Implications of Higher Mode Effects on Seismic Shear Demand of Eccentrically Braced Frames with Short Link Beam

Abstract

The capacity design philosophy primarily focuses on limiting the seismic shear force that will affect the structural system and has been widely accepted in the structural earthquake engineering community. However, several sources in the literature indicate that there are still some problematic points in this design philosophy. The main problem is that the proposed solution in this philosophy considers the first fundamental mode of the structural system but overlooks the effects of higher modes. This problem may lead to increased seismic shear force demand that is attempted to be limited by the capacity design philosophy. A similar problem with cantilever walls was first identified in the mid-1970s, and several solutions, which consider both dynamic effects and capacity design principles, were proposed in the relevant literature. Regarding Eccentrically Braced Frames (EBFs) with short link beam, no specific study on the dynamic amplification of shear force demand has been observed in the literature. However, other studies in different contexts have alluded to the possibility of such a phenomenon in EBFs and other steel frames. This study focuses on the dynamic amplification of shear force demands and determines its range using incremental dynamic analysis and multi-modal push-over analyses. Consequently, it has been demonstrated that limiting the shear force demand of the system, as recommended by the capacity design principles in all steel structure design specifications, is not possible. The use of a dynamic amplification factor is suggested to address non-ductile failure modes and enhance structural reliability. In this context, the current study examines the previously mentioned issue for eccentrically braced frames with short link beam in 4-story and 8-story buildings, representing low-rise and mid-rise buildings respectively. As a result, the seismic shear demands for EBFs found in Incremental Dynamic Analysis are significantly higher than those calculated by using capacity design principles for both 4-story and 8-story buildings. Additionally, the results from Incremental Dynamic Analysis have been comparatively examined with multi-modal push-over analyses. The internal force demands for elements of EBFs, especially in braces, increased due to the higher base shear demands, excluding the link beam.