Performance Based Elastic Design of Braced Frame

Abstract

When building is designed for seismic resistance using elastic analysis, most of the build- ings experiences significant inelastic deformations during large earthquakes. Various per- formance based design methods require ways to determine the behavior of structures during earthquakes. Non-linear analysis plays an important role in the design of new as ell as existing building. It is also necessary to evaluate structural response beyond elastic limit including strength and stiffness deterioration. Nonlinear analysis involves significantly more efforts than linear static analysis. Performance Based Design gained wide acceptance in engineering community. It provides very good framework to assess the building performance and potential hazard it is likely to experience. As a structural material, steel has many advantages like high strength, ductility, uni- formity, toughness. Steel moment resisting frame has very low elastic stiffness and it experiences higher storey displacement. To increase the stiffness and decrease the storey displacement bracings are provided. Performance based design of special concentric braced frame is carried out along with unbraced frame according to ATC 40. The main objective of the study is to overview the effect of different types of bracings and different types of load patterns. Initially, performance based design is carried out of G+3, G+7 and G+11 2D cross, inverted V and V braced frame. The results of 2D braced frame shows that inverted V bracing has higher inelastic energy dissipation capacity as compare to cross and V type bracing. Three different configurations of inverted V bracing for G+3, G+7 and G+11 has been taken for the performance based design. The load patterns like inverted triangular load pattern, uniform load pattern and parabolic load pattern has been taken for pushover analysis. Allowable inter-storey drift ratio and allowable plastic hinge rotation given in FEMA 273 has been used to evaluate the performance of special moment resisting frame and special concentric braced frame. Uniform load pattern shows higher base shear and lower displacement at performance point. For uniform load pattern damage is observed at lower storeys while for parabolic and inverted triangular load pattern, damage is observed at lower as well as top storeys. The capacity curve pattern depends upon the X, where X is the power of h (hX). As the value of X is increases, resultant shift towards upward and capacity of structure will be reduced due to higher number of hinge formation. It has been noted that, as the height of structure increases, rotation of plastic hinge will be increased. For G+3, performance of structure is at operational level and for G+11, it reaches up to IO-LS. Energy Dissipation capacity of structure is compared for each inverted V Bracing Configuration. Damage index has been computed for each load pattern and for each bracing.