimensions and fairly short lengths (very short lengths may be within the wall height). This enables material to be obtained even from very small trees, to be rapidly air seasoned and, where necessary, to be more easily and thoroughly treated with preservatives.2. Production methods can range from on-site construction to factory fabrication of small or large panels.
3. The simplest means of assembly are generally adequate-end nailing or toe nailing of framing members and nailing or stapling or sheathing, claddings, etc. Stud panel walls are generally supported on strip footings direct or through " edge" members of suspended floors, which may he designed as beams to carry the loads to pad foundations, as in the case of widely spaced column and beam structures.
Party walls in light stud framing can be designed to provide high fire resistance and sound insulation but composite construction is required, involving the use of wood-based panels or gypsum wallboards as linings with infillings of mineral quilts or bats or certain granulated barks;19 staggered studs with interweaving quilts, or double partitions with a space separation of 6 inches or more provide the best solutions. The latter permits the introduction of a noncombustible screen of thin brickwork or blockwork between the stud frames (see Figure 1). In this connexion wood-wool cement slabs provide an excellent alternative material. It is generally rated as noncombustible, and has high fire resistance as well as sound absorption and thermal insulation. Moreover, it can be nailed to wood framing or laid in cement mortar. 20
(19 The fire resistance of ¼-inch plywood-lined stud partitions was increased from 12 to 63 minutes by filling the interstud spaces with shredded redwood bark to the extent of 1.7 lbs/sq ft (9).)
(20 The particular virtues of wood-wool slab as a low-cost building material and the need for its increased production and use in developing countries have boon underlined by a Study Group convened by UNIDO (29).)
FRAMED FLOORS AND FLAT ROOFS
The most common framing for flat roofs and lightly loaded floors takes the form of closely spaced joists, spanning between framed panel walls, solid walls or beams. Such joists are sheathed, with boarding spanning across them. This may be plain edge or tongued and grooved sawn and planed boards or plywood or particle board. All these are generally secured by surface nailing (or concealed in some types of tongued and grooved boards or strip flooring). Joist spacing will generally be governed by the maximum permissible span of the boarding, but even softwood tongued and grooved boarding 3/4 -inch thick will span 24 inches without excessive deflection. To ensure that small dimension timbers (say, not exceeding 6 inches in depth) can be used for spans up to 12 feet, the spacing of joists may be reduced if necessary.21
(21 In the Malayan houses described by Burgess (38), 2x4 inch joists at 2 feet centres spanned 8 feet.)
The ratio of depth to thickness of joists is an important factor in economy of design, and the greater the ratio the more economical the design. Two limiting conditions are:
1. Minimum thickness required for fixing purposes (for site work, probably 1 1/2 -inches).2. The lateral restraint provided; where a ceiling is used the ratio may be as high as seven or more but this will be reduced where only bridging or blocking is inserted between joists, and will be down to probably no more than five where only floor boards provide the buckling restraint.
PITCHED ROOFS
Timber roof structures are commonly used in house construction throughout the world and a great variety of structural systems, usually requiring only light framing members, are available. The most useful classification of pitched roofs is probably according to the supporting system used, as the nature and availability of roof coverings will in most cases determine the choice of system. There are three main classes of roofing material, listed at the end of Table 4, which gives the major framing systems used, their characteristics and limitations, and their suitability for the various roofing materials.
Of these roof systems, only the trussed rafters and the built-up purlins are components usually produced in series in the shop. For the rest, precutting, notching and otherwise preparing for site assembly are often undertaken prior to delivery to site.
SURFACE STRUCTURES
Structural diaphragms are thin-walled elements, usually rectangular, designed to resist shear and transfer the load of lateral forces such as wind to the edge members and thence to other resisting elements or foundations. The sheathing of walls and partitions may act as vertical diaphragms and those of floors and roofs as horizontal, inclined or curved diaphragms. The efficiency of connexions to perimeter members and to intermediate members is all-important. For on-site work connexions will generally be made by nailing, the spacing and pattern of which should be determined by calculations or based on tests. Nailing, glue-nailing or stapling are often used for factory-made elements. The construction of diaphragm skins is listed below in order of increasing efficiency:
1. Transverse boarding, at right angles to studs, rafters, etc. Shear resistance low 22 due entirely to the moment couples of the nailing at intersections or friction where single nails are used.2. Diagonal boarding. Although some of the bending moment is taken by the boards, the main resistance to loads is in direct compressions or tension, axially. These stresses are highest at the perimeter and efficient connexions there are vital.
3. Two layers of diagonal boarding, at right angles to each other. This is considerably stiffer and acts almost as a continuous sheet. One layer is designed to take compression axially and the other tension.
4. Plywood sheathing. As with double diagonal boarding. no bending moments are introduced into the perimeter members, which need be designed only for axial loading. Depending on the loading and spacing of framing members it may be practicable to reduce thickness to 1/4 inch or less. 23
(22 But resistance may be adequate under certain climatic conditions (see H.J. Burgess [3X]).)
(23 Shed materials other than plywood may be used if their strength properties are known or the design is determined by tests.)
In structures where a continuous membrane is required as a back support or final surface material, its design as a structural diaphragm usually provides an economical solution and permits the use of simple connexions for the framing members. Continuous diaphragm construction is tending to replace diagonal bracing in light structures. 24
(24 For design data and procedures on diaphragms and stressed-skin panels, see (1) and (24).)
SOURCE: TRADA. Information sheets on the Building Regulations 1965. High Wycombe, Bucks., 1966.
STRESSED-SKIN FLOOR AND ROOF PANELS
Stressed-skin panels generally consist of plywood sheets glued to top and bottom of longitudinal framing members (or sometimes top only) so that the whole structure acts as a series of joined I-beams or T-beams. The spacing of the framing members (webs) is determined by the effective width of the plywood flanges. The continuity of members and skins essential, as is adequate bond. 25 Carefully calculated pattern nailing has also been used successfully. Stressed-skin panels are extremely light relative to their stiffness and load-bearing capacity. The double skin type in particular has a considerably reduced depth compared with normal joist and board construction. The size of panels is limited by available lengths of framing members and plywood (which may however be efficiently glue-jointed). Common panel widths are 3 and 4 feet and lengths up to 16 feet. As in diaphragm construction sheet materials other than plywood (e.g., particle board) may be used. but their structural properties and in particular creep characteristics must be carefully considered in relation to the intensity of stresses.
(25 For suitable adhesives and other relevant information see D. Countryman (40). Note in particular his report on modern gap-filling adhesives suitable for site application and providing a measure of stressed-skin effect.)
Stressed-skin panels have been used to a smaller extent in wall construction, where similar design considerations apply. Their advantage in lightness of construction is counterbalanced by somewhat more costly materials and, generally, limitation of production to controlled shop conditions. Sound insulation is poor due to their rigidity and very light weight.26
(26 See especially a report on recent American acoustic tests on low-cost housing systems of stressed-skin elements (10).)
TABLE 4. PITCHED ROOF FRAMING SYSTEMS
|
|
Characteristics and limitations |
Suitable roofing materials |
|
1. Rafter system (monopitch or double pitch) |
Short spans only, due to limitation of lengths and sizes of unsupported members; max. 16 feet for monopitch and 24 feet for double pitch or close couple roofs, at 16- to 24-inch spacing, considerable thrust on walls unless reduced by collar beams or fully tied at feet of rafters; provides open, unencumbered roof space |
(a) and (c) |
|
2. Purlin systems |
In contradistinction to rafter systems which are generally supported on front and rear walls or beams, purlins run parallel to eaves and ridge and transfer the roof loads to cross walls; 16 feet is generally economic limit of sawnwood purling, span may be increased, without increase in depth of purlin, by strutting from end walls; open roof system |
Generally (b), but may be suitable for (a) where front and rear walls are incapable of supporting roof load |
|
3. Purlins and trusses ¹ |
Intermediate triangulated trusses¹ introduced into a sawnwood purlin system to reduce purlin spans to acceptable max imum. Such trusses may be supported on load-bearing walls or columns. Roof may be left open or ceiling suspended from rafters at purlin supports |
As above |
|
4. Trusses¹ with purlins and rafters |
As above, but widely spaced purlins support common rafters, which in turn support either battens or rigid boarding |
(c) and (a) |
|
5. Trussed rafters |
Closely spaced light trusses (generally 24-inch) providing supports for roofing battens or boarding and for ceiling boards. Trusses regarded as a load-sharing system |
(c) and (a) |
|
6. Rafters and purlins |
A variant of 2, adapted to support rafters and battens; requires heavy section sawnwood purling, but lighter built-up purlins with plywood, or cross-boarded or triangulated webs are often used instead |
(c) |
(a) Flexible sheet materials or mastic roofing, which require a rigid board backing (bituminous felts, asphalt tiles, felts with metal foil facing, thin metal or plastic sheeting mastic asphalts or plastics).
(b) Rigid sheet or board materials, with ability to span between framing members (corrugated or troughed steel, aluminium or asbestos cement sheeting, rigid hollow or sandwich panels of various materials).
(c) Small rigid units, requiring batten support (clay, concrete or asbestos tiles, wood shingles).
¹ Trusses of a variety of shapes, but most commonly W-shape, are used for small spans generally required in housing. Members are kept small, if necessary by using twin members. Nailed, glue-nailed gussets and connectored joints are used, and more recently truss plates.
SHELL ROOFS AND FOLDED PLATE ROOFS
Timber shells are curved diaphragms or stressed-skin structures which derive their stiffness from the curvature of the particular geometrical shape used. Because radii of curvature are generally great in relation to the thickness of timber members required for membranes, there is no difficulty to required shapes by slightly bending or twisting individual boards when fixing them in position. 27 With the exception of edge members. which need to be shaped to the roof curvature. the material used for the structure could be ordinary tongued and grooved floor boards fixed on site.
(27 Of the venous curved shell shapes illustrated in Booth's paper (35), Table 4, hyperbolic paraboloids have been used in house construction. A great advantage is that intersections of the roof with partitions parallel to the walls would be in straight lines.)
Folded plate roofs of factory-made stressed-skin panels have also been designed by the Timber Research and Development Association for use in house construction. Like purlin roofs they span between end walls; however, no experience has been obtained with them in production. To summarize, the basic types of structure reviewed are clearly related to methods of production as follows:
Solid structures: on-site construction. Maximum shop production limited to precutting and machining individual planks.Framed structures: heavy frames are similarly limited to site production, whether wood-based or masonry infill panels are used. Light framing is, however, versatile and production methods cover the entire range of possibilities from full on-site fabrication and erection to a high degree of prefabrication, for the various elements of structure.
Surface structures: may be similarly site produced, with a minimum of factory fabrication of framing members, but stressed-skin panels generally require factory-controlled production conditions.
NONSTRUCTURAL COMPONENT AND USES
The design of nonstructural components and parts is governed by various functional requirements relevant to the particular end use, as well as by available materials and methods of production. Some of the functional requirements affecting in particular surfacing uses such as floors. wall claddings, internal linings of walls, partitions and ceilings have already been considered briefly. The point has also been made that economy in design dictates the use of single parts to perform dual or multiple functions. This is particularly true of finish surfaces in wood or wood-based panels, which can usually be exploited structurally in addition to their primary functions.
Joinery and wood trim constitute an important group of wood uses. Wooden doors, windows, shutters, cabinets, shelving, skirting, etc., are widely used in housing even in wood-deficient countries. The designs of some of these items are largely influenced by traditions and climatic conditions. Increasing attention is being given in many countries to design adjustment according to the needs of rationalized production techniques. The background paper by J. Bim and M. Koukal (32) deals in detail with the various factors affecting the design and production of wooden joinery and includes tables of tropical timbers, showing the particular end uses for which they are most suitable. As with structural components, the general trend in the design of joinery items is toward economy in material content-largely achieved by the use of wood-based panels, and simplification of details of construction to reduce the number of necessary operations.
In modern design, particularly of low-cost housing, many traditional wood trim items such as skirtings, picture rails, architraves, barge boards, eaves, fascias and soffits are reduced to minimal dimensions and often completely eliminated. However, details must be carefully studied to avoid detrimental effects on function or appearance.
Regarding ancillary uses, mention must be made of formwork and scaffolding for which sawnwood, poles and panel products are used on a large scale. Careful structural design and selection of material are essential to guarantee both economy in material and the safety of operatives. Multiple re-uses and reduction of waste in shuttering can best be achieved by panelization, which in turn dictates a measure of dimensional standardization in building.
For the construction of houses, wood and wood-based panel products enjoy a number of advantages over most other materials from the point of view of production and erection techniques. They are light and strong, easy to work, assemble and fix with simple hand tools, and also relatively easy to transport and erect in large sections without the use of costly equipment. This extreme flexibility has enabled a wide variety of production and erection techniques to be developed, attuned to the technological levels of different sectors of the building and wood-working industries which co-exist in most countries, and to the characteristics of the housing markets in which they operate. The methods range from on-site preparation of materials and stick-by-stick assembly of the houses to the most advanced prefabrication and shop-finishing of complete dwelling units, or sections of dwellings, and their subsequent transport and placing on prepared foundations. Between these two extremes there are many intermediate stages, each with a different distribution of labour and skills between factory and site. The adoption of one method or another in a particular set of circumstances depends not only on technical feasibility but to an even greater extent on economic justification.
Basically, all wooden housing systems and methods of shell construction fall into one of three main classes, progressing from least to most prefabrication:
1. On-site assembly from individual members such as random sawnwood materials, panel products, and more complex factory-made components such as doors and windows.2. On-site assembly of factory-made elements such as panels for walls, floors, etc.
3. Factory production of three-dimensional sections or complete house units.
The first category embraces the methods which have evolved traditionally in most countries and regions with abundant supplies of wood. The on-site construction methods which are currently used in North American housing, and account for the great majority of single-family houses built there, are described in detail by R.F. Blomquist (34). Their efficiency depends largely on the availability of standardized materials and skilled carpenters on the site. Progressive reduction of waste and of skilled labour requirements is achieved by precutting to length, notching, drilling, etc., at the mill or workshop.28 The plywood gusseted rigid frame houses described by A.W. Kempthorne (43) were also designed for on-site construction and rely on simple jigs and the use of standard sheet materials to achieve economy in labour and materials. The pole houses described by D.H. Percival (44) are similarly for construction on the site and would appear to require rather greater skill.
(28 The cut-to-length and ready-cut stages were tried successively in Surinam, and conditions for their success in developing countries arc discussed by C.W.P. Tempelaar in his Industrial production of housing in developing countries (47).)
Most prefabricated timber housing systems fall into the second category of panelized construction.29 The wall units are generally of storey height and may be either narrow or wide. Narrow panels are usually standardized in width to rationalize production, and common dimensions are 3 or 4 feet (or metric equivalents). In many systems this enables the provision of doors and windows in standard panels before they leave the workshop. The prefabricators of wide panels generally adopt wall or room length dimensions, offering a limited range of house types and manufacturing panels to suit their selected models. The small panel fabricators, on the other hand, aim at greater flexibility in planning, which increases with reduction of panel width or the introduction of more than one standard width. The gain in planning flexibility and greater ease of handling and transport are offset by the greater number of site joints to be made and weatherproofed, the greater accuracy required in manufacture and the difficulty of incorporating linings, finishes and services within the small units. With large panels a fairly high degree of factory applied finishes and the incorporation of electric wiring and service ducts are possible. In most prefabricated systems, however, even with large room-size panels, the units are supplied with framework and sheathing only and sometimes with applied external cladding. Internal linings, services and decorative finishes are generally left for on-site work. Floor, ceiling and roof elements are less frequently panelized than walls, except in systems of stressed-skin construction.
(29 For a detailed discussion of the "panel and component" and "sectionalized" production method, see J.L. Tucker, Industrialized housing (48). Also special paper by K. Tiusanen, Production of prefabricated wooden houses (49).)
The most advanced prefabricated housing systems are those supplying factory-finished three-dimensional sections, or complete dwelling units (if these are small enough). Variants are units with folding parts in which the core, including the services, is transported as a three-dimensional unit and the wings are folded back during transportation to the site. Excluding mobile homes, this type of production as yet makes a very small contribution to the total number of houses being produced, even in the most advanced industrialized countries. The reasons for this will be discussed later.
PREFABRICATION - ADVANTAGES AND DRAWBACKS
The methods of house production outlined above imply an increasing transfer of labour from site to factory. The advantages of such a transfer are numerous:
1. Labour productivity in the shop is higher due to sheltered working conditions, better organization and supervision, and the breakdown of activities into smaller repetitive operations. Hence total site and factory labour diminishes with the proportionate growth of the latter. 30(30 In the United Kingdom, with panelized prefabrication of timber-frame housing, total man-hours per dwelling were almost halved in the late 1960s compared with traditional brick construction (11).)
2. Apart from foremen, machine setters, etc., only semiskilled labour is required in the shop and is thus easier to train. Moreover, the greater the transfer of operations from site to factory, the lower the amount of skilled labour required for site erection.
3. Only low-cost wood-working machinery, hand tools and simple jigs are necessary and production equipment can be progressively improved with growth in scale of operations.
4. More rapid house completion; the speedy erection of the house shells enables services and finishes to be carried out under cover and completed more quickly.
5. Accelerated turnover of capital, conducive to higher profitability.
6. Reduction of waste, pilferage and similar material losses.
7. Greater accuracy of finish can be obtained (through use of jigs and better work control).
These advantages in prefabrication are offset by a number of disadvantages, which tend to check its development:
1. The higher the degree of prefabrication and pre-finishing the more restricted the choice of end products, 31 and the greater the difficulty in meeting varying local bylaws.2. The capital required for investment in factory space and equipment, as well as for operation, grows in proportion to the extent of prefabrication.
3. Similarly, storage and transportation costs grow with production, and transport difficulties increase with size and degree of finish of the elements.
4. Factory overheads are much higher than on-site costs and can be justified economically only when there is a commensurate reduction in labour requirements for given operations. This generally means that beyond a certain point more sophisticated production equipment and techniques are needed if the operations are to be performed in the shop.32
5. Large semifinished panels and sectionalized houses require mechanized erection equipment, involving further capital investment and skills.
6. The socially desirable transfer of labour from the site to sheltered factory conditions is necessarily achieved at the cost of some labour redundancy. This may be acceptable only when an expanding building programme can absorb this surplus labour or there are alternative employment opportunities.
(31 One company in the United Kingdom which offered a single type of sectionalized house at an attractive price, having achieved a remarkable reduction in overall labour requirements, soon ran out of orders (11).)
(32 H.B. Dickens concluded from studies of Canadian house production techniques (5) that with current designs and materials very little further reduction of overall labour requirements was possible beyond the "closed panel" stage (see Figure 2).)
PREREQUISITES FOR THE INDUSTRIALIZATION OF TIMBER HOUSING PRODUCTION
The development of industrialized methods of timber-house production can flourish only if certain basic conditions are satisfied. The most important of these are:
The existence of materials of adequate quality, consistent grading, and accurate dimensions.
An adequate sized market for the particular houses or house elements.
Proper design related to social, climatic and other local conditions.
Product acceptance by local building authorities, the building industry and the public at large; the development of products must proceed hand in hand with a dynamic promotion campaign.
Adequate transport facilities.
FIGURE 2.- Approximate labour contents of present stages of wood-frame house prefabrication.
SOURCE: Dickens, H.B. Trends in Canadian house production. Ottawa National Research Council of Canada, Division of Building Research, 1969. Technical Paper No. 299.
General. A bungalow, 1000 sq ft in area and using no masonry, is assumed.
Stage (a), All site. Little precutting, little prefabrication except windows (window shop labour not counted here).
Stage (b). Project site. Precut frames; shop trusses; shop cabinetry.
Stage (c). Project prefab. Add to (b): shop walls and shop partitions, clad one side; shop doors (hanging).
Stage (d). Closed panels. Add to (c): shop walls and partitions (closed both sides), some wiring in shop gables, some shop plumbing heating.
Stage (e). Closed panels-and-core. Add to (d): shop roof section; shop ceiling sections (sometimes); more wiring in, wiring "harness"; plumbing wall and subassemblies, kitchen cabinet walls, closet walls, heating " kits" and subassemblies.
Stage (f). Transportable section.
It is significant that in Europe industrialized building systems and prefabrication have flourished in both the centrally planned economies and in the public sectors of market economy countries. In the United Kingdom, for example, industrialized building accounted for nearly half the housing output in the public sector during the past few years, but for only 5 percent in the equally large private sector. Nearly all the timber-frame houses (recently reintroduced in the United Kingdom) were at least partially prefabricated, and supplied to the public sector, mainly municipal authorities and new town corporations.
However, even within public sectors there is generally fragmentation, and lack of uniformity and continuity of orders, and this condition circumscribes the extent of prefabrication which it is possible for building organizations to undertake.33 The number of plants which can benefit from economies of scale and operate on flow-line or semiflow-line production rather than in intermittent batches is limited, and is to be found mainly in industrialized countries with large timber housing outputs.
[33 The largest single order for timber-frame houses awarded in the United Kingdom in recent years was for 700 houses, but included 12 different types. Simple jig assembly of panels and hand nailing were found to be the most economical method in these circumstances (11).]
In the large North American market for single-family houses, probably 80 to 85 percent of these were still erected by on-site techniques in the late 1960s. These conventional techniques, dictated by market conditions, have been modified in the past few decades by an ever-increasing percentage of factory-made, and in many cases factory-finished, components. Hansen's study (8) of a typical Canadian bungalow built by on-site methods shows that the labour cost element was less than one third of the cost of materials, whereas at one time they were assumed to be nearly equal (see Figure 3)." The low labour content shown in this study indicates that modern building materials such as pre-hung windows, sheet materials, roof trusses, cabinets, aluminium siding and soffits have a large built-in factory labour content and considerably reduced on-site construction time over the years."
FIGURE 3. - Cost components of selling price.
SOURCE: Hansen, A.T. The Mark V project. Part 1. A cost study of a tvpical bungalow. Ottawa, National Research Council of Canada Division of Building Research. Housing Note No. 29.
It appears, therefore, that component rather than whole house production has been the main trend of industrialization of house-building in North America. According to E.G. Stern (20), already in the mid-1960s wood component manufacturers were reported to be delivering the following percentages of factory-made components of the total used in housing:
|
|
Percent |
|
Roof trusses |
63 |
|
Door and frame units |
46 |
|
External wall panels |
32 |
|
Partitions |
25 |
|
Window walls |
20 |
|
Box beams |
17 |
|
Roof panels |
7 |
|
Floor panels |
7 |
In addition, there was a marked trend toward the higher finishing of joinery components in the shop. The percentage of preglazed and painted windows, as well as factory finished cabinets and other items, increased.
The industrialization of individual components is easier to achieve than that of entire houses; standardization is more readily accepted and a mass market for certain items (such as doors, windows or trusses) can be created even in a small country. In developing countries with small or insignificant public housing sectors, component production promises to be the main avenue toward industrialization of housing, not only for countries rich in forest resources but also for wood-deficient countries in which the demand for joinery and other wood components will grow apace with their housing demands.
The second precondition for industrialized timber house production is equally decisive. Standard materials of adequate quality are essential and no accurately finished panels or other components for easy site assembly are conceivable without them. The technically successful experiences with various types and degrees of prefabrication in developing countries appear to have been based on the activities of large companies or forest services. Prefabrication in these cases was an extension of high quality sawmilling and planing. Such operations are likely to continue on a scale limited by market characteristics which cannot change rapidly. A significant step forward in industrialization will be the raising of the general level of forest products, without which no prefabrication or component production in a modern sense can be envisaged. To this end governments, industrialists and finance institutions will have to decide on their investment priorities.
What do houses cost, and how are costs affected by the use of wood in their structure? To what extent can improved technologies and increased productivity be expected to contribute to lower costs? How do timber housing costs compare with those of alternative materials? These are questions which must now be considered.
GENERAL HOUSING COSTS
Although there is no satisfactory definition of a standard housing unit, and there are great variations in space standards and amenities both within countries and between them, some idea of the real cost of an average family house of the type approved or financed by governments has been arrived at in a number of international and national studies. One such study carried out by the United Nations Economic Commission for Europe (27) showed that in 13 of the 17 countries examined in western Europe, houses built between 1955 and 1957 cost between 2.7 and 4.9 times the average annual earnings of male workers. In the remaining four countries (Spain, Portugal, Turkey and Greece) the figure depended entirely on the type of housing used for comparison: in Greece, for example, from 2.9 for a minimum " social dwelling" for earthquake victims to 7.5 for a "working class urban dwelling." The United Kingdom figure of 2.7 was lower than that for any other country in western Europe, despite the fact that the United Kingdom dwelling selected for comparison was appreciably larger. In the United States, the figure was lower still (about 2.0). A later United Nations study (28) showed that in eastern Europe housing costs were slightly higher than in the west but, with the exception of Yugoslavia, lay in a similar range. D.V. Donnison pointed out (6) that such comparisons should not be pressed too far, since the findings may be due to the fact that the countries concerned selected modern dwellings for the purposes of the comparison and then related their costs to average incomes of manual workers, which in the poorer countries are largely derived from low-paid rural occupations. Nevertheless, the fact stands out that in real costs the provision of decent dwellings is much higher in poorer than in richer countries, although their standards may be pitched lower. There is no doubt that this factor, combined with the generally low level of investment in housing in developing countries (see Section 1), in large measure accounts for the enormous shortfall in the provision of houses in recent years.
Studies made in the United Kingdom by the Building Research Station (3) suggest that the cost of building a new house has remained close to three times the average annual earnings of industrial workers for the past 40 years, except for short periods of higher costs following each world war. Figures for the United States from 1948 to 1960 show a similar pattern, although American earnings are much higher. Thus, when comparisons are made for one country over a long period of time, or between countries at similar stages of industrial development, the real cost of a house measured in man-years remains much the same.
Donnison (6) concludes from these findings that " the price of an adequate house is likely to keep pace with average incomes, although temporary fluctuations in supply and demand, government interventions imposing higher standards of building and layout and other factors will produce occasional departures from this pattern." This he attributes to the fact that "unlike saucepans, stockings, bread or ballpoint pens, housing is not a single commodity whose price can be expected to fall as productive techniques improve. Housing is a whole complex of commodities and services, forming a central feature in a nation's living standards and largely determines the demand for other goods which depend heavily on the kind of house that must accommodate them, the number of people it houses and the aspirations it encourages. This does not imply that attempts to reduce building costs must be fruitless, but suggests that the fruit they bear will generally be harvested in the form of better rather than cheaper housing"
It is further deduced from these findings that if people's priorities with regard to consumption expenditure remain unchanged, the amount they will be prepared to spend on housing will only rise and fall with the level of their incomes. Any reduction in building costs may be swallowed up either by an improved quality and quantity of housing purchased, 34 or by an increase in the cost of land, or profit of the developers, or by increased taxation on house property. Unless they operate on a sufficiently massive scale to alter the basic preferences and priorities in investment and to secure higher and widely distributed incomes, government interventions in the market are more likely to alter the distribution between the various subheads of the total cost of shelter than to change the proportion of income devoted to the total and raise housing standards and output. This is no doubt particularly true of the market economies in developing countries.
(34 Section 1 shows that this has been a strong trend in industrialized countries.)
BUILDING COSTS AS A PROPORTION OF SHELTER COSTS
Within the above limitations on shelter costs, what is the proportion of building costs which might be affected by the use of timber and by technological advances in production and building techniques? This will vary greatly not only between countries, but within countries according to types of dwelling and residential densities, cost of land and site services, prevailing costs of building materials and components, the builder's overheads and profit (or loss-largely dependent on his costing and management efficiency) and, most important, capital costs, depending on interest rates and terms of loan. J.J. Huson has shown (12) that in typical United States housing built in recent years, both single-family housing for sale and low-rise garden apartments for renting, the building costs represented approximately half of real shelter costs. Cost studies carried out on the construction of typical bungalows in the Ottawa district in the same period showed a similar breakdown of costs, with the building cost component representing 58.8 percent of the selling price (see also Figure 3). It is significant that in both cases the houses analysed were conventional timber-frame structures, competitive in their class of housing with masonry construction.
BUILDING COST ANALYSES
The analyses of building costs given by Huson (12) for a typical two and a half-storey garden apartment block and by Hansen for the Ottawa bungalow (see Tables 5 and 6) are remarkable for some striking similarities, although they represent entirely different house types. In the apartment block the complete shell represents about 45 percent of total building costs. In the Canadian bungalow the shell cost amounted to about 53 percent. Plumbing, electrical and mechanical services, however, represented 25.4 percent of costs in the apartment block and about 21 percent in the bungalow. Finishes accounted for just over 30 percent for the block and 26 percent for the bungalow. The complete shell of the house, therefore, accounted in both cases for only about one quarter of the true housing cost. This indicates the restricted boundaries within which changes in methods or details of construction of walls, floors, roofs and other parts of house shells are likely to affect costs.35
(35 Hansen also reports (8) on a second bungalow built to the same plan and elevations as the first, and designed to incorporate many cost reducing improvements, but the total effect was small.)
TABLE 5. - BUILDING OPERATIONS COSTS OF A TWO AND A HALF-STOREY GARDEN APARTMENT BUILDING IN THE UNITED STATES
|
Operation |
Cost |
Share of total |
Share of labour on-site |
|
US dollars |
Percent |
||
|
Foundation |
4 126 |
2.2 |
30 |
|
Flat concrete |
4 700 |
2.5 |
50 |
|
Termite control |
172 |
0.1 |
80 |
|
Slab flashing |
83 |
- |
60 |
|
Lumber |
28 050 |
14.8 |
0 |
|
Millwork |
4 950 |
2.6 |
0 |
|
Cabinets |
7 900 |
4.2 |
0 |
|
Stairs |
900 |
0.5 |
0 |
|
Rough carpentry labour |
19 125 |
10.1 |
98 |
|
Trim carpentry labour |
6 275 |
3.4 |
100 |
|
Lightweight concrete |
3 000 |
1.6 |
50 |
|
Wallboard |
10 000 |
5.3 |
50 |
|
Windows and glass doors |
2 660 |
1.4 |
0 |
|
Mirrors and medicine cabinets |
800 |
0.4 |
0 |
|
Miscellaneous hardware |
1 500 |
0.8 |
0 |
|
Fixed class |
100 |
- |
0 |
|
Structural steel |
150 |
- |
0 |
|
Masonry |
4 000 |
2.1 |
65 |
|
Ornamental iron |
1 100 |
0.6 |
20 |
|
Roofing |
2 000 |
I. I |
50 |
|
Plumbing |
19 000 |
10.1 |
40 |
|
Electrical |
15 100 |
8.0 |
45 |
|
TV antenna |
500 |
0.3 |
50 |
|
Heating, air conditioning and sheet metal |
13200 |
70 |
30 |
|
Insulation |
4 500 |
2.4 |
50 |
|
Painting |
6 850 |
3.6 |
70 |
|
Wallpaper |
2 500 |
1.3 |
50 |
|
Caulking |
100 |
- |
80 |
|
Appliances |
8 960 |
4.7 |
0 |
|
Ceramic tile |
1 500 |
0.8 |
65 |
|
Floor tile |
1 400 |
0.7 |
50 |
|
Carpet |
11 140 |
5.9 |
30 |
|
Draperies |
1 750 |
0.9 |
10 |
|
Final clean-up |
650 |
0.3 |
100 |
|
TOTAL |
188 841 |
99.7 |
38.2 ($72 255) |
SOURCE: Huson, J.J. Analysis of costs in housing. In Proceedings of the International Symposium on Low-cost Housing Related to Urban Renewal and Development, 8-9 October 1970. Rolla, Missouri, University of Missouri Press
It is interesting to note that in Surinam the cost of Bruynzeel precut or ready-cut house packages constitutes on average about 40 percent of the price of the finished houses (47). Site labour costs were said to amount to about 20 percent of the selling price in Surinam, but considerably more in some of the Caribbean islands where wage rates were higher (compare with 13.4 percent proportion of site labour in the typical Canadian bungalow built by conventional on-site methods). While these comparisons cannot be pushed too far, it is significant that in the developing country as in the industrialized one the labour element of costs is small compared with that of the prepared materials and components delivered to site. This applies of course to well-developed systems of construction and suggests that further economies in such cases are more likely to be derived from a reduction of cost of material by more economical designs and elimination of waste.
The importance of good organization and site management, involving a high degree of standardization and the setting of work norms for individual operations in reducing or containing building costs, is brought out clearly in a report from the Housing Department of Tasmania (18). It was shown there that by the application of standard controls, without any technological change in mode of production of components or the conventional methods of on-site erection (of frame houses), building costs between 1952 and 1963 hardly increased despite a rise of about 50 percent in both wage rates and material costs (see Figure 4).
COST COMPARISONS BETWEEN TIMBER-FRAME AND BRICK HOUSES
Cost comparisons between houses built with different materials and techniques are not easy to establish for a number of reasons, which may be grouped as follows:
It is difficult to equate comparability of performance and to assess the value to be placed, e.g., on better insulation, finish, durability, appearance, etc.The wide range of variations in site labour requirements and building costs, even for the same method of construction, due to differences in plan, house size, details of construction, site and weather conditions, size of contract and, in particular, efficiency of management and labour, in which specialization and continuity play a large role.
For these reasons (particularly those in the second group), estimates of cost based on designs or on proto-type houses, or even on small numbers of houses of new types, can be misleading. In the United Kingdom special efforts were made in the postwar period to develop statistical methods of planning new housing developments on a substantial scale involving hundreds of houses, and of analysing site and factory data on labour requirements and costs, to enable predictions to be made for the performance of similar house types on similar or different sites, with due allowance for improvement rates and within calculated confidence limits (16, 17).
TABLE 6. - SUMMARY OF ON-SITE CONSTRUCTION OPERATIONS AND COSTS OF A TYPICAL BUNGALOW AT GLEN-CAIRN, NEAR OTTAWA
|
Operation |
Share of total on site cost ($ 10586) |
Cost components |
||
|
Labour |
Equipment |
Materials |
||
|
Percent |
||||
|
Earthwork |
2.13 |
11.0 |
89.0 |
0.0 |
|
Sod, final grade |
0.94 |
43.5 |
... |
56.5 |
|
Walks, drives, exterior steps |
2.16 |
15.2 |
0.6 |
84.2 |
|
Foundation walls, (including footings, drain tile) |
7.15 |
29.2 |
... |
70.8 |
|
Basement floor (including base) |
2.78 |
35.6 |
... |
64.4 |
|
Rough carpentry (including sill, beams, columns) |
15.29 |
15.6 |
... |
84.4 |
|
Roofing (including chimney saddle) |
2.41 |
22.8 |
... |
77.2 |
|
Insulation and vapour barrier |
1.64 |
23.4 |
... |
76.6 |
|
Exterior doors, windows (including storms, sash, frames) |
7.81 |
8.3 |
... |
91.7 |
|
Plumbing (including service to house) |
5.29 |
9.8 |
... |
90.2 |
|
Electrical (including range) |
4.78 |
18.3 |
... |
81.7 |
|
Heating |
4.29 |
13.8 |
... |
86.2 |
|
Wallboard (including wall tile) |
6.32 |
41.0 |
... |
59.0 |
|
Interior finish carpentry and millwork |
11.82 |
14.6 |
... |
85.4 |
|
Hardwood floors (including sanding, varnishing) |
4.10 |
20.4 |
|
79.6 |
|
Resilient and ceramic flooring (including underlay or base) |
1.18 |
31.1 |
... |
68.9 |
|
Chimney and fireplace |
5.30 |
34.7 |
... |
65.3 |
|
Brick veneer |
236 |
32 8 |
.. |
67.2 |
|
Aluminium siding fascia and soffits (including strapping and sheathing paper) |
6.10 |
18.7 |
... |
81.3 |
|
Plywood siding and ceiling (carport), and exterior wood trim |
1.62 |
29.0 |
... |
71.0 |
|
Exterior and interior painting |
2.67 |
76.7 |
... |
23.3 |
|
Miscellaneous (clean-up, repairs, deliveries) |
1.86 |
92.3 |
... |
7.7 |
|
TOTAL |
100.00 |
23.5 |
1.9 |
74.6 |
SOURCE: Hansen, A.T. The Mark project. Part 1. A cost study of a typical bungalow. Ottawa, National Research Council of Canada, Division of Building Research, 1967. Housing Note No. 29.
The results of these early developments in the United Kingdom and more recent advances in industrialized house-building are particularly instructive since they offer a guide to the inherent competitiveness of timber-frame housing, measured by the yardstick of conventional brick construction in which that country excelled in quality and low cost. In 1948 timber was in short supply and strictly controlled, consequently few of the many new methods of house construction then developed were based on its use as a structural material, other than in roofs and intermediate floors in two-storey housing which were invariably used in conventional brick housing. Of the nine " new methods of house construction" first studied on an adequate scale as to labour and cost performance, only one was wholly timber framed. 36 Its "improved productive times on an average site" were the best of all the house types studied,37 which included cast in-situ and precast systems, and also compared favourably with the "pooled traditional" average (see Figure 5). The cost of this house, however, was 8 percent higher than the national average for traditional brick construction.
(36 The design was based on factory-assembled wall panels of very light timber framing, machined out of 3 × 1 ½-inch members, with brick veneer to ground floor outer walls and steel sheet cladding applied in the shop to the upper storey panels. Structural design was by whole-house prototype testing, and constituted a tour de force in that the total softwood content was no greater than that allowed for traditional brick houses, that is, 1.6 standards (about 8 cubic metres))
(37 Only one nonwood housing system of a number subsequently studied gave slightly better performance in terms of man-hours.)
FIGURE 4.- Cost indices in timber-frame house building (by direct labour), Housing Department of Tasmania. 1. Building costs. 2. Material costs. 3. Labour rates.
SOURCE: Murphy, G.I. Standard controls for large-scale single unit housing. In Towards Industrialised Building: Proceedings of 3rd C.I.B. Congress, Copenhagen, 1965.
Characteristics of walls
Type 1 9-inch in-situ concrete cavity construction
Tape 2 12-inch in-situ " no-fines" concrete ovally
Type 3 Concrete studs and " weatherboard" panels
Type 4 Post and panel construction in aerated concrete
Type 5 Precast concrete frames with internal and external small slabs
Type 6 Storey height precast concrete units (narrow width)
Type 7 Large crane-handled precast concrete panels
Type 8 Large brickette-faced foamed-slag concrete panels (erected with special travelling gantry)
Traditional 11-inch cavity walls of brickwork
(Note: The ranges shown are curtailed 90 percent confidence limits for use in comparing the systems. Only when two ranges do not overlap can it be stated, with 90 percent confidence, that the difference in productive times is significant.)
FIGURE 5. - Comparison of timber frame with brick (traditional) arid alternative methods of house construction productive times for superstructures (man-hours).
SOURCE: United Kingdom. Ministr of Works New methods of house construction. London, HMSO, 1948-1949. National Building Studies, Special Reports No. 4 and No. 10).
With reference to these costs the Ministry of Works report (16) stated:" It might be expected that the known advantages of this type of construction in ease and speed of erection would be reflected in the cost of the house. The house has been subject in this respect to the same disadvantages as the other nontraditional types. There has been the uncertainty about future business which has made it impossible to spread the overhead charges in a rational manner, and the same uncertainty has so far operated to prevent an efficient line of production in the factory. Some discussions have taken place with the sponsors, as a result of which it is expected to be possible to make economies in the construction of the house, which would save between £60 and £100. In addition, substantial savings should be possible by the application of rigorous costing in the factory, which should make the house definitely cheaper than the traditional house."
This long quotation serves to emphasize the cost penalties under which timber-frame construction, even developed as a fully industrialized system - in the sense that the sponsors operated as an integrated orga nization of designers, manufacturers and builders-was bound to suffer in the initial stages of development. For this reason government support and encouragement were made available to its sponsors (as to those of other selected new methods) not only by assistance with prototype development but by granting special subsidies to housing authorities using the new methods, in order to bridge the cost differential with traditional construction in the first few years in the life of the systems.38
(38 When the subsidies were withdrawn many of the new systems proved unable to compete and went out of production, but not the timber-frame system.)
Toward the mid-1960s a second spate of activity in the development of industrialized methods of building was sparked off in the United Kingdom by the announcement of a greatly expanded building programme, with which the traditional industry was not expected to be able to cope. Of the various methods of construction which now flourished, precast concrete large-panel systems dominated the high-rise sector and in-situ concrete (mainly of the " no-fines" variety) and timber-frame housing dominated the low-rise sector. A number of successful timber housing systems were now operating, largely through licensed contractors, and information which was collected by the Timber Research and Development Association and others showed that they compared favourably in costs with the national average for traditional brick construction (14). See also Figure 6.
The most striking example of the present competitiveness of industrialized timber-frame housing was provided in the 1970 annual report of the London Housing Consortium --- West Group. The group successfully negotiated a joint project comprising 718 dwellings (of two- and three-storey houses) on six sites between the Boroughs of Hillingdon and Islington. For four sites for which full costs were available, substantial savings have
TABLE 7. - SAVINGS ACHIEVED BY USE OF INDUSTRIALIZED TIMBER-FRAME HOUSING ON FOUR SITES IN THE UNITED KINGDOM
|
Site |
Number of dwellings |
Approved estimates for traditional construction |
Accepted tenders for timber-frame system¹ |
|
Pounds sterling |
|||
|
HILLINGDON |
|||
|
Cranford Park IV |
113 |
427 766 |
394 689 |
|
Cranford Drive |
157 |
591 121 |
561 206 |
|
ISLINGTON |
|||
|
Barnsbury Grove |
86 |
442 126 |
380 233 |
|
Mildmay Street |
48 |
237 914 |
205 382 |
|
TOTAL |
404 |
1 698 927 |
21 541 510 |
¹The industrialized building system in this case was a fairly highly prefabricated timber-frame system, consisting of large panels for ail main elements, with brick veneer to ground floor outer walls and shopapplied plastic cladding to upper storey panels. Erection by the main contractor (not the system developer) was crane operated. -² Total saving on estimated cost was £157 417.
SOURCE: Holmes, R. Optimum design and production of two-storey, timber housing, 1970. unpublished thesis based on official housing statistics and information from industry. been achieved on the approved estimated costs if each site had been handled by traditional methods. In Table 7 total savings on the estimated costs are set out.
The large scale of this contract, awarded to a single building contractor, no doubt contributed in part to the savings, which amounted to nearly 10 percent (or £400) per dwelling. However, examples in the past few years have showrt that even on very small projects of 10 or 20 houses wood-frame housing can match conventional building in costs, provided the degree of shop fabrication and the method of erection are suited to the contractor's organization and the scale of the scheme. Two groups of 12 houses each were erected in 1970 by the SEMLAC/ TRADA method.39 In both cases the contractors had had no previous experience in timber-frame housing, yet their tender prices were competitive and one of the contractors conceded that his profit was rather higher than with conventional construction.
(39An open system, in which the shell is composed of relatively small panels of timber and plywood, designed for manual erection in small- and medium-size projects.)
The conclusion which may be drawn from the United Kingdom experience is that a decade or two were required to break the cost barrier and make timber framing competitive with traditional brick construction. In effect, in that period it ceased to be an innovation and became fairly widely accepted by clients and the building industry alike. It should be added that this was achieved despite the fact that during the same period productivity in the building industry generally increased substantially (7). This development should be encouraging to those who do not find timber-frame housing competitive in the first instance-a common experience in countries where this method of building is not indigenous.
COMPONENT PRODUCTION AND COST
Standardization and mass production of individual joinery and structural components lead to cost reduction through: refined design with consequent economy in materials (e.g., through glueing and use of sheet materials in flush door production, or press inserted tooth plates in trussed rafter manufacture); and through reduction of labour requirements by mechanization and division of operations. Despite progress in respect of many items, particularly in the industrialized countries, there is still great scope for further rationalization of the design of components and their standardization to reduce costs still further. Advances in this direction will do much to maintain the position of wood products in housing, as well as promote timber-frame housing.
(1) AMERICAN INSTITUTE OF TIMBER CONSTRUCTION. 1966 Timber construction manual. New York, Wiley.
(2) BOOTH, L.G. 8: REECE, P.O. 1967 The structural use of timber. London, Spon.
(3) GARSTON, ENGLAND. BUILDING RESEARCH STATION. 1962 International comparison of the cost of house building. London, HMSO. Note No. C918.
(4) U.S. DEPARTMENT OF HOUSING AND URBAN DEVELOPMENT and U.S. DEPARTMENT OF AGRICULTURE. 1947 Manual on wood construction for prefabricated houses. Washington,
D.C. (Reprint 1967)
(5) DICKENS, H.B. 1969 Trends in Canadian house production. Ottawa, National Research Council of Canada, Division of Building Research. Technical Paper No. 299.
(6) DONNISON, D.V. 1967 The government of houshig. London, Penguin Hook S.
(7) FORBES, W.S. 1969 A survey of progress in housebuilding. London, HMSO. Building Research Station. Cur rent Paper 25/69.
(8) HANSEN, A.T. 1967 The Mark V project. Part 1. A cost study of a typical bungalow. Housing Note No. 29; Part. 2. Changes to reduce costs. Housing Note No. 30. Ottawa, National Research Council of Canada, Division of Building Research.
(9) HANSEN, HJ. 1948 Timber engineering handbook. New York, Wiley.
(10) HIXON, E.L. 1970 Acoustic evaluation of ten houses in the Austin Oakes Project. In Proceedings of the Inter national Symposium on Low-cost Housing Related to Urban Renewal and Development, 8-9 October 1970. Rolla, Missouri, University of Missouri Press.
(11) HOLMES, R. 1970 Optimum design production of two-storey timber housing. (Thesis)
(12) HUSON J.J. 1970 Analysis of costs in housing. In Proceedings of the International Symposium on Low-cost Housing Related to Urban Renewal and Development, 8-9 October 1970. Rolla, Missouri, University of Missouri Press.
(13) KEATING, E.L. & MILLER, E.S. 1970 Preconditions for and benefits of the development of low-cost housing technology. In Proceedings of the International Symposium on Low-Cost Housing Related to Urban Renewal and Development, 8-9 October 1970. Rolla, Missouri, University of Missouri Press.
(14) LEVIN, E. & MATTOCK, P. 1967 Some economic aspects of timber frame housing. High Wycombe, Bucks., Timber Research and Development Association. Information Bulletin A/IB/5.
(15) UNITED KINGDOM. MINISTRY OF HOUSING AND LOCAL GOVERNMENT. 1952 The density of residential areas. London, HMSO.
(16) UNITED KINGDOM MINISTRY OF WORKS. 1948-1949 New methods of, house construction. London, HMSO. National Building Studies, Special Reports No. 4 and No. 10.
(17) UNITED KINGDOM. MINISTRY OF WORKS. 1959 A study of alternative methods of house construction. London, HMSO. National Building Studies, Special Report No. 30.
(18) MURPHY, G.I. 1966 Standard controls for large scale single unit housing. In Towards Industrialised Building: Proceedings of 3rd CIB Congress, Copenhagen, 1965. Amsterdam, Elsevier.
(19) OLLIS, 1. & JORDAN, J. 1971 Watery Lane project. Architects' Journal 10 February 1971. High Wycombe, Bucks., Timber Research and Development Association. (Reprint)
(20) STERN, E.G. 1968 Research on jointing of timber framing in the U.S. In Modern Timber Joints: Proceedings of an International Symposium on Wood Joints, London, April 1965. High Wycombe, Bucks., Timber Research and Development Association.
(21) STONE, P.A. 1959 The economics of housing and urban development. London, HMSO. (Building Research Station, Garston)
(22) TIMBER ENGINEERING COMPANY. 1959 Timber design and construction handbook. 2nd ed. New York, F.W. Dodge Corporation.
(23) TIMBER RESEARCH AND DEVELOPMENT ASSOCIATION. 1967 Design data for timber structures. Architects' Journal, 8 and 15 March, 12 and 19 April 1967. High Wycombe, Bucks. (Reprint)
(24) TIMBER RESEARCH AND DEVELOPMENT ASSOCIATION. 1967 Timber frame housing design guide. High Wycombe, Bucks.
(25) TIMBER RESEARCH AND DEVELOPMENT ASSOCIATION. 1968 Sound insulation of timber framed party walls. High Wycombe, Bucks. Information Bulletin A/IB/7.
(26) TURIN, D.A. 1969 The construction industry: its economic significance and its role in development. London, University College Environmental Research Group. Paper presented to the United Nations Industrial Development Organization, 1969.
(27) UNITED NATIONS. ECONOMIC COMMISSION FOR EUROPE.1958 The financing of housing in Europe. Geneva.
(28) UNITED NATIONS. ECONOMIC COMMISSION FOR EUROPE.1968 Major long-term problems of government housing and related policies. Geneva.
(29) UNITED NATIONS INDUSTRIAL DEVELOPMENT ORGANIZATION. 1970 Production techniques for the use of wood in housing under conditions prevailing in developing countries: report of Study Group, Vienna, 17-21 November 1969. New York, United Nations.
BACKGROUND PAPERS
(30) ANDERSON, LO. 1971 The wood frame house resists nature's furies. WCH/71/4a/5.
(31) BECKER, G. 1971 The hazard of fungus and insect aback for wood and wood-based material in houses in various regions and means of alleviating it. WCH/71/4a/6.
(32) BIM, J. & KOUKAL, M. 1971 Production of joinery for tropical countries.
(33) BLACK, J.M. 1971 Finishes, construction factors, and design to compensate for effects of weather on wood. WCH/ 71/4a/4.
(34) BLOMQUIST, R.F. 1971 Timber framing for on-site construction. WCH/71/5/4.
(35) BOOTH, L.G. 1971 Timber engineering for developing countries. WCH/71/5/2.
(36) BOYD, J.D. 1971 Problems associated with the use of wood in construction. WCH/71/4a/2.
(37) BURGESS, HJ. 1971 Design aids including computer programs for universal application. WCH/71/5/8.
(38) BURGESS, HJ. 1971 Experience with the promotion of wood in housing in the tropics.
(39) CAMPBELL, P.A. 1971 Performance specifications for the quality control of timber for housing in developing countries. WCH/71/6/2.
(40) COUNTRYMAN, D. 1971 The use of glued and composite elements in housing. WCH/71/3/3.
(41) GUISCAFRE, J. 1971 Le bois dans la construction en Afrique tropicale francophone. WCH/71/6/9
(42) HEWITT, R. 1971 Design standards for wood: a North American view. WCH/711/5/1.
(43) KEMPTHORNE, A.W. 1971 Plywood gusseted rigid frames: a low cost structure for housing. WCH/71/5/7
(44) PERCIVAL, D.H. 1971 Present and potential applications of treated pole and post construction for houses. WCH/71/3/4.
(45) PRANGE, G.F. 1971 Uses of sawn lumber in housing.
(46) SILVERSIDES, R.G. 1971 Fire hazard in timber structures. WCH/71/4a/3.
(47) TBMPELAAR, C.W.F. 1971 Industrial production of housing in developing countries. WCH/71/5/6.
(48) TUCKER, J.L. 1971 Industrialized housing. WCH/71/5/5.
SPECIAL PAPER
(49) TIUSANEN, K. 1969 Production of prefabricated wooden houses. New York, United Nations. ID/61.
1. Problems of design, production and erection techniques and costs were covered by the Consultation. The following aspects of design were examined: urban development and house densities; height of residential building and its effect on the use of wood in elements of structures; planning principles and functional requirements and their effect on the use of wood and structural efficiency; and the potential use of wood in various components. Attention was drawn to the advantages and drawbacks of prefabrication, and prerequisites for the industrialization of production of timber housing Finally, in covering cost aspects, the Consultation dwelt on building costs and cost comparisons between timber-frame and brick houses. The detailed discussions resulted in the recommendations which follow.
2. Many delegates highlighted the lack of training in timber engineering in the curricula of civil engineering courses in universities. The Consultation recommended that, in order to increase the use of timber in building, technical colleges, universities and professional institutions include substantial and adequate training in timber engineering and its utilization in housing in the curricula and training programmes of civil engineers and architects.
3. Methods of structural design and selection of types of structures were discussed. The need to simplify products as far as possible through sound design was stressed. The Consultation noted with interest the computer aids for design developed by TRADA in the United Kingdom, and recommended the establishment by an international agency of a central international clearinghouse for technical information, with particular reference to computer programmes and design aids for timber engineering and building practice. Such a centre should diffuse information through national organizations.
4. Wood-based materials available for use in housing were reviewed and the Consultation was of the opinion that lack of standards and poor grading often deterred specifiers and designers from recommending the use of wood. Consequently, it urged the International Standards Organization (ISO) and regional and national standardization associations to speed up the work of committees concerned with the standardization of dimensions, qualities, testing methods, etc., of finished and semifinished wood products. To accelerate this work it recommended that international groups be formed to put forward draft design codes and standards for consideration by ISO. It was stressed that end users need to be adequately represented in these groups.
5. The adaptability of nailed laminated trusses and their advantages in conditions in developing countries were stressed.
6. Various topics related to production and erection techniques were discussed. The characteristics of the market and demand affected production methods which ranged from traditional on-site methods of production to the prefabrication of complete house shells, and to the use of competitive materials and mixed construction.
7. The need to improve production planning and organization of tasks on construction sites was stressed.
8. The Consultation drew attention to the fact that the versatility of wood in design and in production methods permits the adoption of a whole range of techniques appropriate to the technological level and market conditions in a given country. It considered that all methods, from the most traditional on-site construction using random length materials to whole house prefabrication, would be valid for different circumstances arising in different countries, or for different sections of their market.
9. Necessary prerequisites for economically and technically viable prefabrication were considered to be:
(a) the existence of materials of adequate quality, consistent grading, and accurate dimensions;
(b) an adequate sized market for the particular houses or elements of houses;
(c) proper design related to social, climatic and other local conditions;
(d) a climate of acceptance of the product by local building authorities, the building industry, and the public at large; furthermore, the development of products must proceed hand-in-hand with a dynamic promotion campaign;
(e) adequate transport facilities.
10. The Consultation considered that in general the road to industrialization lay in developments in the following order, and that short-cuts could lead to difficulties and failures:
(a) standardization of sawnwood and wood-based panel production;
(b) development of mass production of simple components for joinery and structural uses (such as doors, windows, shutters, trusses, beams, etc.);
(c) production of panels for walls, floors, etc.;
(d) whole house systems.
11. The Consultation recognized that in order to develop wood housing-particularly for multidwelling urban units-and to enhance its acceptability the use of wood should be encouraged in combination with inorganic materials used either as protective coverings or as binders in mixed materials (such as wood-cement slabs). In the latter connexion, the Consultation urged the investigation of the suitability of the various tropical species and of methods of production which are not capital-intensive.
12. The difficulties arising from comparing costs of various types of buildings and for production processes were noted.
