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The Purposes of Warehouses, and Basic Requirements
Warehouses are intended for the storage and physical protection of goods. In the context of grain storage, 'goods' primarily refers to bagged grain. It may also include materials and equipment required for the packaging and handling of bagged grain, and storage pest control; although, in an ideal situation, such items should be stored separately. The distinction is made between warehouses and flat stores, which are designed rather differently for the storage of grain in bulk and are discussed in Chapter 7.
Locating a Warehouse
The approximate location of a proposed warehouse for grain storage will have been decided already. However, the determination of its exact siting is a matter for engineers. If a large warehouse is planned, it is always prudent to involve local Civil Engineers at this stage.
Several factors need to be considered in selecting a suitable site. In the first instance the topography of the area has to be studied. It is preferable to erect the warehouse on level ground, ideally slightly raised above the surrounding area, which is well drained and not prone to flooding. Low locations must be avoided. If it is difficult to find a level area then the least undulating or sloping area should be selected, and the site should be oriented along contour lines, in order to minimise the amount of levelling and filling in to be done.
It is then important to determine the characteristics of the soil: its load-bearing capacity, resistance to compaction, and drainage characteristics. Never build on black cotton soils, because these are weak and do not have sufficient soil-bearing capacity even for small warehouses. The warehouse and the approaches to it will need to be protected from running water by an effective drainage system, and the site should be able to accommodate such a system.
For easy access and movement of stocks, the warehouse should be sited as near as possible to a main road. It is also important to ensure that the approaches to the warehouse will permit easy movement and manoeuvring of vehicles around it. This means that, in addition to the area to be occupied by the warehouse, there should be plenty of usable space around it. Also, looking to the future, there should be sufficient space for the erection of additional warehouses and utility buildings.
In tropical countries it is very important that the long axes of warehouses are oriented East-West as nearly as possible. This way, the side walls are least exposed to the sun and temperature variations inside are minimised. If the warehouse cannot be oriented EastWest, some benefit may be derived from siting it across the direction of the prevailing wind. The interior can then be effectively cooled by opening all doors and windows at appropriate times.
Finally, bearing in mind that grain in the warehouse will probably be fumigated with gas lethal to human beings (see Chapter 8) from time to time, it is important that the site chosen must be a safe distance from dwelling houses, shopping centres and other working areas.
Standard Warehouse Design (for information on fumigable warehouses, see Annex 4)
All warehouses consist of a floor, walls, a roof, and one or more entrances. However, they can vary considerably in the detailed composition and construction of these basic components; and may include others, such as ventilators, windows, artificial lighting, etc. The various combinations of features possible have to be considered very carefully, together with other factors relating to location, intended use, etc., when planning the construction of a warehouse.
Paramount importance should be attached to ensuring that the quality of the commodity to be stored will not be affected by physical factors such as moisture and heat. Wherever possible and practical, the design of the warehouse should incorporate features which will protect its contents from attack by rodents and birds, and facilitate the use of insecticides.
The warehouse should also be easy to clean and maintain (there is no point in using components which are not readily replaceable or repairable); and it should provide good working conditions.
(i) Foundations and Floor
Unstable clay soils and areas which have been filled in should be avoided wherever possible, because they involve the risk of subsidence. In all cases, it is necessary to dig down to a point where the soil-bearing pressure is 150 kN/m2 or better.
The floor must be able to bear the weight of the grain which will be stacked upon it, and it must also be impermeable to ground water. For these reasons the floor should consist of a slab of reinforced concrete laid upon well compacted hard core, with a moisture barrier sandwiched between the two. This moisture barrier should consist of a layer of bitumen or asphalt, bitumen felt, or a polyethylene film.
The reinforced concrete slab must be made with expansion joints, to prevent cracking (which makes storage hygiene difficult). It should be covered with a cement cap a few centimetres thick, which is rendered smooth and hardened (to prevent powdering). Ideally, the concrete slab should be laid after the roof has been completed: to prevent direct sunshine drying it too rapidly, and possibly causing it to crack.
The floor level must be sufficiently above ground level to ensure that water will not enter the warehouse, even after the heaviest rainfall that can be expected. Consideration could be given to erecting the warehouse on a plinth raised about 1.2 metres above ground level, to facilitate loading and unloading of vehicles; but this alternative is expensive and can add 40% to the cost of construction.
Most modern warehouses are constructed with a framework, usually of reinforced concrete. The supporting pillars are linked together by lower tie-bars, which are themselves secured to the floor slab, and by upper tie-bars, which hold the frame firmly together. It is essential that all joins are secure and accurate, and that the reinforcing rods are well covered with concrete. The walls of the warehouse are built between the supporting pillars.
If the supporting posts are thicker than the walls, it is important that the extra thickness is on the outside of the building so that the internal surfaces of the walls are smooth and free from projections. This facilitates cleaning of the store, and avoids interference with other operations as well.
The walls may be made of breezeblocks, or stabilised earth bricks 15 to 20 cm thick, and should be rendered smooth on both sides. They should be painted white, on the inside to facilitate the detection of insect pests, and on the outside to help keep the warehouse as cool as possible. Alternatively, the walls may be made of a lightweight material such as fibro-cement, galvanised metal sheet, or aluminium sheeting. However, walls of this kind are easily damaged, have poor insulating properties, and are sometimes prone to erosion.
A vapour-proof barrier should be incorporated into the base of the walls, to prevent damp rising and causing damage to the warehouse structure and its contents. Also, a concrete strip about 1 metre wide should be laid around the outside of the warehouse, to prevent rain from eroding the base of the walls below the damp course.
Internal pillars supporting roof frames should be avoided because, as previously stated, they can interfere with pest control and other stock management procedures. Instead, roof frames should be designed so that they transfer the weight of the roof to the supporting columns (in framed buildings), or to the walls if the warehouse is small.
Modern engineering techniques allow very wide 'free-span' roofs (i.e. roofs without internal supporting pillars) to be constructed. However, such roofs are very expensive and rarely used in warehouse construction. A steel portal frame should be used if the span is to be greater than 15 metres. Warehouses less than this width may have reinforced concrete roof frames.
Roof frames made of wood or bamboo are only suitable for warehouses not more than 4 or 5 metres wide. The wood used must be well dried and treated with a preservative.
Roof cladding may be of galvanised steel or aluminium sheeting, or asbestos cement; the latter being more fragile but having better insulating properties. Tiles are not recommended, especially for large warehouses.
The roof should overhang the gables by 0.7 to 1.0 metres, and the eaves by at least 1 metre. This ensures that rainwater is shed well clear of the walls; and obviates the need for guttering and drainpipes, which may become blocked or assist rodents entering the warehouse. The overhang also helps to keep walls cool and protects ventilation openings from rain.
Ventilation openings are necessary for allowing the renewal of air and reducing the temperature in the warehouse, they also allow some light to enter it. If such openings are
located too low down they can be the source of numerous problems: entry of water, rodents, thieves, etc. These problems are avoided when ventilators are placed under the eaves. They should be fitted on the outside with anti-bird grills (20 mm mesh) and on the inside (10 cm behind the grills) with 1 mm mesh screens (removable for cleaning) which will deter most insects.
The number of doors will vary according to the size of the warehouse. If possible there should be at least two doors, so as to be able to rotate stocks on a 'first in, first out' basis. Hoever, this may not be possible or practical in a very small warehouse.
Double sliding doors are recommended. Preferably made of steel, or at least reinforced along their lower edges with metal plate as protection against rodents, they should be sufficiently large (at least 2.5 x 2.5 m) and close fitting. If swing doors are fitted they should open outwards in order not to reduce the storage capacity of the warehouse. It is recommended that the doors be protected from rain by an extension of the roof or a separate cover.
Adequate light in a warehouse is an important factor as far as the safety of workers inside it is concerned. However, there can be problems in providing sufficient natural light while satisfying other technical aspects at the same time.
Many warehouses are fitted with translucent sheets in the roof. However this is considered inadvisable, because it may involve the risk of spot heating of produce in the top layers of stacks underneath. Other warehouses are reasonably well lit by daylight filtering through ventilation gaps left along the tops of side walls. This source of illumination is impaired by the installation of bird-proofing. Non-opening windows set high up in walls may solve this problem; although their sills could harbour pest-infested grain residues, unless they are specially sloped to prevent this happening.
Most warehouse managers find that leaving several doors wide open during the hours of intense sunlight in tropical countries provides adequate illumination of the interior. This is probably the most practical solution when all open doorways are in active use. Otherwise, it does invite the risk of theft, or furtive access by rodents.
Artificial lighting is justified only in warehouses which are regularly worked in during hours of darkness.
Determining the Dimensions of a Warehouse
Before calculating the dimensions of a warehouse it is important to identify the function it is intended to perform. If it intended to be a transit store, and bagged grain will be moved through it quickly, stacks are likely to be low and plenty of working space will be needed. If, on the other hand, it is to be used for the storage of reserve stocks, stacks will need to be as high as possible and only minimum working space will be required.
The dimensions of a warehouse are calculated mainly from:
An example of how the dimensions are calculated, using these parameters, is given on page 162.
(i) The Specific Volume of the Product
This is defined as the volume occupied by 1 tonne of the bagged grain. The term Specific Volume may be similarly defined for grain stored in bulk, which is the subject of the next chapter.
Engineers find it more convenient to use Specific Volume rather than Bulk Density (see Chapter 3) in their calculations. However, even now, there is a tendency to use bulk density data in determining specific volumes for products. This tendency is queried (Clancy (1977) and Hayward (1981a)) because large quantities of bagged grain in stacks or bulk grain in silos become compacted, and occupy less space than bulk density calculations indicate. Clancy found that bulk wheat can pack by 1% or more, while Hayward discovered that the average density of bagged millet exceeded quoted bulk density figures by up to 40%.
While Specific Volume is regarded as a valid parameter, it is recommended that engineers responsible for designing warehouses should collect as much information as possible on the specific volumes of locally important products to guide their calculations. For what it is worth, Table 6.1. gives the specific volumes of a number of products for which warehouse accommodation is frequently required.
Table 6.1. Specific volumes of some bagged grains and grain products.
|Commodity||Specific Volume (m³/t)|
|Beans, peas, lentils||1.30|
|Wheat, milled rice, coffee||1.60|
|Maize, sorghum, decorticated groundnuts, palm seed||1.80|
|Wheat flour, maize meal||2.10|
(ii) Maximum Tonnage
This parameter will depend upon the purpose for which the warehouse is required. The quantity calculated should also take long-term projected requirements into account.
(iii) Stack height
This also depends, in part, upon the purpose for which the warehouse is required (see above). The nature of the commodity and the type of sack to be used are additional factors to be considered.
Some commodities, notably palm seed and cocoa, cannot be stacked very high because they compact easily. Sacks made of woven polypropylene have a tendency to slide on each other, and therefore should not be stacked more than 3 metres high. Jute sacks bind together better, and may be stacked up to 6 metres above the floor.
The height of stacks should not exceed the height of the walls, and a space of at least 1 metre should be allowed between the tops of stacks and roof frames.
(iv) Separation of lots
Maximum use of the warehouse is gained by storing products in one stack. However, it is usually necessary to separate lots; and, for better stock control, gangways have to be provided between and around stacks. Spaces 1 metre wide should be left between stacks and between stacks and the walls. Also, it is customary to provide one or more areas at least 2 metres wide, in which incoming or outgoing stocks can be handled.
Ancillary Buildings and Structures
Warehouses are often constructed without consideration being given to how the storekeeper, equipment and consumable items ( empty sacks, pesticides, etc.) are to be accommodated. The storekeeper is then obliged to section off parts of the warehouse as his 'office' and storage areas for equipment and other items. Apart from thus wasting valuable space, this practice is also hazardous when stocks of grain have to be fumigated. It is always better, therefore, to include an office and other ancillary buildings, adjacent to the warehouse but a safe distance from it, right from the early planning stages.
The provision of toilets and washing facilities for workers is a statutory requirement in many countries, but often overlooked. Larger grain storage installations may also require quality control laboratories, workshops and garages for vehicles, and so on. An incinerator for the destruction of spoiled grain and combustible waste material would complement other pest control measures, and reduce rubbish disposal costs.
In order to make maximum use of the storage space inside a warehouse, it is often advantageous to extend the roof at one end (Figure 6.18. Standard warehouse with a working area at one end.), or along one side, to provide a covered working area for the handling of stocks being received or despatched.
For further detailed information see FAO (1985),
Example of how the dimensions of a warehouse are calculated
In this example it is assumed that a warehouse is required for the storage 1000 tonnes of maize in jute sacks in 4 separate lots. It is also assumed that the warehouse will be rectangular in plan, with the length approximately twice the width. Prom the specific volume of maize (Table 6.1), the total volume of the stock will be:
1000 (t) x 1.8 (m3/t) - 1800 m3
If the sacks of maize are to be stacked 5 metres high, the floor area required will be:
If length (L) = 2 x width (W), when:
2W2 = 360 m2, or W = 13.4 metres
Keeping the example simple, let W = 12 m; then, the area being 360 m2, L = 30 m.
If the stock is to be kept in four separate lots, each measuring 6 x 15 metres, then the following floor space will also be required:
a main handling area, 3 metres wide, along the axis of the warehouse; a gangway 2 metres wide, across the centre of the warehouse; and an inspection space 1 metre wide around the entire stacking area.
The internal dimensions of the warehouse will be:
Width (W) = 1 m + 6 m + 3 m + 6 m + 1 m = 17metres
Length (L) = 1 m + 15 m + 2 m + 15 m + 1 m = 34 metres
giving a total floor area of 578 m².
If the warehouse is to have a trussed roof, the walls should be at least one metre higher than the intended stacking height: in this example 5 m + 1 m = 6 metres.
The percentage utilisation of the building will then be:
Similar calculations indicate that in small capacity warehouses (10 to 30 tonnes) only about 20% of the space is usable. Medium capacity warehouses (50 to 100 tonnes) have only about 30% usable space. Thus the larger a warehouse is, the more economical it will be in terms of cost of construction per tonne stored.
Figure 6.19. Inside view of a standard warehouse.
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