Evaluation of deflection amplification factor for estimating seismic deformations of RC frames

Abstract

Elastic displacement is multiplied by deflection amplification factor (C_d) to get maximum displacement under severe seismic conditions. The elastic displacement is determined either by equivalent static analysis or linear time history analysis. In general code specifies certain value of C_d which may be used to estimate maximum inelastic deformation of building. The approved Bangladesh National Building Code (BNBC) has specified 4.5 as C_d value for Intermediate Reinforced concrete Moment Resisting Frames (IMRF). Theoretically, C_d value should vary according to building height, span, stiffness and other properties. To examine the range over which C_d varies for building in different regions of Bangladesh, in this work, 24 Reinforced Concrete buildings with different vertical and plan configuration has been examined. For analysis of these frames, three-dimensional finite element software ETABS has been used. The 24 buildings considered in this work had bay width of 5 meter and height of 3.2 meter. Building frames of 2, 3 and 4-bay were examined in this work. Each bay has 8 frames having 2, 3, 4, 5, 6, 8, 10, 12 story. At first, structural analysis of these frames has been performed using equivalent static analysis as per BNBC (2020) which is based on zone specified seismic force and weight of the building. Through equivalent static analysis the building is designed and different structural member dimension was selected. Using same geometrical, material and loading data, nonlinear time history analysis was on the same buildings. Nonlinear time history was been done for PGA 0.2g, 0.3g and 0.36g using the imperial valley earthquake. As peak ground acceleration of Imperial valley earthquake is 0.2605g (at 0.43 sect), it was scaled so that PGA becomes 0.2g, 0.3g and 0.36g. For 12 storied building, nonlinear displacement was found 115.2 mm, 100.4mm and 102.5 mm for 2,3 and 4-bay building respectively. Whereas for 8 Story building had displacement was of 63.3 mm, 55.1 mm and 55.8 mm for 2, 3 and 4-bay building respectively. As storey was reduced, top nonlinear displacement was found to decrease whereas as number of bay was increased, nonlinear displacement was found to decrease. Likewise, nonlinear storey drift was found to increase as number of storeys was increased. For 12 storied building, linear displacement was found 25.2 mm, 22.6 mm and 22.8 mm for 2,3 and 4-bay building respectively. Whereas for 8 Story building, top displacement reduced to 14.9 mm, 15.2 mm and 15.4 mm for 2, 3 and 4-bay building respectively. For four storey building, top displacement was found to decrease further. Hence, as storey height was reduced, top linear displacement was found to decrease. Further, as number of bays was increased, linear displacement was found to decrease. C_d value was estimated from non-linear and linear top displacement found from the respective analysis. For 12,10 and 8 storied building had C_d value ranged from 4.57 to 4.23, 4.5 to 3.63 and 4.5 to 3.63 for 2,3 and 4-bay building respectively. BNBC specify a flat C_d value of 4.5 for buildings of all configuration. However, from this analysis work, it is evident that C_dvalue is very much dependent on building height, number of bays etc. C_d values suggested in this work may be useful for practicing engineers and researchers to estimate ultimate displacement of buildings due to severe earthquake from linear analysis of building for different regions of Bangladesh.