Contents lists available at ScienceDirect Journal of Constructional Steel Research journal homepage: www.elsevier.com/locate/jcsr Three-segment steel brace for seismic design of concentrically braced frames Onur Seker a , Bulent Akbas b , Pinar Toru Seker a , Mahmoud Faytarouni a , Jay Shen a,⁎ , Mustafa Mahamid c a Department of Civil, Construction and Environmental Engineering, Iowa State University, USA b Department of Earthquake and Structural Engineering, Gebze Institute of Technology, Turkey c Department of Civil and Materials Engineering, University of Illinois at Chicago, USA ARTICLE INFO Keywords: Three-segment steel brace Seismic design Braced frame Non-linear time history analysis Hysteresis ABSTRACT A three-segment steel brace has been developed and investigated by means of numerical and experimental studies. The objective of the development was to develop a brace member that exhibits stable and symmetrical cyclic response under cyclic loading. The concept was conceived by extending a conventional elastic buckling of a column with variable sections to include post-buckling deformation. The concept was first examined using FEM-based simulations, and tested experimentally with an ensemble of small-scale brace specimens under cyclic loads. Seismic response of CBFs with conventional buckling braces and the three-segment braces were compared and results are discussed in terms of drift, brace and beam ductility demands. The results indicate that the tested three-segment braces specimens were capable of exhibiting stable and symmetrical hysteretic response, as well as dissipating a greater amount of energy compared to conventional buckling braces. Further, the dynamic analyses results point out that substituting the conventional buckling braces with the three-segment braces substantially mitigates the seismic demand on the braced frames. 1. Introduction Seismic response of a concentrically braced frame (CBF) is highly dependent on the inelastic cyclic behavior of its braces. Due to the degradation in compressive strength subsequent to global buckling, conventional buckling braces exhibit an unstable and unsymmetrical cyclic behavior when subjected to an earthquake ground motion ex- citation. The potential issues arising from the unsymmetrical hysteretic response due to the post-buckling behavior can be summarized as fol- lows: (1) Strength loss due to the post-buckling behavior substantially re- duces the overall energy dissipation capacity of a CBF. (2) Substantial difference between the tensile and compressive strengths [1] would impose significant demands on the brace-in- tersected girders [2,3], columns and beam-to-column connections [4] during a seismic event. (3) Isolated stories in a CBF that incorporates conventional buckling braces may be subjected to significant lateral stiffness and strength reduction due to the rapid stiffness and strength degradation in compression subsequent to global buckling. As a result of the non- uniform lateral stiffness and strength distribution along the building height, plastic deformations may accumulate in the relatively weak (or soft) stories as the demand increases [5]. The purpose of this study is to develop a steel brace member with conventional structural shapes that provides significant inelastic de- formation capacity primarily through its yielding in tension and com- pression. 2. Buckling of a non-prismatic column 2.1. General remarks Employing a uniform cross-section along the length (prismatic member) of an axially-loaded member (column member) might not be the most efficient way to resist compressive loads [6]. An investigation of a buckled simple column is given in Fig. 1. A compressed column member buckles globally when it reaches its critical load and begins to deform laterally (Fig. 1b). This lateral deformation induces second- order bending moments, which leads to plastic hinge formation at the mid-length of the brace. As indicated in Fig. 1(c), bending moment diagram for a buckled brace is not uniform, and thus the buckling load, as well as the hysteretic stability of a column member can be improved by increasing the steel material at the middle portion [6]. Ideally, such improvement can be achieved by utilizing a cross- http://dx.doi.org/10.1016/j.jcsr.2017.06.035 Received 1 March 2017; Received in revised form 23 June 2017; Accepted 26 June 2017 ⁎ Corresponding author. E-mail address: jshen@iastate.edu (J. Shen). Journal of Constructional Steel Research 137 (2017) 211–227 0143-974X/ © 2017 Elsevier Ltd. All rights reserved. MARK