How Strong is the Wall?
Masonry construction involves heavy materials. The weight from these materials leads to certain types of stresses. There are three types of forces that may impact on the construction. The first step to cope with these stresses is to understand the forces involved.
There are 3 types of forces to be considered – compressive forces, bending forces and shear forces.
Compressive Force - this force will squash or compact the lowest points of a construction. For a build constructed on piers or pillars, the compression force is relative to the amount of weight it is supporting. The compressive forces are greatest where a foundation sits on the earth below, and the structure sits on the foundation.
Bending Force - when there is a mass of masonry over an opening (e.g. a door or window), bending forces from above can cause the structure to bend or collapse from above. A metal lintel, for instance, may bend downwards from the centre.
Shear Force - when a beam (e.g. lintel) sits on top of two sections of wall, or piers, a third type of downward force can occur which tends to cut through the masonry. These forces can cause cracking in brickwork. Reinforcing is frequently needed against such forces.
For large builds a structural engineer may need to calculate how to build to adequately cope with such forces.
TYPES OF WALLS
There are different types of wall which can be constructed including:
- Cavity walls, dry stone walls, wet walls, load bearing walls, partition walls (non load bearing)
Walls in buildings must:
- Support the upper floors (if any) and roof
- Be able to accommodate windows and doors where needed
- Provide suitable insulation against sound and temperature extremes
- Be able to resist damp
- Have reasonable resistance against fire
Arguably, although it's a matter of taste, a wall should also be well made so that it looks good.
Walls have to be constructed so that they are thick enough to maintain stresses within safe limits for the compressive stresses on the materials in the wall. Wall thickness to height should prevent buckling of the wall under stress and collapse laterally. The materials used have to be fit for purpose i.e. able to withstand adverse conditions or corrosion from salts and pollution.
Cavity walls don't need to be as strong as single layer walls because they gain support from being tied to one another. For very high strength bricks (>70N/mm2) mortar with a 1:3 cement to sand mix should be used. For low strength brickwork, a mix of 1:2:9 (cement to lime to sand) may be used.
Building regulations set out permissible height to thickness ratios for walls of buildings (slenderness ratios). These vary according to the material used e.g. clay brick, stone. Construction of smaller buildings and other structures are governed by factors other than strength such as fire resistance and insulation.
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