Differential settlement among isolated footings of building frames: the problem, its estimation and possible measures

The conventional design of building frames is based on the assumption that the settlement of footings has no effect on the load of the corresponding columns. In reality, the differential settlements among various footings result in a redistribution of the column loads, the amount of which depends on the rigidity of the structure and the load-settlement characteristics of the soil. The present paper attempts to study the effect of the same on design force quantities for frame members of building frames with isolated footings. The nonlinear settlement versus stress relationship arising in case of building frames with isolated footings on clayey soils is attempted to be dealt with two alternative iterative approaches. Results of a few simple three-dimensional frames obtained by the simpler approach are compared with the results of the more rigorous approach and found to be satisfactory. Frames resting on sandy soil are also studied by idealising the soil medium below the footing as linear elastic springs. Various representative case studies are presented for frames resting on sandy and clayey soil. These case studies may help in arriving at design provisions to account for such effects. The simpler iterative approach may be exercised with the help of available commercial frame analysis software and may prove useful in improved design of buildings accounting for the effect of soil-structure interaction. Adequacy of providing tie beams, continuous lintels, and diagonal braces in minimising the redistribution of force quantities are also studied and presented in the paper in limited form. The addition of diagonal braces are found to be effective in substantially minimising the change in force quantities due to the soil-structure interaction.