Seismic Vulnerability & Retro Fit of RC Flat Plate Structure

Flat-plate system has become one of the most popular practices now-a-days, primarily for architectural flexibility, use of space, easier formwork and shorter construction time. However, the structural efficiency of the flat-plate is hindered by its poor performance under earthquake loading. Post-earthquake observations and experimental testing have shown that lateral movements induced by earthquake can make the connections between slabs and columns susceptible to punching shear failures. Though Reinforced Concrete Frame (RCF) is widely used in Bangladesh, flat-plate constructions are also becoming a common practice. BNBC and ACI have suggested that flat-plate frames can be designed either by Direct Design or Equivalent Frame Method. The present study deals with the seismic evaluation of existing flat plate structure using inelastic method (Pushover analysis). The load-deformation curve (i.e. capacity curve) is obtained using ETABS. The performance point has been determined by superimposing the capacity spectrum with the normalized response spectra for 5% damping ratio as mentioned in BNBC. The methods for evaluating the performance level have been determined using procedures presented in FEMA-356 & ATC-40. Many existing flat plate buildings in Bangladesh may not have been designed for seismic forces. Hence, it is important to study their response under seismic conditions and to evaluate seismic retrofit schemes. Based on the seismic evaluation results, two possible retrofit techniques were applied to improve the seismic performance of an existing Nine-storied flat plate building which was considered as a model representative of all flat plate buildings in Bangladesh.