رفتار چرخه ای از قابهای مهاربندی شده با مهاربندی مرکب بخش های مجزا Cyclic behavior of concentrically braced frames with built-up braces composed of channel sections

1. Introduction Concentrically braced frames, CBFs, are frequently used as a structural system to resist earthquake loading. High strength and stiffness as well as economic consideration have attracted engineers to this structural system. Seismic behavior of this type of frames is affected by their configurations; brace properties such as slenderness, section geometry, compactness, and details of brace to gusset plate connections. In the past, different studies have been undertaken on this kind of frames, which can be classified in three distinct categories. In the first category, the seismic behavior of CBFs have been investigated with respect to their configurations, e.g. frames with diagonal, X, and chevron braces or those with super-X. In the second category, details of connection of brace to gusset plates have been the main parameters of interest. The third category of research has concentrated on the effects of properties of the brace section such as geometry, slenderness ratio, and width to thickness ratio on the seismic behavior of CBFs. In the following paragraphs, some of the researches falling in the second and third categories are reviewed briefly. With regard to the second research category, considerable attention has been given to rotation capability of brace end. Brace end rotation can be achieved by accommodating proper clearance length in the gusset plate. Figure 1 shows some variations of different clearance lengths in a gusset plate. As one of the first and major research in this area, (Astaneh-Asl1982, 1988 and Astaneh-Asl and Goel 1984) carried out some experimental full scale tests. In their study, 23 double angle braces, which were connected to gusset plates with different clearance lengths, were cyclically loaded. In these models, the linear clearance length of t, 2t and 4t were tested in which t is the gusset plate thickness. They proposed a 2t linear clearance length in order to achieve a ductile behavior of CBFs. This recommendation has been later stipulated in seismic provisions and it still exists in Seismic Provisions for Structural Steel Buildings (AISC, 2010a). Although complying with the mentioned detail can bring more ductile behavior; it usually leads to big and thick gusset plates. Some other researchers also examined the seismic behavior of concentrically braced frames focusing on connection details. Among these studies, extensive research has been done at University of Washington (Yoo et al., 2008; Leman et al., 2008; Lumpkin et al., 2010; Roeder et al., 2011). More than 30 frames have been tested experimentally. Based on their findings, the common procedures stipulated in (AISC, 2010a) have some shortcomings. To address these shortcomings, three suggestions were made. First, it was suggested that balanced design procedures (BDP) be used to design connections. BDP is based on balancing the damage mechanism of desirable yielding in order to avoid undesirable damage modes. It is obtained through a hierarchical design system of yielding and failure. Second, it was recommended that an elliptical 6t to 8t clearance length be used instead of a linear 2t one. Third, it was proposed that the welding of the interface of gusset to beams and columns be designed based on the plastic strength of gusset plate not that of the brace.