Enhancing seismic resilience of RC building through foundation rocking

Abstract

The incorporation of rocking foundation is remarkably effective construction alternative for safeguarding the superstructure from significant damage caused by severe lateral forces during intense earthquakes.Even though, the mechanism of rocking foundations and its beneficial effects are widely documented, generalized design guidelines and its applicability on the Reinforced Concrete (RC) framed buildings are yet to be explored. Hence, The scope of this thesis work aims to to quantify the seismic force and displacement demands for RC frame buildings on the explicit and combined effects of the rocking foundation and superstructure behaviour with respect to key parameters addressing the seismic force and displacement demands. The objective of this study is to demonstrate the advantageous impact of using rocking foundations on the seismic performance of Reinforced Concrete (RC) framed building. This will be accomplished by comparing the performance of buildings that have conventionally designed foundations, rocking foundations and fixed base counterparts. Rocking at the foundation level is achieved by under proportioning the footings by considering the reduced earthquake loads for footing design. The present research comprises of two distinct objectives, where for RC framed building without shear wall solely the supporting foundations are allowed for rocking. However, for RC building with shear wall solely the foundation supporting shear wall is allowed to rock. Within the OpenSees framework superstructural elements are model as fiber-based modelling with distributed plasticity whereas substructural elements and soil are modelled using Beam on Nonlinear Winkler Foundation (BNWF) modelling. The observations made from eigen analysis indicates the period lengthening for the both structural configurations considered. For RC framed buildings without shear wall, nonlinear static pushover assessments showed that permitting the foundation rocking increases yield and peak displacement by about 9% to 34% without substantial reduction in the strength. Also, the plastic displacement capacity increases as the rocking effect increases. This shows that rocking the foundations in a structure is advantageous for its overall seismic performance. According to the nonlinear dynamic time history analyses, seismic moment transferred from the column to the foundation decreases by 20% to 50%. Due to reduction in the peak roof acceleration and increasing settlement at the base of the foundation with increasing effect of rocking, reduced seismic moment is noticed at the base of the structural members. Similar responses are noticed for the buildings where only the shear wall foundation is allowed to rock. It is found that the foundation of a shear wall can be designed by taking into account 40% of the earthquake loads for zone V design level and 60% of the loads for zone II design level without encountering excessive settlements beyond permissible limits as per Indian standards. From the hysteric responses for the shear wall foundation rocking, it is evident for very strong impact seismic motion, conventionally designed footings tends to experience higher flexural displacement along with higher seismic force demands and settlement demands too. This suggests that an overdesigned footing may not always be beneficial for the superstructure. From the fragility assessment it is observed that the probability of exceeding 25mm settlement increases with increase in foundation rocking regardless of soil type. However, the probability at the collapse prevention level of 60mm is not considerably influenced for the foundation proportions while transitioning from conventional footings to moderate rocking footings. This implies that reducing the dimensions of the foundation may not necessarily result in reasonable settlement limitations being exceeded. The most favourable conditions for foundation rocking is observed to be dense and very dense sand than medium dense sand.