New connection between reinforced concrete building frames and concentric braces: Shaking table tests A. Benavent-Climent a,⇑ , E. Oliver-Saiz b , J. Donaire-Avila b a Department of Mechanical Engineering, Polytechnic University of Madrid, 28006 Madrid, Spain b Department of Structural Mechanics, University of Granada, 18071 Granada, Spain article info Article history: Received 2 December 2014 Revised 8 March 2015 Accepted 9 March 2015 Keywords: Concentric brace Upgrade Brace–frame connection Shear-key Anchor bolt abstract Concentric steel braces and brace-type dampers are often applied to the upgrading of reinforced concrete frames subjected to lateral loads. These braces may develop high axial tension forces, and transferring them appropriately to the existing beam–column joints is a key challenge. This paper investigates a solution for connecting the end-plate of the steel brace with the frame, using (1) shear-key plates fixed to the concrete with anchor bolts, and (2) a low friction material inserted between the end-plates and the shear-key plates. The presence of the low friction material impedes the development of tension forces in the anchor bolts and ensures that they are basically subjected to shear forces. This prevents brittle types of failure (concrete cone failure, pull-out/pry-out failure), and results in a reduction of the number of anchors required as well as anchorage height. The efficiency and validity of the proposed brace–frame connection is investigated experimentally by means of shaking table tests conducted on a 3  3  3m 3 scaled reinforced concrete frame retrofitted with brace-type hysteretic dampers. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction A high proportion of the building stock located in earthquake- prone regions—including the Mediterranean area—was designed before the appearance of seismic codes or in view of rudimentary anti-seismic design criteria [1]. Recent earthquakes (L’Aquila 2009, Lorca 2011) have revealed the poor performance of this type of under-designed building, accentuating the need for seismic assessment and retrofitting [2]. Many damaged buildings had rein- forced concrete frames as the main system of lateral resistance; in such cases retrofitting may involve installing concentric steel braces, either ordinary ones or brace-type dampers. The former solution is very common, but the latter provides for better seismic performance. A number of brace-type dampers are commercially available or under development. Among them, the so-called ‘‘hysteretic’’ dampers are particularly popular because of their low cost in comparison with viscous fluid dampers or viscoelastic solid dampers. In the past two decades, the use of brace-type hys- teretic dampers for the seismic upgrading of existing frames has increased exponentially. When retrofitting reinforced concrete frames with concentric steel braces, the connection between the ends of the steel braces and the existing frame (briefly referred to as ‘‘brace–frame connection’’ herein) is a key challenge [3]. The steel brace may develop high axial loads, and its influence on a possibly damaged frame is a matter of major concern. Several solutions have been proposed for the brace–frame con- nection in the past. The simplest one consists of using steel anchors to connect the end-plate of the brace directly to the concrete, as shown in Fig. 1a. The anchors are thereby subjected to shear V i and tension forces N i . A second proposal [4,5] is to use shear-key plates to fasten the end-plate of the brace, as shown schematically in Fig. 1b. The shear-key plate is adhered to the surface of the con- crete with epoxy resin and fixed with anchor bolts. In this second solution, there is a direct metal-to-metal contact between the end- plate of the brace and the shear-key plate. When the brace is in tension, the direct metal-to-metal contact induces forces perpen- dicular to the plane of the shear-key plate (denoted by V c in Fig. 1b) that tend to uplift and detach it from the concrete surface. Because these contact forces also induce large tension forces N i in the anchors and bending moments in the shear-key plates, there is a need for thicker plates or the addition of stiffeners. Sustaining high tension forces with bolts anchored in the concrete calls for considerable anchoring height and a greater number of anchors. Hence, this solution can prove costly or technically unfeasible. In addition, the typical failure modes exhibited by anchor bolts subjected to tension loads (concrete cone failure, pull-out/ pry-out failure) are brittle. http://dx.doi.org/10.1016/j.engstruct.2015.03.023 0141-0296/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel./fax: +34 91 3363135. E-mail address: amadeo.benavent@upm.es (A. Benavent-Climent). Engineering Structures 96 (2015) 7–21 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct