Numerical analysis of controlled rocking self-centering steel braced frames with replaceable dampers

Abstract

The conventional seismic force resisting system uses the property of inelasticity to dissipate seismic energy which leads to structural damage and permanent drifts of the building after the earthquakes. These residual drifts make the structure difficult to reuse and cost huge financial losses. To overcome this problem, controlled rocking system is developed which helps to minimize residual drift to near zero and also ensures that the structural damages are limited to the replaceable fuses. The controlled rocking system is a steel braced frame consisting of post-tensioned strand and energy dissipator (fuse) and the frame rocks upon its foundation during the shaking of the building. The system enables quick and economical post-earthquake repairs by its self-centering capacity and ensuring that the inelastic damages to structure is limited to the fuses which can be easily replaced later. In the present study, a numerical finite element investigation has been conducted to study the behavior of controlled rocking frame system. A simplified fuse model representing butterfly fuse model is used to model the energy dissipation system. Nonlinearities is used in modeling the simplified fuse system. The developed finite element model has been applied to simulate experimental studies done by past researchers and it has been found that good agreement exists between present analysis and past experimental results, which establishes the acceptability and validity of the present finite element model to carry out further investigation. Then this research aims to modify the previously proposed design procedure and develop new coefficient values and stiffness ratio for better design purpose. In this study a particular configuration of rocking frame system is focused which has post-tensioned strand at the center of the rocking frame and fuses connected with the columns. Considering different aspect ratios, three cases such as three-story, six-story, nine-story are used for case study and nonlinear dynamic analysis are conducted by 10 earthquake ground motions scaled to the level of the Maximum Considered Earthquake (MCE) subjected to the three cases. The findings demonstrate that the modified stiffness ratio gives better prediction of uplift ratio than the ratio used for uplift prediction in the existing literature. Besides the modified coefficients also show better agreements for predicting the member forces for seismic design than the previous method. The configuration of rocking frame used in this study validates the controlled self-centering rocking behavior and can be reused after severe earthquake. As the prediction of the uplift ratio and member forces agree with the numerical analysis result, the modified design method in this study can be used in practical for design purpose.