Journal of Constructional Steel Research 65 (2009) 452–465 www.elsevier.com/locate/jcsr Bracing systems for seismic retrofitting of steel frames L. Di Sarno a,∗ , A.S. Elnashai b a Department of Engineering, University of Sannio, Benevento, Italy b Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, USA Received 14 August 2007; accepted 15 February 2008 Abstract The present study assesses the seismic performance of steel moment resisting frames (MRFs) retrofitted with different bracing systems. Three structural configurations were utilized: special concentrically braces (SCBFs), buckling-restrained braces (BRBFs) and mega-braces (MBFs). A 9-storey steel perimeter MRF was designed with lateral stiffness insufficient to satisfy code drift limitations in zones with high seismic hazard. The frame was then retrofitted with SCBFs, BRBFs and MBFs. Inelastic time-history analyses were carried out to assess the structural performance under earthquake ground motions. Local (member rotations) and global (interstorey and roof drifts) deformations were employed to compare the inelastic response of the retrofitted frames. It is shown that MBFs are the most cost-effective bracing systems. Maximum storey drifts of MBFs are 70% lower than MRFs and about 50% lower than SCBFs. The lateral drift reductions are, however, function of the characteristics of earthquake ground motions, especially frequency content. Configurations with buckling-restrained mega-braces possess seismic performance marginally superior to MBFs despite their greater weight. The amount of steel for structural elements and their connections in configurations with mega-braces is 20% lower than in SCBFs. This reduces the cost of construction and renders MBFs attractive for seismic retrofitting applications. c  2008 Elsevier Ltd. All rights reserved. Keywords: Bracing; Buckling restrained braces; Steel frames; Concentrically braced frames; Moment resisting frames; Ductility; Seismic retrofitting; Performance assessment; Time history analyses 1. Introduction Damage experienced during past earthquakes worldwide demonstrates that steel multi-storey building structures gener- ally exhibit adequate seismic response (e.g. [1]). This is due to the favourable mass-to-stiffness ratio of base metal and the enhanced energy absorption of structural ductile systems em- ployed. Nonetheless, relatively recent earthquakes, e.g. those in the 1994 Northridge (California), 1995 Kobe (Japan) and 1999 Chi-Chi (Taiwan), have shown that poor detailing of connec- tions (e.g. beam-to-column, brace-to-beam, brace-to-column and column-to-base) and buckling of diagonal braces can un- dermine the seismic performance of the structure as whole (see, for example, [2–6]). Fig. 1 shows the distribution of damage level and the damage to structural members and connections with respect to structural type as surveyed in the aftermath of the 1995 Hyogoken-Nanbu (Kobe) earthquake [7]. Damaged ∗ Corresponding address: Department of Engineering, University of Sannio, Piazza Roma, 21, 82100Benevento, Italy. Tel.: +39 0824305566; fax: +39 (0)824 325246. E-mail address: disarno@unina.it (L. Di Sarno). buildings are classified as having unbraced (UFs) or braced (BFs) frames. Thus, considering the two principal framing ori- entations of a building, the surveyed structures include the following designations: UF-UF (unbraced frames in two hori- zontal directions), UB-BF (unbraced frames in one horizontal direction and braced frames in the other direction), and BF- BF (braced frames in both horizontal directions). Beams con- sisted almost exclusively of wide-flange sections, either rolled or built-up. For columns, wide-flange (H) sections were used most extensively; square-tube (S) sections were also utilized in some structural systems. Considering the 988 damaged steel buildings, 432 (43.7%) are UF-UF, 134 (13.6%) are UF-BF and 34 (3.4%) are BF-BF, with 388 (39.3%) having uniden- tified framing systems. These statistics indicate that the ma- jority of damaged buildings had unbraced moment resisting frames (MRFs) as earthquake-resistant system. Fig. 1 also dis- plays the location of damage, namely columns, beams, beam- to-column connections, braces and column bases, as a function of frame type. Major observations from the collected data are as follows [8]: (i) columns in UFs suffered the most damage relative to other frame elements (in terms of the number of 0143-974X/$ - see front matter c  2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2008.02.013