4.1 Variation in natural frequency
The variation in natural frequency due to the effect of soil flexibility, slenderness ratio and thickness of raft are studied for 200m chimney models, modelled with annular raft. The structure is assumed to be resting on three different soil conditions with modulus of sub grade reaction Ks 5E3, Ks 50E3 and Ks 100E3, modelled using elastic model, and the results for H/Db 7 and Do/t 15 are tabulated in table 2.