Civil Engineering
METHODS OF RAFT FOOTING DESIGN
According to IS – 2950:1965, the design criteria of raft footings are given below:
The maximum differential settlement in foundation on clayey soils and sandy soils should not exceed 40mm and 25 mm respectively. The maximum settlement should generally be limited to the following values:
Raft foundation on clay – 65 to 100 mm.
Raft foundation on sand – 40 to 65 mm.
There are two methods for the design of raft foundations. They are:
1) Conventional Method
2) Soil Line Method.
1. Conventional Method
Assumptions:
1. The soil pressure is assumed to be plane such that the centroid of the soil pressure coincides with the line of action of the resultant force of all the loads acting on the foundation.
2. The foundation is infinitely rigid and therefore, the actual deflection of the raft does not influence the pressure distribution below the raft.
In this method, allowable bearing pressure can be calculated by the following formulae:
Where and = allowable soil pressure under raft foundation in (use a factor of safety of three). The smaller values of and should be used for design. and = reduction factor on account of subsoil water.
N = penetration resistance.
If the values of N is greater than 15 in saturated silts, the equivalent penetration resistance should be taken for the design. The equivalent penetration resistance can be determined by the formula:
The pressure distribution (q) under the raft should be calculated by the following formula:
Where Q = total vertical load on raft x, y = co-ordinates of any given point on the raft with respect to the x and y axes passing through the centroid of the area of the raft.
A = total area of the raft.
= eccentricities about the principal axis passing through the centroid of the section. = moment of inertia about the principal axis through the centroid of the section.
, can be calculated by the following equations:
Where and = eccentricities in x and y direction
According to IS – 2950:1965, the design criteria of raft footings are given below:
The maximum differential settlement in foundation on clayey soils and sandy soils should not exceed 40mm and 25 mm respectively. The maximum settlement should generally be limited to the following values:
Raft foundation on clay – 65 to 100 mm.
Raft foundation on sand – 40 to 65 mm.
There are two methods for the design of raft foundations. They are:
1) Conventional Method
2) Soil Line Method.
1. Conventional Method
Assumptions:
1. The soil pressure is assumed to be plane such that the centroid of the soil pressure coincides with the line of action of the resultant force of all the loads acting on the foundation.
2. The foundation is infinitely rigid and therefore, the actual deflection of the raft does not influence the pressure distribution below the raft.
In this method, allowable bearing pressure can be calculated by the following formulae:
Where and = allowable soil pressure under raft foundation in (use a factor of safety of three). The smaller values of and should be used for design. and = reduction factor on account of subsoil water.
N = penetration resistance.
If the values of N is greater than 15 in saturated silts, the equivalent penetration resistance should be taken for the design. The equivalent penetration resistance can be determined by the formula:
The pressure distribution (q) under the raft should be calculated by the following formula:
Where Q = total vertical load on raft x, y = co-ordinates of any given point on the raft with respect to the x and y axes passing through the centroid of the area of the raft.
A = total area of the raft.
= eccentricities about the principal axis passing through the centroid of the section. = moment of inertia about the principal axis through the centroid of the section.
, can be calculated by the following equations:
Where and = eccentricities in x and y direction