Building load

For a typical modern three-bedroom detached house, the total of the dead and imposed loads is about 120 tonnes and most types of ground can easily carry this load using simple foundations. The exact dead and imposed loads can be calculated using simple tables produced by the British Standards Institution.

When designing buildings, engineers are primarily concerned with forces rather than weights and the unit of force most commonly used is the kilonewton (kN); 1 tonne is roughly equal to 10kN. Therefore, a building load of 120 tonnes exerts a force on the sub-soil of 1,200kN. If the load of a building is carried by the external walls and the perimeter of the building is 30m, simple calculation shows that each metre in length (i.e. metre run) of foundation is carrying about 4 tonnes or 40kN. In practice, allowance must be made for the nature of the loading, which varies according to the direction of roof and floor spans. Furthermore, uneven loading can be caused by large openings, such as patio doors. Once the average loading per metre run of the building is established it must then be related to the safe bearing capacity of the ground in order to enable the design of an appropriate foundation.