Effect of Connection Conditions of Inverted V-braces on the Seismic Behavior of Steel Frames

Concentric braced steel frames are commonly used in steel structures to resist the lateral forces especially in structures that are constructed in high seismic zones. Common assumption for the braces connection is pinned, but in practice, the connection of the braces may have partial rigidity, with regard to the details of the connection. Besides, the buckling load of the braces is affected by effective length factor that is affected by the conditions of the end connections. Therefore, changing the condition of the brace connection affects on the buckling load of brace and consequently on the behavior of the frame. In this paper, the behavior of two-dimensional inverted V-braced steel frames has been investigated with different number of stories (3, 5 and 10 stories) and bays (3 and 5 bays), which are different in connection conditions of braces. The lateral resistance of the frames is provided only by bracing system. The end connections of braces have been modeled in either pinned or rigid extreme states with proportional effective length factors. The frames were designed due to gravity and lateral loads according to the specifications of American Institute of Steel Construction (AISC), Allowable Stress Design (ASD), so that there was not any over-strength in the elements of the models. After designing, P Delta effects were considered and plastic hinges were assigned to the members to allow nonlinear behavior of frames according to the specifications of FEMA273. For this purpose, P plastic hinges were assigned to the braces with pinned connections, P-M plastic hinges were assigned to the columns and braces with rigid connections, and M plastic hinges were assigned to the beams. Initial static analyses of the frames were performed by the SAP2000 software. Then nonlinear static and nonlinear time-history analyses were done on the frames subjected to different strong ground motions to reveal the effects of the brace connections and effective length factor of the brace members on the seismic behavior of the steel frames. Triangular lateral loading pattern was applied to the frames for nonlinear static analyses. The results of the linear static analyses show that changing the end connections of the inverted V-braced frames has not considerable effects on the fundamental period, lateral stiffness and internal forces of the members of the frames. Moreover, the results of the nonlinear static analyses of the frames show that changing the brace connections does not affect on the yield point and shear capacity of the frames; but the effective length factor of the braces greatly affects on the behavior of the frames. The frames with lower effective length factors of braces have higher yield point and shear capacity than the other frames with greater effective length factors. Relative differences are about 8% to 67%. Similar results were obtained from nonlinear time-history analyses. According to these results, when the elements remain linear, differences in the response of models with different conditions for brace connections are negligible; whereas under some earthquakes that several plastic hinges were generated in the members, the differences in maximum base shear and roof displacement of the models are considerable.