14 Engineering design criteria

Contents - Previous - Next

The subject of engineering design criteria is a vast one which cannot be satisfactorily covered in a short manual. The following notes are meant to provide a guide for non-technicians so that they can understand the scope of engineering design related to markets. The notes should also be helpful in providing a basis for the preparation of terms of reference for consultants. Technicians may also find the notes useful for highlighting the types of engineering problems that can be encountered in market design. Many of the subjects may be covered by local building regulations and codes, but often these have a strong bias towards the construction of houses and may not be appropriate for the design and construction of markets.

Site investigations

The conclusions and recommendations of the site investigation survey described in Chapter 11 should include consideration of the following:

· the depth and appropriate standards to be adopted for the compaction of embankments and other areas of fill based on the soil type and the type of compaction plant to be used (to be confirmed during construction by undertaking field density tests);
· what measures should be taken for disposal of unsuitable fill material;
· the types of foundations required for different building types (described later in this chapter);
· the modulus of vertical subgrade reaction, dry density and coefficient of permeability of foundation soils;
· the maximum dry density and optimum moisture content of sub-grade soils, from modified Proctor compaction tests;
· the minimum 4-days-soaked California Bearing Ratio (CBR) value of the sub-grade soils and the recommended total thickness of the road pavement above the sub-grade, related to the number of commercial vehicles per day;
· maximum compressive strength of cement-stabilized soils, the proportion of organic matter they contain and whether practical problems and costs could make cement stabilization an attractive option;
· recommendations on the suitability of natural gravels and mixed soils obtained from the nearby quarry sites for use as imported filling material and as the sub-base course for road pavements;
· comments on pore water pressure inside the void spaces of the soil mass and whether vertical or horizontal sub-soil drains are required for the stabilization of the project site; and
· general recommendations on excavating procedures, the depth of cover, surrounds and bedding required for pipes and standards for backfilling of trenches in graded material.

Site preparation and development

On the basis of the site investigation survey, a visual inspection of the site and detailed traverse and topographic surveys the overall physical conditions should be assessed. The implications that this analysis may have for the site's development should then be defined, requiring either adjustments to the site layout and building designs or special consideration to be given in the detailed design of site preparation and infrastructure contracts. The main characteristics that should be considered in making this assessment will relate to the site's drainage and topography.

Site drainage analysis. The analysis of drainage should assess whether a site is low lying and how the surface water is presently drained, particularly the relationship of the outlet's invert to the present ground level. Sites with a large catchment area and discharge flow may require the provision of an improved site outlet and on-site storage for peak period storm water flows.

The analysis should also consider whether temporary drains and sediment control structures will be needed during the construction period and if stilling basins, cascades and trash trapping will be required as part of the permanent drainage system. This may be applicable when a site is located at the bottom of a slope, off which there may be a substantial surface water discharge, possibly carrying earth and other loose material which might block the market's drains. Trash trapping may also be necessary at the outlet from the site.

Sub-soil drainage may be needed in order to accelerate primary settlement because of a high water table near the existing ground and extremely soft ground conditions with a low bearing capacity (such as clayey-silt alluvial material). There are generally two forms of sub-soil
drainage that are used: either horizontal "French" drains which are laid adjacent to main drains and sewers, retaining walls and ground floor slabs of buildings; and vertical drains, often used where raft foundations are proposed.

Topographic analysts. The topographic analysis will assess whether it will be necessary for the level of the site to be raised. This may be needed to provide stable soil conditions for new building works, to provide adequate cover to the new drains discharging through the site and to ensure that the final level comes above high flood level. A 25-year return period is often assumed. Raising the site level will require fill material, either obtained by excavating slopes on the site or, more usually, by importing suitable fill material from elsewhere. This needs to be compacted by heavy plant, the usual minimum standard is 95 percent Proctor. To aid construction in soft ground conditions the fill may be laid on a geo-textile blanket.

Moderate gradients across a site can be accommodated by sloping roads and parking areas (see Table 14.1) and by using small changes in level within buildings, providing ramps where necessary. However, where a steep cross-slope exists, it may be necessary to form the site by excavating level platforms. In this case, earth retaining structures may be needed. These are typically constructed as pre-cast concrete crib walling, reinforced concrete or masonry retaining walls or reinforced earth slopes.

Roads and parking

With the exception of areas designated to buildings, landscaping or reserves for future facilities, the entire area within a market site will need to be paved in order to provide the maximum degree of traffic manoeuverability and to facilitate site cleaning. The main characteristics of the road system will have been established during the site planning process described in Chapter 13 A hierarchy of roads will have been defined and the number of parking spaces at peak periods calculated. The purpose of the detailed engineering design will be to refine the broad assumptions used in the site plan, often necessitating layout adjustments. The road layout will form the main base map for the construction contracts and, for this reason, it is essential to undertake the road design before other engineering services

Table 14.1 Appropriate road design criteria

Sources: Mittendorf; Lynch and Hack; Tutt and Adler; De Chiara.

Geometrical design. The road system of markets will need to accommodate a wide range of vehicle types, from the smallest cars and pick-ups to large trucks, fire appliances and refuse collection vehicles. Appropriate geometrical design criteria that can be adopted for the design of small and medium-scale markets are illustrated in Table 14.1. The geometrical design of roads is a specialized activity and in the case of a
complex urban market it will not be possible to develop the design without technical advice. Typical problems that will require such assistance could be the application of traffic models to predict flows, the design of a main junction at the site entry/exit and the detailing of complex loading bay arrangements.

Pavement design. A high standard of road construction is always required in markets and the road pavement should be designed on the basis of the California Bearing Ratio (CBR) data for wet conditions, obtained during the site investigations, and the peak projected traffic levels (usually expressed as standard 8,200kg axle loads). Specialist advice is generally required to determine the thickness of the road pavement. A suitable design procedure for tropical roads is set out in the publications of the UK Department of Environment (Technical Memorandum H6/78 and Road Notes 29 and 31).

Where adequate supplies of local crushed stone are available "Macadem" construction would normally be adopted, with a compacted sub-grade, sub-base and base courses, linen finished with a tack coat and surfaced with a pre-mixed bitumen based material. Alternatively, concrete road pavements can be used, but they tend to be more expensive. Where traffic loads are light and subsoils are suitable, soil stabilization techniques can be adopted using lime or cement as the stabilizer. Roads would be usually provided with integral side gutters and precast concrete herbs. Parking areas may be constructed to a slightly lower standard of design.

Surface-water drainage systems

The process of preparing layouts of the surface-water drainage system will, in tropical countries at least, have a significant impact on the detailed site planning, To prepare derailed drainage schemes there are many published technical works and it will also be critical to employ the local knowledge of public works departments. Examples of such publications for tropical areas are the Malaysian Department of Irrigation and Drainage's procedures and the regulations of the Singapore Drainage Department (1978. Code of Practice on Surface Water Drainage, Ministry of the Environment, Singapore). The following notes, however, will be of use in making an initial assessment.

Catchment and discharge The first step is to review available mapping of both the site itself and the areas immediately around it. The direction of the site's main. outlet (or more than one outlet) to a natural water course should be identified and the catchment area of the site should be defined (the area of land whose surface water will drain into the site). The next step is to calculate the total peak discharge of water from the site. The most useful technique is to use the Rational Formula:

where:
C = run-off coefficient
I = rainfall intensity in mm per hour
A = catchment-area in hectares, including the site.

The run-off coefficient is selected from standard tables and will depend on the extent of paved areas and building coverage. For markets this is normally taken as a high value, such as 0.9, because the sites are normally flat, impermeable and have fully paved surfaces, and because the future land uses around market sites are likely to be heavily urbanized.

Rainfall intensity Comprehensive historical information on rainfall intensity is usually obtainable from agriculture or irrigation departments, but because of the urban location of markets the best source of rainfall data is often from the local airport. An assessment will also have to be made of the maximum recorded intensity of rainfall per hour for a range of design storm intensities, such as I in 5, 10, 25 or 100 year storms. For most public buildings a level of service of at least 1 in 25 years return period would be appropriate. This should ensure that during major storms there is no inundation of the market buildings and that road access is still possible.

With complex catchment-area shapes and where it is necessary to allow for some degree of storage within the channels a version of the Rational Formula modified to meet local conditions should be used.

Drain design. Covered or uncovered reinforced concrete rectangular drains are typically adopted, with a small dry weather flow channel in the bed of the larger drains to cater for a self-cleansing velocity. A freeboard of 10 per cent of the channel depth is often used in design as a safety margin to cope with high intensity short duration storms. The calculation of drain sizes is usually based on the use of the Continuity of Flow Formula:

Q (run-off:) = A x V
where:
A - cross-sectional area of channel in m²
V = velocity of flow in m per second (taken as maximum of 3 and minimum of 1.8 m per second for self cleansing)

and Manning's Formula:

where:
R = hydraulic radius of the channel in metres
S = gradient of the channel as a percentage
n = Manning's roughness coefficient (which can be assumed as 0.014 for normal insitu concrete lining to the channels).

Cut-off drains on the site boundaries may need to be provided to control the inlet of water into the site. The alignment of other main drains is likely to follow the pattern of buildings and roads, with a minimum of crossings. Some drains are likely to have only a minimal slope and wider cross-sections need to be provided, particularly at the site outlet, to cater for back-water effects if the existing outlet is constricted and to provide a level of on-site storage at times of peak discharge.

At the detailed design stage it may be necessary to fully investigate the possibility of improving the site outlet by increasing the size of the existing outlet or by introducing a new outlet.

Water supply

The majority of the water use at a market will be for washing purposes. The water will need to be to a similar standard to that for drinking water. Local standards may exist for calculating demand which reflect climatic conditions and cultural habits. These should be used if available.

An approximate estimate for water demand at ultimate development of a typical 10,000 m² market, based on Nepal Water and Sewerage Board standards (Drinking water installation and drainage requirements in buildings in Nepal, page 88), is as follows:

From this calculation it will be possible to estimate the size of any incoming mains or borehole by converting the water demand into a flow rate. Assuming that the market in the example above operates over a 16 hour day then the flow rate would be equivalent to 1.3 litres per second. Because the calculation has been based on the total floor space the flow rate is broadly equivalent to a peak flow On a net site this would require a 50 mm diameter polythene pipeline.

Tabulations of pipe diameters for different flow rates, materials and gradients are given in the Ross Institute Bulletin No. 10 (Cairocross, S. & Feachem, R. 1978. Small water supplies. London School of Hygiene and Tropical Medicine). Specialist advice should be sought for the design of pipelines on very net sites (less than 1 in 50 gradient) or where there are high water pressures.

Depending on available pressures and the reliability of the supply the site may need to be served by an underground reservoir or a high-level tank (in pressed steel or concrete shell). Pumping may be required to raise the water to the tank level, so that the site can be served by a gravity distribution system. Any tank or reservoir should hold at least one full day's supply. The main tank would serve a reticulation system of ring loops, which will make maintenance simpler as parts of the system can be cut-off. Distribution should be to overhead tanks in individual buildings, stand pipes and to a separate fire hydrant system.

Fire-fighting systems

The large amount of organic material in markets means that they present a substantial fire risk and special provision should be made. A number of markets have been completely burnt to the ground in major fires.

Fire hydrants. The market site should be served by a series of above ground fire hydrants, spaced at approximately 30 metres intervals in loop systems encircling the main building and around the site periphery. The hydrants should be located in the pavement areas to protect them from damage by vehicles and be served by connections from a gravity fed overhead storage tank, thus guaranteeing a water supply for fire fighting. In designing the water supply system a minimum fire-fighting flow of 34 litres per second (450 gallons per minute) should be aimed for.

Fire prevention in buildings. Smoke detection and alarm systems should be installed in all the main market buildings. In order to avoid the need for a costly overhead sprinkler system the buildings should be compartmented by limiting the distance between fireproof walls to a maximum of 60 metres and to an area of less than 1,000 m². Key buildings with a higher fire risk should be provided with secondary alternative means of escape in case of fire and compartmented to a higher standard of fire resistance. Cold storage buildings should also be fitted with gas detection equipment.

All buildings should be provided with internal emergency equipment to the following minimum standards:

· 1 fire bucket per 100 m² of floor area (or part thereof);
· 1 fire extinguisher per 600 m² of floor area (or part);
· first aid kits and tools (asbestos blanket, hatchet, gloves, etc.) for each building or compartmented section; and
· internal fire hydrants to open-market sheds, served from overhead gravity fed tanks to a minimum pressure of 3 kg/cm². The hydrants should be provided with wall-mounted hose reels to serve a maximum radius of 30 metres.

These fire safety requirements have been generally based on the Indian Code of Practice for fire safety of industrial buildings: general storage and warehousing, including cold storage (IS: 3594, 1967) and Recommendations for providing first aid - fire fighting arrangements in public buildings (IS:2217, 1963). These principles should provide a reasonable basis for design, but local codes may exist and adjustments to meet these standards should be made. Consultation with the local fire brigade is always essential. The enclosed nature of market sites may make it necessary for the fire brigade to have special facilities for access.

Sewerage

For most sites it should be possible to use a water-borne gravity system (typically 150 - 200mm diameter pipework) but in some cases, where the site is very flat and where suitable locations for the treatment plant are limited, it may be necessary to pump the sewage. Local methods for estimating the peak sewage flow and the hydraulic design of the sewers should be available. On very flat sites it may be necessary to use a pressurized pumping mains. The design of this can be based on the Colebrook-White equation, which is published in the design tables of the UK Hydraulic Research Station. As new markets are often on filled sites with potential long-term differential settlement problems, it is desirable to be cautious in the structural design of pipelines, particularly their bedding, surround and relationship to rigid structures.

Septic tanks. Assuming that a mains sewerage system is not available then sewage would normally be taken to one or more septic tanks located within the site boundary. An appropriate method in tropical areas for estimating their size, is given in the Indian Code of Practice for the design and construction of septic tanks, part 11, large installations, (IS:2470, 1971). A typical size for a market with an annual turn-over of 40,000 tons might be around 26m³ capacity (7.7 x 3.4 x 1.7m depth). The partially treated effluent would be discharged into the main surface water drain at the outlet from the site.

Sanitary fittings The provision of sanitary fittings can be estimated on the basis of the Indian Code of Practice for drainage and sanitation (IS: 1172, 1971) and Layout for regulated market yards for fruits and vegetables (IS:1787, 1961). These standards are shown in Table 14.2.

To estimate the total number of fittings it is necessary to make a number of assumptions about the usage of facilities. The following calculation is based on the requirements for a medium-size wholesale market yard:

· assuming that the average out-going transaction size is a small pick-up load of one ton, the average number of people involved with each transaction is 1.5 (0.25 sellers, 1 buyer and 0.25 market staff) and market users are 75 per cent male, then the maximum number of market users per fitting at ultimate development would be:

· number of water closets:
= 280 tons (daily throughout) x 1.5 (usage factor) x 2 per 50 users
= 17 water closets
· number of urinals:
= 280 tons (daily throughout) x 1.5 x 0.75 (males) x 2 per 50 users
= 13 urinals

Table 14.2 Standards for the provision of sanitary fittings at markets

Source: Indian Codes of Practice

Electrical services

The provision of power, particularly artificial lighting, is an important infrastructure component as it enables the fullest and safest use to be made of the market's facilities. Larger market sites will need to be served with their own high-tension supply (usually 11kV), which for economic reasons is often an overhead supply mounted on pylons. A transformer is normally required, typically 300 kVA, which can be a double-pole mounted type or one located within a building.

The transformed low voltage supply should run in encased cable ducts to a main switch board, with distribution cables to sub-switch boards in the individual buildings. For ease of maintenance all external cables should be ducted through cable trenches and internal wirings should be concealed in conduit wherever possible.

External lighting, For security reasons and so that the effective working period of the market can be extended, all internal roads and paved areas need to be adequately lighted by means of high level luminaires, either tungsten halogen, high-pressure mercury or sodium vapour. The best arrangement of lighting is to mount lamps or floodlights on the face of the market buildings, with pole mounted lamps on the site perimeter. Large open areas, such as parking areas, may require cable-suspended fittings.

The spacing of the lanterns will depend on the height at which they are mounted and whether they overhang the road area. For example, lanterns mounted at 8 metres and overhanging 2 metres would be spaced 25 - 30 metres apart. preferably in a staggered arrangement. Detailed design criteria are contained in the British Standard BS 5489 (1980. Code of Practice for street lighting London, BSI).

As an alternative, the German guidelines (DIN 57528) recommend the following standards of averagè illuminance and uniformity:

Building lighting Internal lighting levels to buildings need to be to a high standard of illuminance, with a minimum of 500 lux and preferably 1,000 lux to counteract the brighter natural lighting likely to be found outside. To minimize heat gain high-efficiency fluorescent fittings should be used for artificial lighting, with the building's ventilation arranged so as to draw heated air out. For food displays the colour of the tubes should preferably be warm, but some cultures have a strong preference for the use of cool tubes.

Mechanical ventilation. Although the correct location of buildings on their sites (see Chapter 13) and the choice of an appropriate building form and roofing material (see later in this chapter) will assist in the establishment of a satisfactory internal climate, mechanical ventilation may still be required.

In the humid tropics and coastal areas market buildings will tend to have completely open sides to maximize air movement. This may still not be sufficient to provide comfortable conditions if wind speed is low and solar radiation high. In arid, desert climates the frequent occurrence of sandstorms may prevent natural ventilation systems, such as cooling towers, from being used.

Roof extract fans are, therefore, usually provided for the main market sheds, typically reducing the internal temperature by 3° - 5° C. For offices and other facilities ceiling fans are often installed, although wall-mounted fans tend to be more effective. Air conditioning, even for just the market's offices, is not likely to be economically viable in most less-developed countries. Design standards for ventilation are contained in the British Standard BS 5720 (1979 Code of Practice for ventilation and air conditioning London, BSI).

Telecommunication systems

The telephone is essential for a modern wholesale market. In the USA, for example, around 40 percent of all transactions are made directly by phone. It allows rapid communication between wholesalers, retailers and exporters and also acts a management and extension tool. With the development of market information systems the telephone is the major means by which price information is transmitted to producers.

A major market will require the installation of its own switchboard (PABX system) which would be housed in the main management office. It should have sufficient external lines for the installation of computer modems and facsimile equipment. Public telephones will be required within a market, often accommodated at a post office, which may also provide telegram facilities.

Solid waste equipment

The local or municipal authority is already likely have a system for collection and disposal of solid waste. but this may be oriented to the collection of small-scale domestic waste, probably using compression type refuse vehicles. For markets this is not usually an appropriate system as it may involve the refuse collection staff of the market in additional handling.

Container interchange An ideal system often used in markets is one using container interchange, based around skip lifting vehicles. Skip volumes can vary from around 3 - 9 cubic metres The skips should be located at strategic locations in the market for gradual filling, usually from hand cart loads. They should be collected at the end of the working day and empty skips left For the next collection.

The best method for estimating waste generation is to base the calculations on local survey data, if this available. If not, the following method of calculation (based on projections made for the Sansai market in Chiang Mai, Thailand) provides a reasonable basis for estimating the number of skips that might be required:

· Assume Sansai's rate of waste generation is similar to existing markets in Chiang Mai (overall density ranging from 180 - 260 kg/m³, average density 220 kg/m³, market waste density 200 kg/m³)

Other types of solid waste equipment Another method for handling solid waste collection, popular in European markets, is to use metal or plastic containers (paladins) on castors. However, these only have a limited volume (around 1 cubic metre) and require the refuse collection vehicles to be equipped with a lift and tilt mechanism. Medium-size containers of 2-3 cubic metres capacity are also sometimes used, but again require special fork lifts attached to the refuse vehicle. Keeping the paladins and containers clean can also cause a major problem, unless special facilities are available for automatic cleansing.

The general cleaning of road and floor surfaces within a market is also very important. In most less-developed countries the only economic solution is to use a combination of manual cleaning with brooms and small-wheeled collection carts. Where labour costs are high, mechanical methods should be used, either small, hand-operated cleaning machines or vacuum-operated vehicles equipped with brushes for kerb cleaning.

For small secondary wholesale markets the most convenient collection vehicle may be a tractor and trailer combination. In this case refuse would be collected in the market at fixed enclosures (usually constructed of rendered masonry or concrete) and then manually transferred into the trailers. Often this service can be arranged with a local contractor, typically a farmer who already owns the equipment.

Contents - Previous - Next