When should the flexible foundation approach be used?

It should be used when the relative stiffness factor (K) is less than 0.5, meaning the raft will bend significantly under loads, causing non-planar soil pressure distribution.

How is the soil modulus of subgrade reaction determined?

It is typically evaluated using a standard plate load test (IS 1888) and then corrected for the size of the full raft foundation using formulas provided in Appendix B.

What happens if the center of gravity of the loads does not coincide with the centroid of the raft?

Eccentricity is introduced, causing an unequal pressure distribution on the soil. The raft must be proportioned to keep this eccentricity as small as possible to avoid excessive differential settlement.

IS 2950

: 2000

Code of practice for design and construction of raft foundations, Part 1: Design

CurrentFrequently UsedCode of PracticeBIMGeotechnical · Soil and Foundation Engineering

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This code establishes the criteria for the design of raft (mat) foundations. It provides detailed guidance on evaluating soil-structure interaction, selecting between rigid and flexible methods of analysis, calculating the modulus of subgrade reaction, and properly proportioning the foundation to limit differential settlements and ensure safe stress distribution.

Code of practice for design and construction of raft foundations, Part 1: Design

Quick Reference Values

relative stiffness factor limit for rigid analysis> 0.5

relative stiffness factor limit for flexible analysis< 0.5

minimum factor of safety against uplift1.2

recommended minimum thickness at edge150 mm (subject to shear checks)

Key Formulas

Relative stiffness factor (K) = (E_c * I) / (E_s * b^3)

Radius of effective stiffness (L) = [E_c * I / (k * B)]^(1/4)

Characteristic length (lambda) = [k * B / (4 * E_c * I)]^(1/4)

Practical Notes

Punching shear (two-way shear) under heavily loaded columns frequently governs the thickness of the raft and must be checked strictly as per IS 456.

The modulus of subgrade reaction (k) given in tables or obtained from plate load tests (30 cm plate) must be scaled to the actual dimensions of the raft.

The rigid method of analysis assumes the raft acts as a rigid body; this is only valid when the structural stiffness is high and column spacing is relatively close.