Seismic Performance Of RC Frame Structure Strengthened With Redundant Steel Braces

Pancake collapse of the global story is one of the typical failure modes of building structures subjected to strong earthquake ground motion. The formation mechanism of the pancake collapse of the global story can be attributed to the vertical irregularity of the structural layout, which could lead to the seismic elastic-plastic demand concentrations of the weak- and soft-story and the overall buckling collapse of building structures. It is an effective means of the prevention and control of the structural pancake collapse by improving the seismic design redundancy and the displacement capacity of structures. It is an effective strengthening measure for the existing structure to increase the shear capacity and the deformation capacity of frame columns by adding the redundant braces and transferring the seismic demand of the weak- and soft-story seismic demand. This research aims to prevent and control the pancake collapse of building structures under strong earthquake action, which is specific earthquake damage mode. The vertical irregularity planar reinforced concrete frame structures are used to perform the seismic dynamic elastic-plastic time history analysis, and the performance parameters, such as the horizontal sheer force and vertical axial force of frame columns, the maximum lateral displacement and the story drift, are used to evaluate the seismic performance of structures. The quantitative influence of the arrangement of the redundant steel braces for single- and across-story strengthened structure on the pancake collapse is studied. The results are as following:(1) The reinforcement of the redundant steel braces can effectively reduce the shear demand of frame columns. The influence of the steel braces on the shear demand of frame columns is more significant with increasing in the vertical irregularity factor, and the variation rule between the side-column and the median-column is similar. The steel braces cover the full arrangement is the most effective strengthening way.(2) The reinforcement of the redundant steel braces has a great influence on the maximum lateral displacement and the story drift. The reduction amplitude of the lateral displacement decreases with an increase in the story height, which can reach the maximum reduction at the bottom story. The maximum reduction of the story drift appears in the median story.(3) The reinforcement of the redundant steel braces has a disadvantage influence on the axial force distribution, which can lead to an increase in the axial force of frame columns. The influence on the side column is larger, but the influence on the median column is small. The number of the redundant steel braces should be fully considered. The excessive axial force caused by the unreasonablearrangement should be avoided, so as to affect the structure of the vertical bearing capacity.(4) Compared with the single-story reinforcement with steel braces, the across-story reinforcement has a better strengthening effect, which can produce a more homogeneous distribution of the structural internal force and displacement, and has a more significant reduction of shear demand of frame columns and story drifts.