A balanced design procedure for special concentrically braced frame connections Charles W. Roeder ⁎, Eric J. Lumpkin, Dawn E. Lehman Department of Civil and Environmental Engineering, University of Washington, Seattle, WA 98195-2700, United States abstract article info Article history: Received 13 December 2010 Accepted 26 April 2011 Available online 14 June 2011 Keywords: Seismic design Braced frames Gusset plate connections Special concentrically braced frames Ductility Inelastic behavior Concentrically braced frames (CBFs) are stiff, strong structures that are suitable for resisting large lateral loads. Special CBFs (SCBF) are used for seismic design and are designed and detailed to sustain relatively large inelastic deformations without significant deterioration in resistance. Current AISC Seismic Design Provisions aim to ensure the brace sustains the required inelastic action, but recent research showed that current SCBF design requirements lead to variable seismic performance, unintended failure modes, and limited deformation capacity. To improve the seismic response of SCBFs, a balanced design procedure was proposed. The premise of the design methodology is to balance the primary yield mechanism, brace buckling and yielding, with other, complementary ductile yielding mechanisms, such as gusset plate yielding. This balance process maximizes ductile yielding in the frame thereby maximizing the drift capacity of the frame. Further, the undesirable failure modes are balanced with the yield mechanisms and the preferred failure mode, brace fracture, to ensure that the frame fails in the desired manner. To achieve the objectives of the design methodology namely maximum drift capacity, and adherence to a desired yield and failure hierarchy, rational resistance checks and appropriate balance factors (β factors) are used to balance each yield mechanism and failure mode. These factors were developed, validated, and refined using the measured results from an extensive test program. An SCBF connection design example to illustrate the application of the balanced design method and to demonstrate differences from the current AISC design method is presented in an appendix. © 2011 Elsevier Ltd. All rights reserved. Introduction Concentrically braced frames (CBFs) are stiff, strong structures, which are suitable for resisting wind and seismic loading. CBF typically are composed of diagonal bracing members connected to the beams and columns with gusset plate connection. The bracing may be placed in a number of different configurations and geometries. CBF members are initially designed assuming truss action, i.e. the members only carry axial load. Special concentrically braced frames (SCBF) are employed in high seismic regions and use a Response Modification Factor, R, to reduce the seismic design force. During large, infrequent earthquakes, SCBFs must sustain cyclic, inelastic tensile yielding and compressive buckling deformation of the brace without significant deterioration of stiffness and resistance for earthquakes exceeding the reduced design force. Brace buckling and tensile yield are the primary yield mechanisms of the system, which permit the frame to sustain the inelastic deformation and energy dissipation demands needed to provide collapse-prevention performance. Although the brace is the primary member in the SCBF system, the connections play an important role. Gusset plate connections typically connect the brace to the other framing members, as shown in Fig. 1, because they are easier to construct and design than fully restrained brace-end connections. Design of gusset plate connections to achieve the design objectives requires significant effort [1], and recent research has shown that they may not lead to the intended response. Experimental investigations into modified design methods have demonstrated small changes can improve the seismic performance of the system [2]. These improvements can be implemented in practice using a balanced-design procedure (BDP). Balanced design has been proposed as a method to control the sequence of yielding and to increase inelastic deformation capacity in past seismic research (e.g., [3]), but it has not been widely used in practice because of the complexity and uncertainty in achieving the required component balancing in practice. In part, this uncertainty stems from the limited experimental data available to verify the design method and associated expressions. Here, this limitation is overcome using a large data set on braced frame system that simulates modern construction. The resulting BDP is robust and available for immediate used by practitioners. This paper develops, presents, and justifies a simple BDP for SCBF gusset plate connections. The BDP identifies all possible yield mecha- nisms and failures modes associated with the SCBF system, and balances their resistances to optimize the seismic performance of the system [4]. The BDP adapts currently accepted design expressions and uses balance factors (β factors) for each yield mechanism and failure mode to assure that (1) the primary yield mechanism occurs and is followed by Journal of Constructional Steel Research 67 (2011) 1760–1772 ⁎ Corresponding author at: 233B More Hall, University of Washington, Seattle, WA 98195-2700, United States. Tel.: +1 206 543 6199; fax: +1 206 543 1543. E-mail address: croeder@u.washington.edu (C.W. Roeder). 0143-974X/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2011.04.016 Contents lists available at ScienceDirect Journal of Constructional Steel Research