۱- Introduction

۲- Case-study EBF buildings

۳- Seismic sequences

۴- Results

۵- Summary and conclusions

References

Abstract

The goal of this paper is to examine the seismic response of eccentrically braced frames (EBFs) under artificial narrow-band mainshock-aftershock sequences by means of detailed analytical models representative of buildings designed under the Mexico City Code criteria. These analytical models take into account the nonlinear behavior of the links including a failure criterion. Relevant results for engineering practice showed that strong aftershocks could significantly increase interstory drift demands once the link fails, while surrounding members (adjacent beams, columns) behave nonlinearly, which is opposite to the design philosophy. In addition, it was noted the nonuniform distribution of hysteretic energy along-height of the links, which do not take fully advantage of the energy dissipating capacity of the shear links.

Introduction

Eccentrically Braced Frames (EBF) have become an attractive earthquake-resistant structural system in many countries worldwide since it provides high levels of both elastic stiffness (similar to concentrically braced frames) and ductility (similar to moment-resisting frames). In EBFs, the seismic energy induced to the building during earthquake loading is dissipated through the inelastic behavior of the links, while the remaining elements (beams, columns, and braces) are expected to behave elastically. Currently, the design procedure for EBF is prescribed in the 2016 AISC Seismic Provisions for Structural Steel Buildings [1], which specifies the link design, link rotation limits, and link overstrength factors, among other issues. Particularly, the link rotation is limited to 0.08 rad for links behaving in shear (i.e. for links with length equal or smaller than 1.6Mp/Vp, where Mp and Vp are the plastic bending moment and the plastic shear strength of the link). A comprehensive review of relevant experimental and analytical research carried out on steel eccentrically braced frames is presented in Ref. [2]. The first reported worldwide failure in EBFs was observed in St. Asaph Street parking structure in the city of Christchurch as a consequence of the February 22, 2011 (Mw = 6.3) earthquake that struck the Canterbury region in New Zealand. A detailed forensic examination revealed that three main factors led to the unsatisfactory performance of this structure [3,4]: a) the intensity of the ground shaking (several times the intensity that was expected during a design-level event), b) the frame geometry, which severely amplified the imposed seismic demands, and c) observed fracture in the links from an erection (fit-up) error, since the link stiffener was not located (as specified) directly above the brace flange, producing a severe strain concentration. Although not examined in the aforementioned study, it should be noted that the February 22, 2011 seismic event was part of a sequence of strong earthquakes that hit the New Zealand’s South Island that began with the September 3, 2010 (Mw = 7.0) Canterbury earthquake. Therefore, this lesson motivates examination of the behavior of EBFs in seismic regions under strong earthquakes (mainshock) and, in general, under seismic sequences.