Effect of Differential Settlement on Frame Forces - a Parametric Study

The dynamic nature of loads, seasonal variation of soil properties, uneven soil strata below footings or inappropriate design of foundation makes differential settlement inevitable. Differential settlement is largely responsible for developing forces or changing the existing forces in the structure and is often considered as the underlying cause of many structural failures. A structural engineer has to take into account these forces while designing the structure. Different structural parameters like stiffness of columns and beams and number of storeys and bays affect the response of the structure to differential settlement. In the present study, a plain portal frame subjected to constant differential settlement of 10 mm is analyzed for different cases using a structural analysis software i.e. STAAD Pro. Different parameters of the frame are varied to understand their influence on frame forces developed due to differential settlement. The parameters taken into account in this study are length of beam, height of column, moment of inertia of beam and column and number of storeys and bays. It is found that the forces in the frame developed due to differential settlement decrease on increasing beam lengths and column heights. Also, decreasing the moment of inertia of beams and columns is effective in reducing these forces. In addition, differential settlement tends to affect forces more in lower storeys as compared to higher storeys. The frame forces are more prominent for bays which are near to the support subjected to settlement. The increase or decrease in frame forces due to differential settlement can be attributed to the change in stiffness of the members. Hence, the study concludes that the stiffer members tend to develop higher forces for constant differential settlement in frames.