Building structures and materials
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The subject of building and structural design is, like that of civil works design, not something that can be adequately covered in a short manual. The following notes may assist administrators in understanding the types of issues that will need to be considered by the projects architects and engineers when they are developing the design of the market.
Selection of building materials. In less-developed countries the selection of appropriate building materials is as important for the construction of markets as for any other building type. The term appropriate in this context means that, in selecting construction materials, there should be a balance struck between cost, utility and the optimum use of resources. It is generally preferable that local materials and technology are used, but this should not be at the expense of providing the most functional solution. The use of traditional or intermediate technologies does not mean that design will be simpler; the opposite may often be the case.
Reviews of suitable intermediate technologies are contained in Spence, RJ.S. 1983. (Building Materials in Developing Countries. Chichester, John Wiley and Sons) and Stulz, R. 1981 (Appropriate Building Materials Zurich, Swiss Centre for Appropriate Technology)
Typical construction. The typical construction of a low-cost market building would be a steel sheet or brick clad reinforced concrete (RCC) frame, completely open on the sides, providing unloading facilities for producers and a raised loading platform for wholesalers and retailers The roof would generally be of steel trusses, insulated and clad with colour bonded pressed-steel sheeting or, if available, some form of lightweight fibre sheeting. To provide comfortable and well-lit conditions in the centre of the building, high level pressed steel or timber louvre ventilators and "clerestorey" glazing should be provided at ridge level. Glazing in northern latitudes should be "north lights", in southern latitudes "south lights" and near to the equator by completely shaded unglazed openings.
Other facilities on a site might be of a more permanent nature. Construction of the building might be of conventional masonry, with a light RCC frame, or of calculated load-bearing brickwork, with steel roof trusses, clad with clay tiles or colour-bonded, pressed-steel sheeting. The construction of specialized facilities such as cold storage, would be basically similar to the main sheds, except that raft foundations may be required because of high loading. Insulation should be provided to the entire ground floor, chamber walls and roof.
Foundation design, The main criteria for structural design of foundations will be established from the site investigation survey. This will determine the appropriate bearing pressure that should be adopted in design and the types of foundations that will be required for different building categories, such as strip or raft foundation or isolated footing supported by long or short piles. The site investigation will also give recommendations for the depth of strip and raft foundation; the minimum length of pile foundations; and, if the site falls within an active seismic (earthquake) zone, measures which should be taken in the design of column footings and their inter-connection by tie-beams.
Depending on soil conditions, the foundations should ideally be conventional isolated pads under the main columns, with connecting strip footings, except for sections of the building where a raft foundation may be required because of higher loadings, such as under a main water tower or tank.
Structural design of buildings. Market buildings are essentially simple sheds and in their design the most important element to consider in the structure will be the choice of an appropriate system of roof construction. The selection of cladding materials is discussed below, but equally influential will be the decisions made about how the roof is to be supported. Ideally the span and width of structural bays should be as large as possible to provide an unobstructed operating space. Spans can range from 8 metres up to 24 metres, with bays at spacings of 6 metres to 12 metres. Buildings with depths greater than the maximum practical spans can be obtained by providing intermediate supports, often coinciding with a break in the roof plane for ventilation Although the shed form may be simple there are many opportunities for refinement in its detailing and elegant and economic structural forms are possible.
Structural loading assumptions. It is usual for national standards to exist for standard loadings and these will need to be adopted in design. The following figures (based on British Standard CP3: Chap V: Part 1:1967) may provide a useful supplement to these standard figures:
|· Steel roof cladding||480 N/m²|
|· Steel trusses||100 N/m²|
|· False ceilings||140 N/m²|
|Distributed imposed - live loading|
|· Pitched roof||0.75 kN/m²|
|· Produce display/sales||4.00 kN/m²|
|· Cold stores||5 00 kN/metre height (minimum 15 kN)|
|· Other stores||2.40 kN/metre of height|
|· General offices||2.50 kN/m²|
|· Hostels||1.50 kN/m²|
|· Public halls (fixed seats)||4.00 kN/m²|
Roof structures. Common construction materials for roof structures are steel or timber trusses and reinforced concrete, supported on masonry, concrete, steel or timber columns. Practical and cost considerations will, however, restrict the range of options that are available. The presence of termites and wood-boring beetles, for example, may limit the use of structural timber. The modules required for sales spaces and storage areas and the geometry created by the parking bays will need to be studied in selecting suitable dimensions for the structural bays.
The ultimate decision, however, will be determined by a number of external factors. This will include the capacity and level of local technology what materials are available, the ability of local fabrication workshops, transport constraints and site erection facilities. In some regions the need to cope with seismic. movement or hurricane force winds will be the major determinant of the roof form. The final choice will invariably be made on economic grounds. For spans over 6 metres steel trusses will tend to be the most economic solution.
Roof cladding. For markets, the most important building component that will need to be considered is the cladding material selected for the roof. Roofs will account for the largest proportion of the total building costs and are likely to be the most complex technical problem encountered in the building design. Market roofs must be durable, noncombustible, easily maintainable and must be able to perform under all the internal and external climatic conditions imposed on the buildings.
Hot arid and composite climates will require roofs with a high thermal mass (9 - 12 hours time lag and a U value of not less than 0.85 W/m² °C). Tropical upland climates will require pitched roofs of a similar thermal mass, but a lower time lag of around 8 hours. In the warm humid tropics the need to cope with high levels of rainfall will tend to favour lightweight pitched roofs, insulated to achieve a thermal mass time lag of less than 3 hours (U value of not less than 1.1 W/m² °C).
The choice of roof materials is very wide and needs to be considered with the structural design. For flat or pitched roofs in arid climates, in-situ or pre-cast reinforced concrete finished in hot mastic asphalt or a cold-applied bituminous compound are the usual choice. Other options, which may be particularly applicable for secondary markets in rural areas, include fibre-reinforced soil roofs with animal dung waterproofing and reinforced brickwork finished with asphalt or tiles. Traditional pitched roof materials that might also be appropriate for rural markets include thatch, which has fireproofing problems, and clay or stone tiles, which are heavy and thus only generally suitable for short-span structures.
Most modem urban markets favour the use of light-weight
corrugated sheet materials, such as galvanised steel which has a
poor performance thermally, low surface reflectivity and is not
very durable, particularly in coastal areas. Other options
include plastic coated sheet stell aluminium
and asbestos cement. These perform more satisfactorily, but generally require foreign exchange for their purchase. Local production of corrugated, asphaltic or fibre-reinforced cement sheets may exist and these should be investigated.
None of the sheet materials can be used on their own and roof insulation will be required. Fibreglass, aluminium foils and polyurethane boards are very effective but may be too expensive. Locally produced insulating boards, using waste straw or other residues, are the ideal materials if available.
Materials supply. Before making any decisions on basic construction methods, a review should be undertaken of materials availability and supply. It is normal and desirable to design a market on the basis of the maximum use of locally available and manufactured materials and components. Certain items, however, will invariably need to be imported and the responsibility for their import should be considered in the preparation of project documents.
Cement is a material which is often in short supply and notwithstanding the existence of cement factories, it may need to be imported so as to guarantee a supply. Because of the difficulties of quality control the import should always be made the sole responsibility of the main contractor. The import of specialized fittings, such as street lights and fire hydrants, should also be made the sole responsibility of the main contractor.
The construction of market buildings often assumes a large steel component, or roof trusses, window frames, louvres, gratings, frames, partitions, water tanks and roof cladding. Although there may be a high level of capability for fabricating these components locally, steel shortages may have a significant impact on a project's progress. Significant cost and time savings can be made through the bulk purchase of steel internationally. It can then be supplied to selectively tendered local sub-contractors for the fabrication of components.
Where there are likely to be wide variations in the quality of locally available materials (such as earth-fill material, sand and gravel) and components (such as bricks and tiles) the tender documents should incorporate a sub-contract for a local firm to undertake geotechnical and materials testing.
Geotechnical and materials testing programmes. Geotechnical and materials testing should also be undertaken during the construction period so that the settlement of a site during filling can be monitored and appropriate adjustments can be made to infrastructure design, depending on the variations in site conditions and the different properties of materials being used on the site.
A typical list of the types of investigations that might be undertaken during construction is shown in Table 14.3.
Table 14.3 Geotechnical & materials testing during construction
Field density test by sand replacement method
Modified Proctor test in laboratory
Laboratory CBR tests
Field CBR tests
Mackintosh probes (for building foundations)
Consolidation test for primary settlement (T90)
Plate load test
Cement setting times
Cement soundness test
Cement compressive strength
Concrete cube tests
Concrete mix design (28 days strength)
Brick compressive strength
Brick moisture absorption
Los Angeles abrasion test/impact test
Tests on materials from sand and aggregate sources
Engineering design and supervision
Final design, preparation of tender documents and tendering procedures is the responsibility of a government public works department or a locally appointed firm of architect/engineers. Either could also act as the design and supervision (D&S) consultants for the project's duration and be responsible for management of the construction contracts.
Appointment of consultants. Although government staff might be appointed without any formalities it would take around 4 - 6 months for the mobilization of a consultant D & S team. Assuming that there are sufficient, experienced local consultancy firms to take on the role, the following steps would need to be followed for their appointment:
· preparation of D & S terms of reference;
· short-listing of local consultancy firms, including visits to their offices and to work for which they have been responsible;
· invitation to bid, including newspaper advertisements;
· review and evaluation of bids; and
· recommendations and approval to appointment by client body
In some countries there are variants on this procedure where the practice is to use contractors for both design and build. This is sometimes referred to as, a turn-key contract. This method usually has the advantage of faster construction periods and may allow greater financial control during the contract, but it does require that the client body has thoroughly worked out what it wants. A common method to ensure that the client's interests are protected is to appoint an independent project manager.
Scope of design and supervision activities. For large-scale projects, such as an urban wholesale market, a high level of on-site control will be needed. A full-time resident engineer is necessary on the site, assisted by inspectors and a site architect. The following activities would be undertaken by the D & S team and, if they are to be a private firm of consultants, would form the basis for their terms of reference:
· topographic surveys and geotechnical investigations, if not already undertaken by other consultants or government department;
· review of designs undertaken in previous design stages;
· final design and preparation of tender documents (specifications, drawings and bills of quantities) for site construction;
· advertising contracts and pre-qualifying contractors;
· tendering by short-listed contractors (local competitive bidding), reviewing of bids and letting of the contract packages;
· site supervision of construction contracts, including checking of setting out, general quality and financial control, valuations, certification for interim payments and preparation of final accounts;
· preparation of bid documents for materials and equipment procurement, followed by tendering (international and local competitive bidding), reviewing and letting of the contracts;
· submission of monthly technical progress reports on construction design, contract budgeting, equipment procurement and physical progress; and
· preparation and agreement of final construction accounts and handing over of building and sites to client body at the end of the defects liability/warranty period.
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