Contents lists available at ScienceDirect Thin-Walled Structures journal homepage: www.elsevier.com/locate/tws Full length article Experimental and numerical investigation of FRP-confined SHS brace members under cyclic loading Parisa Shadan, Mohammad Zaman Kabir ⁎ Department of Civil Engineering, Amirkabir University of Technology, #424 Hafez Avenue, Tehran, Iran ARTICLE INFO Keywords: Bracing member Cyclic loading test FRP strengthening Square Hollow Section (SHS) Energy dissipation capacity Finite element analysis ABSTRACT In this paper, the enhanced resistance of Steel Hollow Section, SHS, braces, strengthened using Fiber Reinforced Plastics, FRP, was investigated. In this direction, a number of braces with two different slenderness ratios was reinforced with externally over-wrapped CFRP sheets and tested under cyclic loading. It is shown that the adopted strengthening method can preclude the local buckling which yields to improve ductility and energy dissipation capabilities of braces. The proposed technique also minimizes brace strength reduction during the load cycles. Furthermore, in the current work, a numerical simulation was generated in conjunction with da- mage criteria of materials. The validity of numerical analysis was accurately verified by the experimental results. 1. Introduction Concentric Braced Frame (CBF) is extensively used as a lateral load resisting system. Nevertheless, during intense ground motions the for- mation of local buckling at mid-length of the brace, can lead to the fracture [1,2]. This phenomenon is more popular among square hollow section (SHS) braces, however, the use of steel hollow sections in concentrically braced frames has attracted considerable interest, mainly due to their high radius of gyration [3,4]. As indicated by the previous studies, for instance, increasing the width to the thickness ratio with preventing local buckling can improve the fracture life of brace [5–7]. So, the adopted strategy for precluding local buckling by the seismic provisions such as AISC 341-05 [8] and Iranian National Building Code (Part 10) [9] is to limit width-to-thickness ratio of braces. Additionally, using innovative sections with more resistance against local buckling was suggested and examined by some researchers [10–12]. As a result of numerous researches on local buckling prevention [13,14], the limits for width-to-thickness ratio has been restricted over the time [15]. Meanwhile, a huge number of the existing buildings were designed based on an outdated version of provisions. Then, the defi- ciency of compactness should be amended for them [16]. One method for strengthening the stability of steel members is welding or bolting steel plates to the wall of members. However, in- flexibility, an increase in dead load and residual stresses caused by welding are some of the problems that threaten the strengthened ele- ments [17]. Recently, fiber reinforced polymer (FRP) is widely em- ployed in strengthening of steel structures [18]. The most considerable advantages of FRP composites are a high strength-to-weight ratio, good corrosion resistance and absolute shape flexibility [17,19,20]. The influence of strengthening with FRP on the stability of thin- walled sections was investigated to some extent. The first studies on the use of FRP for stability strengthening were buckling improvement of plastic hinges [21,22]. Beams with slender section was also strength- ened with FRP and results showed that FRP composites are thoroughly capable of precluding local buckling incidence [23,24]. Shaat and Fam [25,26] reported the growth in column strength and stiffness after ap- plying FRP laminates. They suggested the use of unidirectional FRP sheets in the longitudinal direction for long columns and in the trans- verse direction for short columns as an efficient strengthening strategy for them. The effectiveness of FRP over wrapped in the short column was also established in other studies [27,28]. Kabir and Nazari [29] expressed that the buckling strength of cracked tubes increased when FRP patch was applied to cracked areas. FRP wrapping was also em- ployed to circular steel tubes and it was observed that FRP application can prevent elephant's foot buckling, as well [30,31]. However, few studies have focused on FRP strengthening of braces. In two different studies, Harries et al. [32,33] employed FRP plates to the flanges of T-section braces and results indicated that the FRP can control local buckling of flanges before debonding. In a study con- ducted by Gao et al. [34], applying FRP layers in longitudinal direction could increase the axial strength of long braces. Despite a number of studies on this issue, there is a lack of research on the effectiveness of FRP strengthening of SHS performance as a brace member. In this study, the hollow SHS sections were confined by FRP composite sheets. Here, with the aim of preventing local buckling at SHS braces, implementing of FRP over-wrapped in the transverse https://doi.org/10.1016/j.tws.2018.04.021 Received 1 December 2017; Received in revised form 3 April 2018; Accepted 30 April 2018 ⁎ Corresponding author. E-mail addresses: p_shadan@aut.ac.ir (P. Shadan), mzkabir@aut.ac.ir (M.Z. Kabir). Thin-Walled Structures 130 (2018) 132–147 0263-8231/ © 2018 Elsevier Ltd. All rights reserved. T