Seismic Evaluation of Dual Steel Frames with Brace Buckling Consideration

This paper investigates the seismic behavior of short dual systems–integrated moment and braced frames-considering the braced frame failure. Modeling of beams, columns, braces, and probable types of braces buckling are discussed in detail. Multi-story 3-bay moment frames were designed according to AISC-ASD 89. To ensure the shear performance of dual frames, moment, and shear displacement of frames are calculated and compared. To evaluate shear displacement of dual frames, shear displacement of moment, and braced frames are calculated separately and added together. These frames can be considered as a cantilever beam so moment displacement of dual frames is dependent on column sections. Therefore, the moment-area theory was used to determine the moment displacement. Buckling of the X-braces can be occurred in two modes depending on the lateral constraint of middle connection that is supplied by moment stiffness of tension braces and middle connections. When moment stiffness of tension brace is considerable and middle connection of X-braces has adequate rigidity, the buckling of braces occurs in second mode. Otherwise first mode overcomes the brace buckling. In this study, the results are obtained by the consumption of adequate moment stiffness of tension braces and middle connection rigidity. In order to consider the stiffness degradation that is caused by gravity load, P-Δ effect, corotational theory is used and pushover curves are compared considering the ultimate and residual strengths. Braced frames buckling includes elastic and inelastic depending on the slenderness ratios. Euler and Belayesh relations were used to evaluate critical strength of braces in elastic and plastic regions respectively. Nonlinear static analysis was conducted on the frames considering braces failure using OPENSEES platform with displacement control pushover analyses on all designed frames. In other words, according to consumption of rigid diaphragm of all stories, one of the roof nodes was pushed up to 3% of the frame height. The load pattern of pushover analysis was chosen adverse triangular. The results indicate that brace failures occur at 2% drift of corresponding stories. In this paper during the pushover analyses, 16, 64, 144, and 1024 pushover analyses were done on 2-, 3-, 4-, and 5-story dual frames subsequently. Pushover curves corresponding with the same story braces failure in each frame are averaged. The results of pushover analyses on short dual frames indicate that the best performance of frames, considering the ultimate and residual strengths, are obtained with brace failures occurring in higher stories.