عملکرد لرزه ای اتصال RBS آکاردئونی/ وب (آکاردئونی/ جان تیر) Seismic performance of the Accordion-Web RBS connection

۱٫ Introduction Reduced Beam Section (RBS) moment connection is one of the most economical and practical prequalified connections among the post-Northridge ones. RBS connections were developed on the basis of a new concept named ‘‘Weakening’’. By this concept, the flexural strength of the beam is reduced in the vicinity of the column face, compared to other nearby sections, by beam section reduction, and this forces a plastic hinge to be formed at the reduced location. The localization of the plastic hinge reduces the fracture vulnerability of beam-to-column complete joint penetration (CJP) welds, and therefore the connection inelastic deformation capacity is improved. A conventional type of RBS moment connection was developed by flange shaving and is known as a ‘‘Dogbone’’ connection; it was first developed during a research project. The project was sponsored by Arbed (a Luxembourg based steel producer) on the basis of Plumier’s idea [1] in order to provide energy dissipative zones for structures. However, the connection was not commonly used until SAC programs for reduction of earthquake hazards in steel moment frame structures. SAC has reported the results of 45 well-documented RBS connection tests, conducted after the Northridge earthquake [2]. The main variables in these tests were the shape of flange cut (straight cut, tapered cut or radius cut), depths of columns and beams and web connection. According to the results, the radius cut RBS connection showed successful behavior; the connection behavior depended on the beam depth; bolted and welded web connections had relatively similar behaviors. The SAC Phase II program, including 17 large-scale tests, was planned in order to improve the performance and reduce the economical costs of RBS connections [2]. The main achievement of these tests related to the assessment of beam instabilities such as web and flange local buckling and subsequent lateral–torsional buckling. Such instabilities cause degradation in the connection strength at large rotation demands. In addition to SAC programs, other investigations were followed on conventional RBS connection. Chi and Uang et al. [3,4] studied experimentally the cyclic behavior of RBS moment connections with deep wide-flange column sections and proposed a seismic design procedure. In similar research, the seismic behavior of RBS moment connections to deep wide-flange columns was studied by Zhang and Ricles [5,6] with regard to the composite floor slab effect. The effect of panel zone strength and beam web connection method on the seismic performance of RBS connections was widely investigated by Lee et al. [7]. These results showed poor behavior of a bolted web connection compared to a welded one. Meanwhile, a criterion was proposed to achieve a balance panel zone that increases the plastic hinge rotation capacity. Nakashima [8] et al. studied analytically and numerically the lateral–torsional instability and lateral bracing effects of wide-flange steel beams subjected to cyclic loading. Furthermore, a statistical study, addressing the cyclic instability of steel moment connections with reduced beam section, was presented by Uang and Fan [9]. Shen et al. [10], Kitjasateanphun et al. [11] and Jin and El-Tawil [12] conducted analytical studies on the seismic performance of steel moment-resisting frames including RBS connections. The results confirmed that RBS frames can economically provide proper seismic performance in regions of high seismic risk. Other than ‘‘Dogbone’’ connections, a few reducing details have been proposed for RBS connections, some of which are as follows: 1. Drilling a set of holes on the beam flanges to reduce the flange contribution to the beam moment capacity [1,13]. 2. Making large holes in the beam web to eliminate the web contribution in the moment. The detail was named Reduced Web Connection [14]. 3. Reducing the beam web depth in a limited region to decrease the distance between flanges and, consequently, reduce the beam flexural strength. The formed connection was named a wedge detail [15]. Scientifically parallel to the above-mentioned innovative RBS connections, a new RBS connection is presented for steel momentresisting frames in this paper. In the proposed connection, the beam section is reduced using corrugated plates instead of a flat web at the expected location of the beam’s plastic hinge in the vicinity of the beam-to-column connection. Here, a design procedure is developed for the proposed connection based on the expected behavior of a plastic hinge with a corrugated web as well as general recommendations of AISC seismic provision [16]. Furthermore, an experimental program is conducted on two relatively identical designed specimens in order to investigate the seismic behavior and cyclic response of the new proposed connection. In this regard, the connection is studied analytically to verify the performance of the proposed connection and for profound insight into its seismic behavior.