Structural design of the foundations for a building is done by the structural designer of the project and is quite a specialized field within itself.
The construction personnel should be generally aware of the various requirements which a foundation has to fulfill.
This article explains the topic of foundation engineering from the execution point of view.
Basics of foundation design :
Structural Design is a balancing act between the disruptive and the stabilizing forces.
There are 3 elements in the foundation design.
A. The forces of nature :
A.1 Wind load :
Higher is the wind velocity higher is the wind pressure on the column which shall increase the turning moment on the footing .
A.2 Seismic loads :
Earthquake causes a lateral force to act on the footings and is parallel to the ground level.
The effect is similar to someone pulling the rug from under your feet.
Based on the past history of earth quakes and the geological data , seismological zones have been defined. The relevant seismic coefficient is taken into consideration making provision for the destabilizing forces due to earthquakes.
A.3 Subsoil water pressure :
The upward pressure of the sub soil water tends to destabilize the footing by uplifting.
This needs to be counterbalanced properly.
B. The character of the soil :
B.1 Topography :
Building code specifies that the footing must rest to a depth of minimum 900 mm from the natural ground level.
B.2 Strata at various depths:
The type of strata decides the Bearing capacity .
Safe bearing capacity is the ability to sustain that load while keeping the settlement within permissible limits.
C. The character of building :
The elements which have a direct bearing on the design of footing are :
C.1: Height of building :
More is the height of building , more is the loads transferred by the column to the footing and it is designed .
C.2: Column span :
The distance between two adjacent columns :
C.2.1 : Determine the bending moments at the joints which affect the foundation design.
C.2.2 : Decides whether to go for isolated column footings or combined footing.
C.3 : Loading conditions on the columns :
Whether load axis is symmetric or eccentric also has an effect on the design.
C.3.1 Dead loads :
Determined from the density of the various materials used for the construction .
RCC : 2500 Kg / CuM.
Red bricks : 1600 Kg / CuM.
AAC blocks : 600 Kg / CuM.
C.3.2 Live loads or superimposed loads:
For a residential building , this is to be considered for a minimum of
200 Kg / Sq.M .
D : Other considerations :
D.2 For end columns of a building it may not be some times possible to excavate a regular footing since it may project into the adjoining property.
At such times it is suggested to have a she footing.
Posted from WordPress for Android