Prior to the design of a foundation, the expected loads to the foundation should be assessed as accurately as possible, this loads is called design load. In general a foundation has to bear the design load as described below.
Dead load – This includes the weight of all permanently materials to the structure. This includes beams, floors, walls, columns and fixed service equipment.
Live load – All loads which are movable from one location of the structure to other location are live loads. Loads due to people, furniture, equipment, machinery and so on are the live loads. It is difficult to assess live load accurately.
Wind load – All exposed part of any structure are subjected to wind loads. The wind load is a function of wind velocity and height of the structure.
Snow load – In cold climate snow accumulates on roofs and exterior flat surfaces. Thus the weight of snow has to be taken into account in the design. The snow load is generally taken as 100 Kg.m2.
Earth pressure – Earth pressure acts laterally on the buried part of a structure. This is calculated using the principles of lateral earth pressure theory. It is normally treated as dead load.
Water pressure – Water pressure should also be taken into account while designing water retaining structures such as cofferdams, dams, barrages etc. Water pressure may also cause uplift pressure on the base of the foundation and should be taken into account.
Earthquake load – The force due to earthquake may act horizontally, vertically and torsionally in any direction. It should be determined from relevant codes.
Computation of Design Load
A foundation shall be proportioned for the following combination of loads.
- Dead load and live load
- Dead load and live load and wind or earthquake loads
Dead load includes the weight of column/Wall, footings, foundations, overlying fills but excludes the weight of displaced soil.
If the wind or seismic load is less than 25 % of combination of dead and live load, it may be neglected and foundation is designed for dead and live load only. When wind or seismic load exceeds 25 % of the sum of dead load and live load, the foundation shall be designed for combination of (ii). The foundation pressure should not exceed the safe bearing capacity of soil by more than 25 %.
Prior to foundations resting on granular soils, the settlement should be computed using the combination (ii), as settlement occurs in a very short period. In clays, however, the settlement should be computed corresponding to permanent loads. The permanent loads are usually taken as dead load plus half 0f the live load.