Seismic Damage Assessment & Retrofit of Hybrid Structures

Abstract

Pakistan is highly seismically prone country, which faced devastated earthquakes in history. Often the construction in northern areas of Pakistan is done with composite materials for gravity load bearing systems, which is not suitable for these areas. Hybrid structures are those with two or more different lateral load resisting systems. Hybrid buildings may be conceived at the design stage itself or could be the result of modifications or expansions in existing buildings. Seismic response of hybrid buildings is different from other structural systems as there is discontinuity, both in the lateral and vertical load transfer mechanisms. The response under lateral loads becomes complex due to explicit strength, time period and stiffness discontinuities resulting from the use of different structural systems. To evaluate the seismic damage of hybrid building, a 6 story sample building with its 4 bottom stories reinforced concrete frame and top two stories of steel frame was selected. The Non-linear static analysis was carried out with a maximum drift of 2.5%. The results of analysis are compared in terms of plastic Hinge formation, Capacity Curve, Capacity Spectrum and Lateral deformations. The hinge results shows that in Non-linear static analysis the hinges formation in Zone 2B is at Life Safety level and in Zone3 it was found at the collapse level. From the capacity curve the un-retrofitted building found deficient in stiffness and observed large displacements. After retrofitting with Shear wall and Steel Braces the hinges found at Operational Level for zone 2B whereas for zone 3 hinges were found at immediate occupancy level. The results are also compared in term of capacity curves and lateral displacement which shows that after retrofitted with shear wall and steel bracing the lateral capacity of the hybrid building increases 47% and 42% respectively. Whereas the lateral displacement of the hybrid building was reduces 37% and 42% after retrofitted with shear walls and steel bracings respectively.