J.A. LISKA
J.A. LISKA is chief of the Division of Wood Engineering Research, Forest Products Laboratory, Forest Service, U.S. Department of Agriculture, Madison, Wisconsin, United States. This paper has been prepared from the background papers listed at the end, and the contributions of the authors are gratefully acknowledged.
WOOD, WHEN WISELY USED, can last indefinitely, as demonstrated by houses in the United States that are giving excellent service after more than 300 years, and wood structures in Japan that are in superb condition after more than twice that time. It is primarily when wood is misused that problems develop.
In the educational system in the United States, wood as a construction material receives scant attention in the training programmes for architects and engineers. Without an understanding of the attributes and basic characteristics of wood, it cannot be used effectively. Designs for steel or concrete cannot be translated to wood, and ignorance of wood's properties results in building designs that are often a comedy of errors. Wood is not always misused. There are those architects, engineers and builders who are educated to wood's capabilities and, because they love and understand it as a construction material, use it efficiently and effectively.
Where wood has been supplanted by other construction materials it is all too often because of consumer dissatisfaction with its performance. The consumer is not interested in the cause of the problem, and wants only to eliminate it or hide it from sight, and has a tendency to accept substitutes readily. Unfortunately, the cure often creates a situation more serious than the basic problem. For example, in the case of paint coming off the exterior of a house because of excess interior relative humidity, the consumer may see only the paint problem and try to correct it by covering the building exterior with an impervious material. He has not solved the difficulty of excess moisture, and it is only a question of time until this reveals itself again.
Excess moisture is a major problem in the effective utilization of wood. This moisture can result from excessive relative humidity in a dwelling interior and an unsatisfactory vapour barrier in the walls, a poor system of space ventilation, an unsatisfactory structural design that traps rather than sheds water, or an inadequate treating or finishing system. There may be paint failures, staining of wood surfaces, or actual decay of me wood with resultant loss of strength and structural integrity. If wood is well understood as a construction material, potential problems can be eliminated. When wood is used in contact with the ground or under conditions where it will attain a high moisture content, decay-resistant wood species can be used, or the wood can be treated with preservatives to resist the most severe environmental conditions. Proper design of structures with adequate roof overhangs, attention to design details to eliminate water traps, and proper protection for severely exposed elements can contribute to trouble-free use of wood.
Wood is a combustible material and there is no existing treatment that will make it noncombustible. Nonetheless, in large sizes wood burns relatively slowly, and may offer more protection under severe fire than other materials which may lose strength and structural integrity. In smaller sizes, wood can be treated to make it more resistant to flame spread. It must be recognized that a major hazard to life results from the hot and poisonous gases given off from the burning of the contents of a building, even though the structure is fireproof. Thus, it may be more desirable to give greater attention to detection and warning systems and to adequate means of egress, rather than to the combustibility of the materials in the structural framework. Composite construction may be developed to offer required levels of fire endurance and construction details incorporated to retard spread of fire.
In the United States, wood has been losing markets to competitive materials as exterior covering for houses largely because of poor performance of exterior finishes. Part of the problem is due to moisture, either external or resulting from high interior relative humidities. Poor performance may also result from selection of species, grain orientation, or both, incompatibility of finishing systems, and construction or maintenance practices. The potentials of nonfilm-forming finishes such as stains and water repellents that emphasize the basic beauty of the wood are being demonstrated by research, as also are treatments that stabilize wood against ultraviolet deterioration and thus provide a better substrate for finishes.
One of the attributes of wood is its ability to be easily and rapidly assembled into sound structural elements with mechanical fastenings. This is particularly true of softwoods. When hardwoods are used, they are frequently assembled in the green condition; while this provides for greater ease of assembly, it is also likely to result in splitting of the wood elements with subsequent loss in joint strength as drying occurs. Hardwoods can be assembled dry if they are predrilled, or if slender high-strength steel nails are utilized, usually with the aid of heavier hammers. Fastenings have been improved through research to provide nails with deformed shanks with higher holding power, high-strength steel bolts, timber connectors, truss plates and power-driven staples. The staples offer advantages in assembling hardwoods since their more slender legs reduce splitting tendencies both during driving and subsequent seasoning.
On-site glueing has not been effective as a means of component assembly largely because of use difficulties and lack of quality control. In recent years, mastic-type adhesive systems have been introduced that offer considerable promise. The better of these systems can be used at low temperatures, on wet and rough wood, and seem to be durable and resistant to high temperatures. Used with a limited number of mechanical fastenings to provide glueing pressure, they may be an answer to more effective structural use of hardwoods.
Wood structures can resist earthquakes or hurricanes. The basic concept is that all components of the structure must act together. It is recognized that the slight movement possible in mechanical joints permits the structures to give slightly under shock loadings without serious structural damage. It is also evident from studies of structures after severe earthquakes or hurricanes that if the mechanical fasteners are used in accordance with good design practices they will provide the needed structural integrity. Suitable fasteners or fastening systems must tie the floor structure to the foundation, walls to floors, and roof to wall. The structural elements are then covered with correctly fastened sheet materials to provide the structural unit which can resist these severe loadings.
Water and wave damage can be severe in hurricane areas. This can be minimized if the structure is built on and firmly attached to piles, or if a pole-type structure is built where the poles form continuous structural elements from the embedment in the ground to the roof structure. The other building elements are firmly attached to the poles to give a structure resistant to the most severe wind and wave forces.
Wood's excellent potential as a structural material can be negated by the attack of and deterioration from fungi and insects. Therefore. if wood is to be effectively used in housing construction, practices that minimize these hazards must be followed: building techniques that prevent undue wetting of elements; the use of heartwood of naturally resistant species; the treatment of less resistant species with preservatives; suitable treatments for the eradication of pests. When these protective measures are taken, wood and wood-based materials can be used effectively under nearly all service conditions.
FUNGI
Fungi of a number of species are able to attack wood and, through enzymatic processes, degrade and decompose its cellulose and lignin, thus decreasing its mechanical strength properties. Other fungi may cause surface staining, which may not seriously impair the strength of wood but decreases its economic value and creates a suspicion of more serious degradation.
Three types of wood destruction result from fungal attack-brown rot, white rot, and soft rot. The hyphae of the brown and white rot fungi grow through the cell walls in small holes and decompose them from within. whereas soft rot fungi are mainly confined to the central cell-wall layer. Brown and soft rot fungi attack the cellulose in the wood, while white rots simultaneously degrade the cellulose and lignin. Soft rot fungi normally change the colour of wood only slightly although a dark brown colour sometimes results, while brown rot and white rot give it a brownish or whitish colour.
Where decomposition takes place, loss of wood results and mechanical strength decreases. Strength losses are much more rapid than loss of mass and may reach 50 percent at mass losses of 5 to 10 percent. All properties are not affected to the same degree; those involving energy absorption are most seriously affected, while the decrease in stiffness is much less rapid.
Fungi need food, moisture, and favourable temperature conditions in order to grow. The minimum moisture content for growth is 22 to 23 percent, although many fungi require moisture nearer the fibre-saturation point. Many fungi, however, are able to survive for long periods under too wet or too dry conditions until a favourable environment for growth again exists. Most fungi are active at temperatures from several degrees above 0°C to 35°C with an optimum temperature of 30°C. Some species can tolerate higher or dower temperatures, and fungi with high tolerance to changing environmental conditions are the main destroyers of timber in interior use.
INSECTS
The primary wood-destroying insects are termites and beetles. Termites (white ants) are widespread in tropical and subtropical countries and 50 of some 2 000 species cause major economic damage. Termites need an almost saturated humidity environment in order to live; thus, when they leave their nests in the soil they build sheltered galleries. These provide visual evidence of their attack on structures. Optimum temperatures are 26°C to 30°C and, while tropical termites require 20°C to 22°C temperatures, those in temperate climates can endure temperatures near freezing point.
Most wood-destroying termite species are soil dwelling (subterranean), nesting either in soil or in attacked wood. Some species, however, can live without access to the soil and can cause serious damage to wood construction and furniture.
Termites damage wood by gnawing holes or galleries in it until the member is little more than a shell. The softer springwood normally preferred. Deterioration can take place very rapidly, which means constant vigilance of untreated species is necessary.
The most important wood-destroying beetles are the Cerambycidae (longhorn), Anobiidae (furniture or deathwatch), and Bostrychidae and Lyctidae (powder post) beetles. Destruction is caused mechanically by the larvae which gnaw galleries through the wood and live on the wood substance. Destruction varies, since a generation from egg to adult varies from several weeks to some years, depending on species, food, and climatic conditions.
Many beetle species need a wood moisture content above the fibre-saturation point. Others develop under drier conditions and these are most damaging to wood. Lower moisture contents are from 8 to 12 percent, depending on species. Optimum temperature for many species is 26°C to 30°C. Temperatures above 36°C are lethal to all species but critical exposure time may vary.
PREVENTION AND CONTROL METHODS
Since adequate moisture is a major requirement for the growth of fungi and many insects, care should be taken to keep moisture away from wood and wood-based materials. Proper construction techniques should be used to eliminate trapping of water and to facilitate drying out of wood that does become wet. Suitable ventilation should be provided for all areas where condensation of moisture can take place. Coatings and water-repellent treatments can also be used to retard moisture absorption.
Poured concrete foundations, walls, and floors free from cracks are a barrier against termites. Metal shields between termite-proof piles or walls prevent access of termites to the upper part of the building. Periodic examination for termite galleries should be made.
In some countries protective soil poisoning to eradicate termites is a common practice. This must be renewed at proper intervals to maintain toxicity. In the United States a combination of termite attractant and poison on stakes spaced at 5-foot intervals around the structures has been found a promising prevention measure. Nontoxic dusts like silica gel or fine clay cause insects coming into contact with them to dehydrate and die. Contact insecticides in rooms not inhabited by humans or used for food storage can protect timber from beetles if the female is killed before egg-laying.
The heartwood of some timbers has high natural resistance to attack by fungi and insects. It is important that such species be identified in the developing countries so the wood may be used where high hazard conditions prevail. Sapwood of all species is normally vulnerable to attack.
WOOD PRESERVATION
It is recognized that the use of pesticides can create problems. They can be injurious to humans, domestic animals, plants, fish and wildlife. Thus they must be used selectively and carefully.
To be effective, the preservative should be applied with sufficient penetration into the wood and have sufficient retention, and the flammability and strength properties of the wood should not be changed materially. In some wood products the preservative must be compatible with the adhesive system or with subsequent finishing operations. Compatibility with fastening methods and other materials used with wood may also be necessary.
Water-soluble compounds meeting the basic requirements of a preservative include those of boron, fluorine, zinc, copper and arsenic. Resistance to leaching by water is often provided by the addition of chromium compounds. In the oily wood preservatives tar oil creosote plays a most important role where its odour and colour are acceptable, as in piling and other exterior uses. Odour and colour are reduced and may even be neglected in preservatives that consist of carrier oils with the addition of fungicides and insecticides. These additions may consist of special fractions of creosote. The most important organic fungicide in many countries is pentachlorophenol. Copper pentachlorophenate, copper and other metallic naphthenates, tin compounds and organic mercury compounds are also used. Against wood-destroying insects, synthetic contact insecticides like dieldrin, chlordane, and organic phosphorus ester compounds arc very efficient.
Efficiency and permanence vary between groups of preservatives and with different organisms. Toxic limit thresholds also vary with the preservative and the organism, and above-threshold retentions are desirable for most conditions of service. For example, development of brown rot fungi can be prevented by loadings of I to 2 kilogrammes/cubic metre of wood of boron and fluoride compounds, and by about 3 kilogrammes/cubic metre of a chromium-copper-boron mixture. Soft rot and bluestain fungi are more tolerant to a number of compounds than brown or white rot fungi, while toxic limits against beetle larvae are generally lower than those for fungi. Arsenic compounds are the most efficient water-soluble substances against termites.
The best creosote products have thresholds against many brown and white rot fungi in the range of 5 to 10 kilogrammes/cubic metre, but this threshold may rise to 30 kilogrammes/cubic metre or more for more tolerant species. Pentachlorophenol has thresholds for many fungi of from 2 to 3 kilogrammes/cubic metre, but against some beetles the threshold is more than 10 kilogrammes cubic metre.
TREATING METHODS
The primary requisite for successful preservation is sufficient penetration of the toxic substances into and distribution within the wood. When preservative fluids are applied, an exchange with the air in the wood tissue takes place; the ease with which this exchange takes place varies with species. Fluids may be injected into the wood by pressure methods, applying vacuum or high pressure. Nonpressure methods involve wood's adsorption and capillarity to achieve penetration. Diffusion is also important in the case of water-soluble salts. Time requirements for treatment vary widely. Spraying or dipping can be carried out in a few seconds or minutes, pressure methods take hours, soaking in open tanks takes a few days, and the diffusion process needs weeks. Pressure treatments provide the maximum degree of protection, soaking or dipping processes provide some penetration, while spraying or brushing are superficial surface treatments.
In the United States a promising nonpressure method uses double diffusion. Green wood is soaked first in one chemical solution and then in a second. The two chemicals diffuse into the wood and react with each other to form an insoluble toxic compound.
ERADICATION OF WOOD-DESTROING ORGANISMS
The damage from various organisms may be halted by eliminating one of the factors necessary for their growth, such as moisture, or by poisons or preservative fluids applied in place. Gases can be applied for killing wood-destroying insects if the rooms can be made gasproof or the structure covered with a plastic tent. The gas most commonly used is methyl bromide. However, this is highly toxic to humans and a more promising gas treatment uses sodium methyl dithiocarbamate. Insects and fungi can also be killed by heat, either from hot air or high frequency in the wood. Under special conditions, gamma radiation may be used.
Once the damage has been halted, a decision must be made as to whether the wood elements are sufficiently strong to remain in place. Prompt recognition and halting of damage will, of course, minimize the costly removal and replacement problem.
While no known treatment can make wood noncombustible, research has shown that under many conditions it may be safely used as a construction material and may perform better than noncombustible materials.
EXPOSURE TO HEAT
When exposed to a sufficiently intense source of heat wood will burn, and it is desirable to avoid its use where burning or flaming may be a hazard to life. Long-term exposure to elevated temperatures may cause deterioration with resultant loss in strength. Since this is an important consideration in structural use, it is a good rule to avoid using timber in situations where its temperature is likely to exceed 75°C to 100°C for extended periods of time.
STRUCTURAL REQUIREMENTS
Timber can function as a structural, protective or decorative material in construction, and in each category the associated fire hazard and protective measures required differ. Desirable fire behaviour also depends on whether it is regarded in terns of occupant safety or survival of the structure and contents.
Fire resistance is not solely the property of a material. but includes the quality of construction that will ensure the confinement of fire or, failing that, to ensure that the building will withstand a burnout without collapse. It is measured in terms of stability, integrity, and thermal insulation: stability, to ensure structural survival; integrity, to prevent passage of flames and hot gases to other parts of the building; and insulation, to ensure that combustible materials in the area are not ignited by conducted heat. Not all of these criteria need to be satisfied in all situations. Structural elements (beams and columns) must meet stability criteria, while walls and floors may need to meet all three criteria to give acceptable performance.
Timber structural elements may possess a high degree of fire resistance. For example, a 12-inch-square European redwood column 10 feet long and designed to carry a load of 90 tons would have a fire resistance of approximately 45 minutes. An unprotected steel column designed for the same load would fail in 15 minutes. The wood column would fail by burning and charring and the steel column by softening due to exposure to high temperatures. The performance of both could be improved by enclosing them in an insulating material such as vermiculite plaster.
The rate of charring of timber subjected to standard heating conditions is about 1/40 inch per minute for the most frequency used structural softwoods. This varies to some extent with species, particularly hardwoods which may exhibit considerably greater resistance to charring.
Tests on Douglas fir and pine floor joists showed the existence of a good relationship between fire resistance, imposed stresses, and joist sizes which allowed prediction of fire performance from calculations. Addition of a timber floor to the joists adds to their fire resistance, and tongued-and-grooved boards will perform better than those with plain edges. A ceiling beneath the joists will also add fire resistance. In traditional small houses, joists with a wood-lath and plaster ceiling below and plain-edge boards above will satisfy integrity and insulation requirements for 15 minutes and will resist collapse for 30 minutes. Greater resistance may be obtained by the addition of 1/2-inch plasterboard over the ceiling or 3/16-inch plywood or dense hardboard over the floor. Floors give a higher degree of fire resistance if joists and decking are constructed with protective insulation.
In walls and partitions, wood can provide a considerable range in fire resistance. If the walls are sheathed with nominal 2-inch tongued-and-grooved boards, fire penetration time is about 10 minutes. A double layer construction of nominal 1-inch boards with a sheet of asbestos paper separating them is effective for 25 minutes. The weakness of such structures is in the joints, and fire resistance can be improved with the use of panel materials. Timber frame partitions covered with 'l.- to 5/8-inch plywood can resist penetration for periods of 10 to 25 minutes. If wall cavities can be filled with a mineral fibre insulation that will remain in position to protect frame and facing. fire resistance can be increased to nearly I hour.
Timber usage for structural elements such as beams and columns has been limited by the availability of high-quality timber. With the development of glued-laminated construction this trend has changed, and glued-laminated wood columns, beams and arches are increasingly used. With suitable adhesives the fire resistance of glued-laminated construction is similar to that of solid wood elements of comparable sizes. Actual fire resistance of columns will be influenced by shape, species, type of adhesives, and loading condition. Fire resistance of beams may be calculated assuming the rate of char is constant during fire.
PROTECTIVE REQUIREMENTS
Building protection implies protection from weather as well as fire. In roof construction, for example, the roof must provide protection from weather as well as preventing fire penetration from the burning of an adjacent building or ignition by radiation. Good protection of timber decking roofs can be obtained with a suitable covering, such as asbestos-based bitumen felt faced with aluminium foil.
Buildings with external coverings of combustible materials must be spaced apart to avoid spread of fire from one building to another. Research work on radiation has resulted in design criteria that establish safe separation distances. Where these cannot be maintained, fire resistance in the external wall constructions must be provided.
Doors present a special problem in protection, since the access they provide may constitute a breach in an otherwise fire-resistant construction. Ideally, the door assembly should have the same fire resistance as the partition in which it is contained. It should prevent penetration of smoke and toxic gases and hinder the passage of fire to other parts of the building. Doors which meet stability and integrity tests are termed fire-resistant, whereas if they perform satisfactorily except for the occurrence of small orifices they are termed fire-check doors.
Assuming the same standards of workmanship, hardwoods and softwoods are equally suitable for doors. The greatest weakness is around the edges, for once the stop is consumed flames can pass between the door and the frame. One method of prevention is the use of intumescent paint in these areas, which swells under the influence of heat and closes gaps.
DECORATIVE REQUIREMENTS
When wood is used as a decorative material it usually performs no structural function so the primary problem is to provide resistance to ignition and flame spread. The flame-spread resistance can be improved by the application of fire-retardants, either by impregnation or surface treatment. Many such treatments are water soluble, and others are affected by ultraviolet radiation, so outdoor permanence is lacking. Treatments also may detract from appearance, but studies are under way that may minimize these problems.
FIRE-RETARDANT TREATMENTS
The main effect of fire-retardants on timber is to retard ignition and limit surface flaming. In a fully developed fire, however, fire-retardant treatments have no significant effect on the rate of decomposition and charring of wood. Thus, these treatments are necessary to restrict flaming where it would be a hazard to life.
NONTECHNICAL CONSIDERATIONS
Technical problems involved in the use of wood as a structural material and related to fire hazard are universal. Interpretation and application of research data related to these problems involve feasibility, economics, degree of risk involved, and level of protection available. Economics require that attention be given not only to safety, but also to building survival, extent of damage, and repair costs. All these involve a subjective approach, and vary from country to country.
CHARACTERISTICS
Finishes for wood may be placed in two categories, each having its own characteristics and attributes. Film-forming finishes provide a film, layer, or coating on the wood surface, and include paints, varnishes and lacquers, as well as overlays. Penetrating finishes-fungicides, preservatives, and pigmented stains-leave no layer or surface film.
Finishes such as paint, the most widely used, provide most protection against sunlight and offer the greatest selection of colours. When these coatings are nonporous and intact, they retard moisture penetration and thus reduce paint peeling, staining by extractives, and dimensional changes. If coatings become cracked, however, they offer none of this protection, but leave the wood even more vulnerable to decay since coatings are not preservatives. Film-forming finishes are also susceptible to blistering and peeling problems. Costs are usually higher and more care needs to be exercised in their application than for penetrating-type finishes.
Paints include oil-base or solvent systems and latex or water-base finishes, and form an opaque surface coating. Transparent coatings such as phenolic, alkyd, polyurethane, or epoxy varnishes may also be used as surface coatings. While they greatly enhance the beauty and grain of the wood they lack exterior permanence in sunlight. Synthetic resin film overlays that are glued to the wood surface offer great promise as lasting surface finishes.
Without any finish or treatment the surface of wood exposed to weather changes slowly to a grey colour and becomes roughened. Further changes with time will be negligible unless decay sets in. A simple fungicide or preservative penetrating finish can inhibit the fungal growth or mildew that causes the grey colour, and greatly enhances the appearance of the wood. Colour can be achieved with penetrating pigmented stains without hiding the wood grain or surface texture, or otherwise covering the wood surface.
DETERIORATION
Painted surfaces normally suffer little deterioration in the first two or three years of exposure. After this time the effect of moisture, type of substrate, paint porosity, or age may induce paint failures. These include cracking, curling, checking, flaking, blistering, and peeling of the paint film.
The weathering of finishes is a process that includes photodegradation by sunlight, leaching, hydrolysis, and dimensional change effects of water and the discoloration and degradative effect of microorganisms. Degradation by sunlight involves photo-oxidation, resulting primarily from the ultraviolet or high-energy portion of the spectrum. Photo-oxidation produces checking and embrittlement in clear coatings, shallow checking in paint surfaces resulting in chalking or erosion, and roughened surfaces on wood. Heating and cooling of the wood surface due to absorption of radiant energy cause moisture movement in the wood that also influences deterioration.
One method for retarding the adverse effects of photodegradation is the polymer-pigment overlay for wood. The polymer is transparent, does not absorb ultraviolet and thus has an indefinite life. The pigment provides permanent ultraviolet absorption to protect the wood.
The most widely employed construction feature to retard sunlight deterioration of finishes is the roof overhang. A 4-foot overhang gives almost full protection to the upper two thirds of a one-storey wall. Since this can reduce maintenance costs, the added cost of the overhang can be easily justified. Vertical siding may also offer some advantages since the boards shed water more effectively and are less perpendicular to the incident sunlight than conventional bevel siding.
MOISTURE EFECTS
Water, either outside or inside the structure. can affect the performance of paint coatings. Rain and dew enter the wood through weathering cracks in the finish, particularly at the ends and edges of boards, and result in blistering and peeling of paint. Even seasonal changes in relative humidity can cause checking of the wood surfaces and cracking of the finish. Roof leaks, inadequate drainage and ice dams on the roof can allow water to enter the side walls and cause paint failure, discoloration from movement of water-soluble extractives, and decay.
Interior water can attack paint films by diffusion through the walls. While this water can come from such things as shower spray or faulty plumbing, the primary source is water vapour inside the building. This moisture is attracted to the cold surfaces of the outer walls during the winter season. It passes into the walls and condenses in the wall cavity or on the exterior siding as water or frost. With warmer weather the moisture causes paint blisters and peeling.
The typical home has many sources of water vapour. The normal living habits of a family of four can contribute as much as three gallons of water per day to the atmosphere. Unvented heaters, clothes driers and humidifiers add additional moisture, as do improperly designed and vented crawl spaces.
Fortunately, most of these problems can be controlled or minimized by good construction. Properly designed roofs and guttering with correctly installed flashing can eliminate leaks. Ice dams can be largely eliminated by reduction of heat losses from dwelling interiors and adequate venting of attic space. Wide overhangs will reduce the wetting of exterior walls from rain and dew. Water repellents can be used to treat decay-susceptible species, particularly where water may be trapped and on the ends of boards. Good architectural practices can be followed to avoid undue water entrapment.
The primary deterrent to damage from interior moisture is a good vapour barrier, properly installed and maintained. In new construction, a polyethylene film placed on the warm side of the wall will be very effective. In older constructions, painting the interior wall and ceiling surfaces, especially with aluminium paint, will reduce moisture transmission. Proper ventilation of attic and crawl spaces will eliminate moisture vapour buildup. Excessive interior relative humidity in the winter season in more severe climates should be avoided. If the interior is humidified for comfort, a proper balance must be maintained between inside relative humidity and exterior weather conditions, if serious paint problems are to be avoided.
The potential damage applies to paint films. Penetrating finishes without a surface film do not trap water in the wood and thus are virtually free of these problems.
FUNGAL ORGANISMS
Fungi, growing under favourable moisture and temperature conditions, can result in discoloration of paint surfaces. Usually this does not damage structure or finish but does detract from appearance. Where excessive moisture is trapped, conditions may be favourable for decay which could cause structural problems. Normally this is avoided by the use of heartwood of decay-resistant species in high-hazard areas or by treatment of the wood with water-repellent preservatives. Fungal growth on finishes may be inhibited by the addition of fungicides to the finish itself.
WOOD-FASTENER RELATIONSHIP
There is considerable information available on the characteristics of a wide range of fasteners, particularly as they are used with softwoods. In the developing countries, however, most house construction will utilize hardwoods and the factors that influence fastener use with these species must be analysed. Problems are related to the density, hardness and shrinkage of hardwoods, as well as to economic and social factors, and levels of training and experience.
In contrast to the softwood structural timbers of the northern hemisphere, with densities of about 25 pounds per cubic foot at 12 percent moisture content, hardwoods from developing countries average 35 pounds per cubic foot with a range of from 20 to more than 65 pounds per cubic foot. While nail-holding power will increase with density, this assumes that a well-nailed joint has been produced. This may be difficult in hardwoods. At moisture contents of 12 percent or less, it is very difficult to fabricate nailed joints in high-density hardwoods. To compensate for this, hardwood timbers are frequently assembled green, but then problems arise from subsequent shrinkage and distortion. Harder nails are frequently used in hardwoods to minimize nail-bending tendencies and are often driven with a heavier hammer to overcome driving resistance. Sometimes the shank of the nail is lubricated with soap or grease to facilitate driving. When common nails are used with hardwoods, an accepted practice is to predrill a lead hole of approximately 80 percent of the nail diameter. This has no effect on the bearing strength of the joint and does result in well-made joints with fewer splits.
Splitting can markedly reduce the effectiveness of the joint. It may result from the nail-driving operation, or from subsequent wood shrinkage if green wood is used. Splitting is minimized by predrilling or by the use of a compensating number of smaller gauge nails. Nail points also affect splitting, and a blunt-pointed nail that tears the fibres rather than wedges them apart will reduce splitting. Such nails may be purchased or can be made by blunting the point of the common nail before it is inserted in the wood.
Shrinkage of hardwoods tends to be two to three times that of structural softwoods. Thus, when several nails are placed in a joint, they impose severe local restraint and splitting is likely. This can be minimized by spacing the nails uniformly across the joint to force several small splits that have less effect on joint strength than one or two large ones.
Shrinkage of framing members can result in nail pops or nailhead protrusion from the surface of wall linings or floors. Seasoning of framing before attaching covering materials will minimize this, and it can also be reduced by using short nails with deformed shanks to give needed holding power with less penetration.
Fastening by means of adhesives has been used extensively for softwoods, primarily under conditions where adequate quality control can be maintained. Glueing of hardwoods can present several new problems related to greater shrinkage and warping tendencies. It is probable that good glue joints can be produced in hardwoods below 40 pounds per cubic foot in density, if they are properly seasoned to uniform moisture content and surfaced just prior to glueing When any question of glueing capability is raised it may be solved by tests of the species and adhesives to be used. Techniques are available for glueing fire-retardant and preservative-treated timber and these should be closely followed if quality glue bonds are to be obtained.
Very recently in the United States a mastic-type adhesive system has come into use. The better of these adhesives can apparently be used on wet wood and at low temperatures, and still produce good joints. The better joints also appear durable and resistant to elevated temperature deterioration. They need additional study, but may provide an effective method of assembly for hardwoods.
EFFECT OF EXPOSURE CONDITIONS
The performance of fasteners can be affected by changes in the moisture content of wood members, severe loading conditions such as wind, earthquake, or blast, and corrosive atmospheric conditions. Where fasteners are exposed to changing humidity conditions where wood members shrink and swell, some reduction in joint strength may be anticipated. At high relative humidities or high moisture contents fastener corrosion could take place, with resultant wood staining and a decrease in fastener strength. Where corrosion or staining may be a problem, corrosion-resistant fasteners should be used.
When mechanical fasteners are properly used they enable wood structures to perform well under the action of hurricane winds or earthquake shocks. It is essential that fasteners of the correct size and number be used to fabricate individual elements and to tie the elements together into a structure that will perform as a single unit to resist these forces. In some instances special fastenings such as framing anchors may be necessary. Alternately, galvanized mild steel strapping can be made to serve the same function, usually at less cost.
NAIL CHARACTERISTICS
Nails made of mild steel wire should be satisfactory for most uses in developing countries, provided fastening techniques are adjusted for species characteristics. Hardened steel nails may be necessary for fastening wood to concrete, and corrosion-resistant nails may be required for specific uses. Normally the use of more costly nails of high carbon steel, stainless steel, aluminium, copper, and so on, will not be justified.
Nail shape can influence fastener performance and should be investigated. In general, however, common wire nails with round shanks will be satisfactory. and the cheapest for construction use. If high resistance to withdrawal is necessary, particularly after exposure to wetting drying cycles, nails with deformed shanks may be used. The nailhead should be consistent with fastener use. Where nails are used to attach flooring or trim and are driven beneath the wood surface, a small-headed nail (Australian bullet head or American finishing nail) is required. For fastening covering materials to walls, flat-headed or countersunk fla-theaded nails will be satisfactory. Spring-head nails or nails incorporating a washer or seal under the head are the most suitable for attaching corrugated iron or similar roofing Nail points influence ease of driving and service performance. Sharp points separate wood fibres as they wedge their way into position and thus have greater splitting tendencies. Blunt-point nails are harder to drive but tend to shear the wood and minimize splitting. The simple diamond point is most satisfactory for all-purpose use.
Nail surfaces can be treated to increase withdrawal resistance or improve appearance. Galvanizing is probably the best general-purpose treatment for appearance, and gives corrosion resistance.
OTHER WOOD FASTENERS
In addition to nails, other mechanical fasteners can be used for wood assemblies, including staples, screws, timber connectors, bolts and sheet metal plate fasteners. Staples, normally driven with a stapling gun, can result in less splitting either during driving or during seasoning of green lumber, because of their thinner legs. More staples need to be used to give the same lateral resistance to the joint. Screws are normally not important in wood construction. When used with hardwoods, they must be driven in predrilled holes and are often lubricated for ease in driving Timber connectors, including split ring, shear plate and tooth plate, are used with bolts for structural elements. As a rule these are too massive to be used in house construction, but may be required in low-rise commercial structures. Bolts are used as fasteners but are little used in house construction except to join prefabricated components or in foundation anchorages. Sheet metal plates, or truss plates, arc extensively used in the fabrication of light monoplanar roof trusses. Where many houses are to be built, prefabrication of roof trusses using these fasteners may be economical. The plates are usually galvanized and may be made with projecting pointed teeth, punched for use with nails, or both.
Anderson (1) concludes that good construction practices can minimize the damage to light-frame structures resulting from hurricanes or earthquakes. While it is impractical, if not impossible, to design wood structures to resist the forces of a tornado, proper construction techniques can ensure minimum damage to such buildings on the periphery of the tornado's path.
HURRICANE RESISTANCE
Hurricanes, with winds that can approach velocities of 200 miles per hour, combined with effects of high water and wave action, impose severe loadings on any type of structure. At wind velocities up to 100 miles per hour, damage is largely confined to windows and roofs; from 125 to 150 miles per hour extensive damage can result to the roofs, walls and foundations of poorly built structures. Foundation damage usually results from wave and water action, although lack of proper anchorage of the house to the foundation can result in severe house damage even though the foundation retains its structural integrity. Siding materials may be damaged by water action but more often the walls are harmed by wind-blown debris. Wood and plywood covering materials of many types offer a resistance to such damage that cannot be obtained with brittle materials. Roof damage, to the covering material or to the roof structure itself, is a most common effect of hurricane forces. Wood shingles and shakes provide excellent resistance to wind and proper anchorage of structurally adequate roof members will eliminate or minimize damage to the roof structure.
The primary consideration in the building of a hurricane-resistant structure is to ensure that all components are well tied together, to enable the structure to perform as a single unit. This requires proper embedment of foundation elements, anchorage of floor system to foundation, proper ties between walls and floor, and adequate fastening of roof structure to walls.
Anderson describes other good construction details that will ensure hurricane resistance:
Treated pile or post foundations with sufficient embedment depth and bracing to provide racking resistance.Beams or girders bolted and strapped to the foundation piles or posts as a support for the floor system.
Wood floor framing anchored to beams and foundation with bolts or heavy strapping.
Plywood or diagonal board subfloor and sheathing. Extension of wall sheathing over floor joist headers to provide a good wall-floor tie.
Anchorage of roof trusses or ceiling joists and rafters to walls with metal strapping or commercial anchors.
Plywood or diagonal board roof sheathing to provide lateral rigidity and racking resistance.
Wood shingles, wood or plywood siding, and shuttering.
SHOCK RESISTANCE
Earthquakes of varied intensity occur each year throughout the world. In March 1964, an earthquake of intensity 8.6 on the Richter scale, the strongest ever recorded on the North American continent, occurred in Alaska. Despite the severity of the earthquake, loss of life was low and structural damage to wood-frame structures was negligible.
Most homes were well anchored to full basements and, despite the shock loading and its long duration, suffered little serious damage. In some areas the earth settled, cracked open, or slid on underlying layers to eliminate all support for the foundation structure. Even in such situations the structural integrity of the wood-frame houses was phenomenal; severely dislocated houses were so structurally sound that they could be put back into service on new foundations.
When the walls frame structures were constructed to provide good racking resistance, and particularly when they were well anchored to floors, the unit action provided excellent resistance to damage. Structural rigidity of walls was provided by well-recognized procedures: let-in braces with horizontal wood sheathing, diagonal wood sheathing, or plywood sheathing, all well nailed to framing. Roof structures suffered little damage when the framing elements were well tied to the walls. Some of the details and techniques suggested by Anderson to ensure earthquake-resistant wood construction are:
Good corner construction in exterior walls to aid in providing racking resistance.Good nailing of sheathing and siding to framing members to give structural rigidity.
Nails in proper size and number to provide design strength. Such fasteners apparently cushion shock loads.
Unification of structure through proper ties between wall, floor, and roof elements.
Anchorage of structure to foundation.
BACKGROUND PAPERS
(1) ANDERSON, L.O. 1971 The wood-frame house resists nature's furies. WCH/71/4a/5.
(2) BECKER, G. 1971 The hazard of fungus and insect attack for wood and wood-based material in houses in various regions and means of alleviating it. WCH/71/4a/6.
(3) BLACK, J.M. 1971 Finishes, construction factors, and design to compensate for the effects of weather on wood surfaces. WCH/71/4a/4.
(4) BOYD, DJ. 1971 Problems associated with the use of wood in construction: methods of fastening. WCH/71/4a/2.
(5) SILVERSIDES, R.G. Fire hazard in timber structure. WCH/1971 71/4a/3.
1. Wood and wood-based products give excellent performance as structural materials when properly used. Most problems result from misuse, either as a result of improper design or poor construction and protection practices.
2. This particular part of the report is concerned with technical questions influencing the utilization of wood. It must therefore be interpreted so as to be consistent with the environmental and social conditions of any particular region.
3. During the various meetings, concern was repeatedly expressed over the feet that most architects and engineers were not fully competent to design with wood. All too often they designed structures that did Dot reflect an understanding of wood properties, of environmental conditions of the country or of the specific desires of the people.
4. In many areas there appeared to be a bias against the use of wood that was often reflected in codes of practice. Education of architects and engineers, and training of technicians and builders were recognized as factors that must be given proper attention if wood is to be efficiently utilized as a structural material.
5. Fire was a problem of universal concern and this was reflected in personal and country prejudices against the use of wood in construction. While recognizing that wood is a combustible material, the Consultation noted mat remedial and protective measures are avail able which render wood no more hazardous than many other building materials. When these measures are properly applied there are many situations where wood can be used safely in low- and high-rise buildings.
6. While biodegradation of wood is an asset as far as maintaining an acceptable forest environment is concerned, deterioration by organisms is a factor of major concern to the use of wood in structures. The Consultation frequently stressed the need for effective protective methods against decay and insect attack. Methods for chemical treatment should be simple, relatively inexpensive and easy to use, particularly with naturally impermeable wood species and incompletely dried timber. The importance of construction practices to minimize these was emphasized.
7. The natural durability of species to fungal and insect attack under various climatic conditions must be evaluated in order to utilize wood effectively in construction, or to promote its use in international trade. Procedures for determining natural durability should be standardized worldwide to establish confidence in the results obtained. Proper design of structures is important along with the use of preservatives in ensuring acceptable building performance. While the cost of preservative treatments may be used as an excuse for avoiding their use, overall economic considerations generally favour preservation.
8. The cost of treatment must be weighed against the possible cost of repair and reconstruction during the life of the structure. Treatments are needed that are efficient, relatively inexpensive, easy to use, and which are applicable to most species including tropical hardwoods, and effective against a variety of hazards. In this connexion the possibilities of diffusion treatments should be considered.
9. As regards internationally accepted standards, the Consultation took note that a standard was being drafted by IUFRO for field tests and for the evaluation of preservatives. This proposed standard was to have been published in the journals of various countries in 1971.
10. The Consultation recommended to governments that they introduce appropriate codes of practice for the use of naturally durable timbers or the effective protection of wood and wood-based materials in construction, when and where conditions require it. Information on satisfactory preservation procedures should be provided to architects and engineers.
11. The Consultation recommended to IUFRO and individual forest products research groups that they continue and accelerate their work toward internationally standardized procedures for determining the degree of natural durability of wood species, their permeability with respect to treating procedures and the effectiveness and performance of preservative treatments.
12. The Consultation expressed the view that, in general, existing codes of practice were much too conservative where the use of wood was concerned, either through bias or ignorance. It was suggested that the use of performance concepts in codes should be encouraged and that national authorities should develop this type of requirement specification.
13. The fact that present building codes as applied to wood construction are more conservative than necessary often leads to higher costs in building programmes. The Consultation therefore recommended that controlling agencies move toward the wider use of performance-type specifications based on objective tests, the most up-to-date information, or established performance capability. It also recommended that governments en. sure that timber interests are fully represented in the development of codes for building materials and construction.
14. There was a lively exchange of experience from many countries regarding fire hazards in wood buildings compared with buildings constructed of other materials. Wood deserved to be given full consideration as a structural material, it being recognized that good construction techniques and planning could prevent the spread of fire, as could also the use of protected constructions, such as those using gypsum board as an internal wall covering material. Restrictions in building codes regarding the use of wood might be lightened by providing reasonable and sound technical data to effect changes in these codes.
15. The use of wood in construction should be related to the size of the structure' and appropriate fire protection measures should be related to the hazards actually involved.
16. Requirements for construction that recognized the fire hazard should relate to the whole building, or group of buildings, and be cognizant of the conditions under which fire can take place. Proper design must include full provision for adequate exits.
17. The Consultation drew to the attention of governments its view that the- dangers of fire arising from the use of wood in buildings are often exaggerated, and recommended that the present provisions in building codes be reviewed to ensure that satisfactory and economical techniques are not being excluded by over-conservative requirements. Codes of practice should relate to the entire building or group of buildings and should recognize the specific situations existing in particular countries and the conditions under which fires can occur.
18. It is recognized that wood structures can perform very well under adverse environmental conditions when the wood components are properly assembled with mechanical fasteners, and that the use of these fastenings requires a minimum of expertise. Publications describing proper techniques for the effective use of mechanical fastenings, including nails, metal plates and bolts, are readily available and should be widely utilized.
19. The Consultation gave consideration to proper finishing techniques for wood structures. The purpose, performance, and problems related to film-forming finishes and the penetrating stains or water-repellent treatments were thoroughly described. Construction techniques and site maintenance were discussed as methods for achieving good finish performance. It was suggested that the use of nonfilm-forming finishes be carefully considered and that construction practices be such that wood could perform effectively with a minimum of treatment. Finishes are not preservative treatments and should not be considered as such. The Consultation recognized the need for finishing techniques that would provide the ultraviolet resistance necessary in the tropics and at high altitudes.
20. The Consultation drew to the attention of IUFRO and individual forest products research groups the unsatisfactory state of knowledge in the matter of finishes for wood in exterior use under adverse conditions, and recommended that research studies be intensified and that the knowledge developed be passed on to the user.
21. The Consultation recognized the need for fundamental information on the properties of available species of wood, so they might be effectively utilized in local construction and as materials of trade and commerce. While testing techniques to define strength properties were well established, in many instances the evaluation programmes might be limited, because of cost, to establishing potential areas of use. More complete data could later be provided with increasing use.
22. The Consultation considered the development of adequate grading rules necessary for proper marketing of species. The cost would be more than offset by savings resulting from the more economical and safer use of wood.
23. The Consultation drew the attention of governments to the need for fundamental data about the properties of all available species from their forests and recommended that they implement procedures to achieve this objective, consistent with the cost involved and the utilization contemplated.
24. The Consultation drew the attention of governments to the fact that grading and proper use of wood at a moisture content consistent with proposed utilization could lead to cost economies, and recommended the wider introduction of grading rules and standards for the structural use of wood. Where applicable, grouping of species for greater ease in utilization was to be commended.
25. The consensus of the Consultation was that lack of training of architects and engineers in the proper use of wood in housing, and in construction generally, was a major factor contributing to the less than optimum use of resources. Educational systems do not generally provide adequate training in wood construction and utilization. Equally important was the training of technicians and skilled workers.
26. The need for more efficient and rapid transmission of technical and research data on the use of wood was emphasized as a prerequisite to good structural utilization of wood.
27. In other sections of the Consultation there is further reference to the need for:
(a) more effective training methods and opportunities in developing areas for designers, builders and operatives;(b) the more effective exchange throughout the world of technical and research data on the use of wood in housing;
(c) more suitable methods for assisting developing countries in specific problems that arise concerning the availability of practical information, techniques of building and the use of wood generally.
