Performance assessment of retrofitted RC frames with different patterns of steel bracing

Abstract

In recent years, many RC buildings in Bangladesh are retrofitted using different types of bracing.However, there is a lacking of guideline or design philosophy in order to obtain the most efficient retrofitting system. Hence, performance assessment of steel bracing patterns for different frames as a retrofitting system is required to be investigated. In this thesis an attempt is made to assess the seismic performance of different bracing system: X bracing, Inverted V bracing, ZX bracing and Zipper bracing.Nonlinear static pushover analyses are performed to evaluate the performance of bracing system.In the pushover analysis method, force and displacement demands are estimated by FEMA-356 criteria. Nonlinearity are assigned to the beams, columns and braces by means of concentrated hinges. Hinges are assigned at both ends of each column, beam element and at mid-span of braces. For column, coupled hinges, which yieldsbased on the interaction of axial force and bi-axial bending momentsat the hinge location, are used. Axial hinges are assigned for steelbraces in tension/compression and moment hinges areassigned for the beam elements. Displacement controlled pushover analysis for all the frames are carried out. The target displacement used for each building is 4% of the total height of the building recommended by FEMA 356. In this study, two load pattern are used for pushover analysis,which is recommended by FEMA 356. For the two load pattern, two load displacement curves (capacity curves) are obtained. Two capacity curves provides two base shear or capacity and two maximum story displacement. Performance of the frames are evaluated in terms of average of this two capacity and average of the two maximum story displacement. In order to understand the performance (post elastic behavior) of braced frame four multistoried factory buildings situated in Chittagong are selected for case study.The building are chosen in such a way that the factors which may influence the seismic performance i.e. different story heights, different number of bays; can be investigated. Furthermore,in order to understand the effect of bays on seismic performance of bracing system, another 24 model of different buildings consisting of 3, 4 and 5 bays considering two lateral load patterns are analyzed. The effect of buildingheighton seismic performance is also investigated by analyzing 32 model of anotherbraced buildings consisting of 6,7,8 and 10 stories.After analysis all the case study frames,ZX or Zipper bracing is found most efficient. Again, ZX and Zipper bracing also show better performance as compared to other bracing systems in the general study, where the buildings consist of different number of bays and story heights are considered. Finally,the effect of section type (i.e. T-section, I-section and Tube Section) in bracing system is evaluated for a six storied buildingby means of 24 models of building frames. From the analysis it is found that, T-section is suitable for X and inverted V bracing. On the other hand, Tubeor T section is found most efficient for ZX and Zipper bracing system.