Evaluation of steel moment frame seismic behavior With concentric brace associated by initial looseness

Although, concentric braces have proper stiffness, but ductility and energy absorption of these are low and under pressure and lateral forces exhibit poor performance and also structural ductility will reduce due to buckling when these braces are used for retrofitting of structural frames.Several methods have been proposed for improving of concentric braces performance, especially increasing energy dissipation capacity. The uses of connections with especially performance such as connections with the initial loosening at the end of concentric braces are among these methods.The main objectives of this paper are comparison of the seismic behavior of concentric brace frames with and without initial looseness and calculation of optimal looseness besides the review of analytical studies conducted on no friction brace looseness.For this purpose, the braced frames with and without initial looseness have been modeled and studied using nonlinear static and time history analysis and then the behavior coefficient was calculated.The studied models in this research are concentric moment frames with and without initial looseness. As briefly, the method which is used in this research based on the principle that the moment frames resist against the seismic lateral load at first and then (if it is necessary) braces are activated for carrying lateral load when the lateral displacement is increased and exceeded from the certain level. To achieve this goal, at the junction of the moment frame and brace initial looseness with a certain amount is applied.After reaching to the desired displacement in frame, the bolts at the junction of the moment frame and brace reached to the end of slotted joint and brace will participate with moment frame in bearing lateral forces. In this case the amount of lateral force that can be tolerated by the concentric moment frame with initial looseness is significantly more than the brace without loosening.Based on the results obtained in this investigation the use of loosening in concentric brace frame connection, in addition to reduction of operational complexity of friction dampers, improves seismic performance and also increases seismic energy dissipation capacity of frames which were studied. Also behavior coefficient obtained for this type of frames is greater than behavior coefficient of conventional concentric brace frames.The study of desired frames in this paper indicated that the optimal value of looseness (L_b) should be in the range of 3/4 L≤L_(b )≤L to achieve maximum flexural capacity of moment frame and maximum axial resistance of brace simultaneously. Also, if the ratio of concentric brace stiffness to the stiffness of moment resistance frame is about 1.0, in the concentric moment frame with initial looseness, energy absorption and dissipation capacity is more than the other axial to moment stiffness ratios. In this state the value of load resistance and also behavior coefficient increase although the coefficient of ductility and reduction coefficient due to ductility don't follow any particular trend and are almost constant.It has been observed also that at the all studied models in this research, frames with looseness connection caused an average 17% energy dissipation more than frames without looseness connection.