Impact of recent research findings on eccentrically braced frame design

Abstract

Recent experimental work performed at the University of Texas at Austin and analytical work performed at the University of California, San Diego address EBF design issues. From these studies, the following conclusions can be made: Failure of short links constructed of A992 steel and designed according to current provisions is likely dominated by web fractures that initiate at the stiffener weld termination points. Short link failures due to severe web buckling, as observed in past tests, was not observed in recent tests. Despite the change in failure mode, the short links were found to satisfy the inelastic rotation requirements. Link rotation capacity can be improved by terminating welds connecting intermediate stiffeners to the link web at a distance of five times the web thickness away from the k-line of the link section. Commentary in the 2005 AISC Seismic Provisions recommends avoiding welding in the fc-area. The experimental and analytical studies reported herein as well as results from previous tests consistently support that the flange width-thickness limit of 0.38 √E/Fy (compactness limit) is adequate to control flange buckling in short links. However, it is not clear whether the relaxed flange width-thickness limit is justified for intermediate and long links. The 2005 AISC Seismic Provisions permit the use of the compactness limit for short links. Intermediate links are susceptible to strength degradation associated with combined flange and web buckling. Link rotation capacity is dependent on the loading protocol used for testing. The EBF protocol in the 1997 and 2002 AISC Seismic Provisions was overly conservative. The revised protocol developed in this program is more reasonable to represent demands arising from actual earthquake ground motions. The link loading protocol in the 2005 AISC Seismic Provisions is based on the revised protocol. ASTM A992 links have similar overstrength to A36 links tested in the past. The flanges of links with e/(Mp/Vp) < 1 may carry shear forces that should be accounted for in computing ultimate shear strength. Further details of the studies reported herein are provided in the references and other forthcoming publications.