Performance-based plastic design of special truss moment frames

T he special truss moment frame (STMF) is a relatively new type of steel framing system suitable for high seismic areas. The frames dissipate earthquake energy through ductile special segments located near the mid-span of truss girders. STMFs generally have higher structural redundancy compared to other systems because four plastic hinges can form in the chords of one truss girder. The redundancy can be further enhanced if web members are used in the special segments. Simple connection details are needed for the girder-to-column connections. One other advantage of using STMF systems is that the truss girders can be used over longer spans, and greater overall structural stiffness can be achieved by using deeper girders. In addition, the open webs can easily accommodate mechanical and electrical ductwork. As a consequence, this system is gaining popularity in the United States, especially for hospital and commercial buildings. Research work carried out at the University of Michigan led to the formulation of design code provisions (Goel and Itani, 1994; Basha and Goel, 1995; AISC, 2005a). Current design practice generally follows elastic analysis procedures to proportion the frame members. Therefore, it is possible that story drifts and yielding in the special segments may not be uniformly distributed along the height of the structure and may be concentrated in a few fl oors causing excessive inelastic deformations at those levels. Thus, the intended deformation limits and yield mechanism may not be achieved when an STMF is subjected to strong earthquakes. In recent years, seismic design has been gradually moving toward a more direct performance-based design approach, which is intended to produce structures with predictable and controlled seismic performance. To achieve this goal, knowledge of the ultimate structural behavior, such as nonlinear relationships between forces and deformations, and the yield mechanism of structural systems are essential. Therefore, the desired global yield mechanism needs to be built into the design process. In current practice, the performance-based seismic design for a new structure is carried out in a somewhat indirect manner. It usually starts with an initial design according to conventional elastic design procedures using applicable design codes, followed by a nonlinear static pushover assessment (ATC, 1996; ASCE, 2000). Usually, an iterative process between design and assessment follows. Moreover, as mentioned in FEMA 440 (ATC, 2004), this procedure still has diffi culty in predicting reasonably accurate structural behavior during a major earthquake when compared with the results from nonlinear response history analyses. While further refi nement is needed in current practice to move toward more reliable performance-based design methodologies, this paper presents a direct performance-based design approach that basically requires no assessment such as nonlinear static (pushover) or dynamic analysis after design. Based on an energy (work) concept (Leelataviwat, Goel and Stojadinović, 1999; Lee, Goel and Chao, 2004; Chao and Goel, 2005), the proposed approach determines the design base shear by using the code-specifi ed elastic design spectral value for a given hazard level, a preselected global structural yield mechanism, and a target drift. In addition, the design lateral force distribution employed in the proposed method is based on nonlinear dynamic analysis results using a large number of ground motions (Somerville, Smith, Punyamurthula and Sun, 1997). The chord members in the special segments are designed according to the plastic design method (Chao and Goel, 2006), while the members outside the special segments are designed by using a capacity design approach. A complete detailed design procedure was developed and the design steps are summarized in a fl owchart in Figures 10 and 11, which could be easily applied to develop a computer program. Shih-Ho Chao is assistant professor, department of civil and environmental engineering, University of Texas, Arlington, TX.