Inelastic Performance and Economical Assessment of Concentrically Braced Steel Frames by Nonlinear Static (Pushover) Analysis

ABSTRACT: Concentric bracing system is one of the most economical systems being used to provide lateral stability for steel structures during earthquake by inelastic behavior. Although inelastic response of structures is affected by their height and structural system, these issues are not considered for the design of concentrically braced frames (CBFs) in the current design codes. The previous research work on the economical comparison of steel bracing systems has compared their elastic response only, regardless of their plastic range. This work is aimed to study the inelastic behaviors and compare the weights of different CBFs (X-, V-, Inverted V- and Diagonal braced frames) in order to supply comprehensive information for design procedures. Inelastic responses of the 4-, 8- and 12-story X-, V-, Inverted V- and Diagonal braced frames were assessed by the nonlinear static (pushover) analysis mainly based on FEMA 440 (2005). A new methodology was proposed for the economical comparison of the frames (subtracting the weight of a benchmark frame from the frame weights to calculate the pure bracing system weight) to overcome the inaccuracy of the procedures being used by the previous studies (using the total frame weight instead of the bracing system weight). By conducting pushover analysis, it was found that the failure progress of all the frames was mainly due to the buckling of compression bracing members, but with some differences due to story height and frame type. Diagonal, Inverted V-, X- and V-braced frames generally have the highest to the lowest initial, elastic and postiv yield stiffness respectively. The changes in nonlinear responses of the frames due to the changes in the story height follow special and predictable rules and are generally has less effective on the results than the frame type. V-braced frame was found to have the highest target displacement point. V-, Inverted V-, X- and Diagonal braced frames were found to be in order the lightest to the heaviest systems. The available economical comparison methodology for the bracing system was found to seriously undermine the differences among the results of the comparison whilst the methodology proposed in this work was observed to give more reliable results. By estimating the energy dissipation per weight of all frames from the obtained results, it was observed that Inverted V-bracing system is the most efficient type for 4-story frames; V-bracing system is the most efficient for 12-story frames; X- and Diagonal bracing systems are the third and fourth efficient bracing systems respectively. ……………………………………………………………………………………………………………………………………………………………………………………………………………………