School buildings in developing countries
Practical Action
might need to be dug as a series of steps with the footing covering more than one of the
steps.
Walls
Some of the materials and types of construction used for walls have been discussed
previously. For low-rise buildings the simple masonry wall is by far the most common type of
construction. The use of a structural frame with precast infill panels is unlikely to offer any
advantage unless a very large school building programme is envisaged with the components
manufactured at a central facility. A variety of types of bond is possible for erecting the
bricks or blocks. One particular type is the rat-trap bond, originally developed in India and
being more economical in the use of bricks and blocks and mortar than most other types of
bond. Drawings of the rat-trap bond and other economical types of brick and block bonding
construction can be found at:
http://www.saud.ku.edu/ngore/nilsweb/cinvablocks/kucinva/strength2.html
or http://janathakshan.net/files/rat_trap.pdf
Masonry materials are bonded by a cement-based or lime-based mortar. For the wall to
function properly as a structural element it is important that the mortar is not significantly
stronger or harder than the material which it bonds and it can even be significantly weaker. A
mortar based only on cement and sand can also be troublesome for letting water into the wall
and for causing spalling (the breaking off pieces from the surface) of the bricks or blocks.
However, a mortar based only on lime needs to be applied with quite a high level of skill for
good effect, takes a long time to harden and is best applied in a very thin layer. To overcome
these disadvantages of both lime and cement it is common practice to mix lime and cement
for a mortar, for example in proportions of one part lime to one of cement to six of sand, or
one part cement to two of lime to nine of sand. If a low strength mortar is required, for
example if using relatively low strength stabilised soil blocks, soft stone or field fired bricks it
can be made from one part lime to two parts pozzolana, such as some types of volcanic ash
and ground up fired clay, to nine parts sand. As the mortar material is usually more
expensive than the bricks or blocks it is used to bond it is good practice to make the mortar
joints as thin as possible, and to only mix sufficient material that can be used within about
half and hour, or somewhat longer if using pozzolanic cement.
One of the most difficult aspects of masonry construction is the making of openings for doors
and windows. It is important to plan in advance of the construction where these openings
would be situated and what their size would be. Before building the wall wooden pieces can
be cut out and set up to show where the door would be. The same would need to be done for
window openings once the wall has been taken up to the course adjacent to the bottom of the
opening. It might sometimes be necessary to cut or break bricks or blocks to fit in the door or
window openings, but with planning the positions carefully this can be minimised. It is
necessary to cover the top of the opening with a rectangular lintel, arch or corbelling, as
mortar joints are extremely weak under bending stress and would give way easily when subject
to this stress, as occurs at top of an opening. Steel reinforced concrete is the material
normally used to make the rectangular lintel.
Walls are built up to a height of 2.6 to 3 metres above the floor level for each storey. Local
building practice or building codes, if available, would determine the height for the top of the
wall. It would not be advisable to choose a height below 2.5 metres, even though well above
almost everyone's head height, as some people would feel uncomfortable with a low ceiling.
A height close to 3 metres would need to be used if electric lights are to hang in the room.
The exact height would also be determined by the dimensions of bricks and blocks used in
the construction.
In areas where there is a risk of earthquakes special measures for the walls would be needed
to increase safety. These would include a ring beam - a beam the sections of which are
joined together and extending continuously over the top of the external walls, and bracing or a
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