A Modified Equation for Expected Maximum Shear Strength of the Special Segment for Design of Special Truss Moment Frames

Chao, Shih-Ho; Goel, Subhash C. (2008). "A Modified Equation for Expected Maximum Shear Strength of the Special Segment for Design of Special Truss Moment Frames," Engineering Journal, American Institute of Steel Construction, Vol. 45, pp. 117-125.

Special truss moment frame (STMF) is a relatively new type of steel structural system that was developed for resisting forces and deformations induced by severe earthquake ground motions. The system dissipates earthquake energy through ductile special segments located near the mid-span of the truss girders. The other elements outside the special segments, such as truss members, girder-to-column connections, and columns, are designed based on the expected vertical shear strength ( ) of the special segment and are expected to remain elastic during a major earthquake. As a consequence, overestimation of in the special segment can result in significant over-design of the elements outside the special segment. This study shows that the equation for expected shear strength in the current AISC Seismic Provisions can be quite conservative, thereby leading to considerable over-design of members outside the special segment. Based on more realistic assumptions, a modified expression for is proposed in this paper, which results in a better estimation of the expected shear strength while maintaining an adequate safety margin. The proposed expression was validated by using previous experimental results as well as nonlinear static and dynamic analyses (to determine the seismic demand in the special segment). A design equation of for STMF using multiple Vierendeel panels in the special segment is also proposed.