Effect of isolated Footing settlement on structural response under lateral loads

Abstract

The structural frames are normally analyzed by assuming frames with fixed base, and role of foundation and soil properties, on member action is neglected. It implies that foundation of frame being analyzed is very heavy and soil on which the structure is going to be constructed is rock like material, therefore settlement at the base of frame is considered zero. Actions thus obtained are used in designing the foundations and other structural members like beams and columns. Since base is considered fixed which may not always be true the actions in members and loads transferred at foundation level in this manner do not depict the true structural behavior. A proper and economical design of foundation results in elastic behavior, which makes the foundations vulnerable to settlement under loads. When uniform gravity loads are applied this settlement is less at base of vertical members located at corners of a frame and more under vertical members located in interior spans. Whereas when lateral loads are applied settlement in bases at far ends is very high as compared to the bases located at the near ends. Theses settlements results in member actions, which are different from those obtained when analyses is carried on assumption, that frame has fixed base. Also different types of foundations have different effect on member actions. Response of foundation in terms of settlement in the structure varies with nature and direction of loading. The cross sectional properties of foundation depend upon the soil properties. When the structural frame and the foundation are modeled together and analyzed, real response of the structural frame can be observed. Due to advancement in computational technology i.e. availability of high-speed computers and efficient softwares it has become possible and economical now to conduct analyses in this way. This work is focused on analyzing frame and isolated foundations together and studying the results in terms of foundation reactions and member actions. All results were than compared. It was concluded from the comparison of results that modeling of structure along with foundation affects its response. Under lateral forces the accumulation of stresses at certain points in the foundation resulted in large settlements. The settlements, which occurred in the modeled foundation directly, affected the member forces in the structural members. The research work can be helpful for the practicing engineers to achieve the more realistic response of the structure and more economical and safe design of reinforced concrete structures under gravity as well as seismic loadings.