Evaluating the influences of connection forms on the cyclic performance of RSFBs

Highlights

• The influences of connection forms on the cyclic performance of RSFB and deformation of support and BRB through pseudostatic tests and numerical simulation.

• •The changes in the BRB and support connection forms have little influence on the overall structural cyclic performance; however, these changes have larger influences on the BRB and support deformation and failure modes.

• •Enhancing the out-of-plane stiffness in the column-BRB joint generates a uniform deformation in the energy dissipating portion of the BRB core plate.

Abstract

A rocking steel frame with buckling-restrained braces (RSFB) with excellent deformation controlling and energy dissipating capacities is proposed to retrofit conventional steel frames. Notably, previous studies have shown that the seismic performance of structures with buckling-restrained braces (BRBs) was influenced by the connection forms of the BRBs within the structures. In this study, a series of pseudostatic tests were conducted on the RSFBs to investigate the influence of the BRB and support connection forms on the structural seismic performance. An out-of-plane deformation is observed in the transiting portion of the compressional BRB because of the failure of the out-of-plane restraints. However, the BRB on the tensional side works normally, and the RSFBs do not fully lose their carrying capacity. The rocking steel frames in all test specimens remain elastic and demonstrate uniform inter-story drifts. Finite element models of the test specimens with full out-of-plane restraints were established and simulated using ABAQUS software to complement the tests. The simulation results show that the BRB and support connection forms have little influence on the overall structural hysteresis response. However, the BRB connection forms affect the deformation of the BRB core plate and the out-of-plane deformation at the BRB ends due to different out-of-plane stiffnesses. Moreover, the supports with various connection forms exhibit different damage degrees and distributions.

Introduction

For conventional concentrically braced frames (CBFs), the damage concentration effect and soft story mechanism occur under horizontal earthquake loads due to the buckling of steel braces [[1], [2], [3]]. Some scholars retrofitted the CBFs by rocking steel frames (RSFs) to reduce the damage concentration effect of the CBFs [[4], [5], [6], [7]]. The RSFs control the lateral drift profile of the CBFs under horizontal earthquake loads through the approximately rigid rotation of the RSFs.

The rocking mode of existing RSFs can be divided into two types: (1) rotation around the bottom of side columns [8,9] and (2) rotation around the bottom of the central column [10,11,[13], [14], [15], [16]]. For the first type, the energy dissipating components can be set in the bottom of the side columns to dissipate energy, as shown in Fig. 1(a). However, in the first type, the bottom of the side columns may collide with the foundation at rocking, and this collision can be avoided by using the second rocking mode. Therefore, some scholars have investigated the seismic performance of RSFs with the second rocking mode. Blebo and Roke [10] proposed an RSF by removing the side columns in the first story and by using a pinned support to connect the central column and foundation, which rotates around the pinned support, as shown in Fig. 1(b). Takeuchi et al. [11] set buckling-restrained braces (BRBs) [12] at two sides of the first story in the RSF rotated around the bottom of the central column, and simulation results showed that the proposed system has excellent deformation controlling and energy dissipating capacities, as shown in Fig. 1(b). Blebo and David [13] used a similar method to that in Ref. [11] to set the BRBs in the RSF proposed in Ref. [10]; this method enhanced the structural energy dissipating capacity. Pang [14] conducted a series of experiments on RSFs with BRBs (RSFBs) and compared the seismic performance of the RSFB structures before and after replacing the BRBs. The experimental results showed that the damage in the RSFB was concentrated in the BRBs, while the RSF remained elastic. Jia et al. [15] noted that the collapse resistance capacity of a conventional steel frame was enhanced by adding an RSFB. Jiang et al. [16] validated the seismic performance of RSFBs through pseudostatic tests and simulations and proposed some suggestions to improve the out-of-plane stability of the RSFBs.

The abovementioned studies show that BRBs have been widely used in RSF systems. However, the experiments for the structures with BRBs showed that the BRB connections may have a significant influence on the structural and BRB seismic performance. Tsai et al. [17] conducted pseudostatic tests on buckling-restrained braced frames (BRBFs) with bolted BRBs, and the seismic performance of the test specimens cannot be fully used due to the buckling of the BRB gusset plates. To solve the problem in Ref. [17], Chou et al. [18] and Tsai and Hsiao [19] welded edge stiffeners in the BRB gusset plates to enhance the out-of-plane stability in the BRB ends. Fahnestock et al. [20] proposed a retrofitted beam-column-BRB joint of the BRBF with pinned BRBs to reduce the out-of-plane deformation of the BRB ends [21]. Based on the aforementioned research results, some scholars [[22], [23], [24], [25], [26], [27], [28], [29]] investigated the influence of the BRB connection forms on the performance of BRBs and structures with BRBs.

The common connection forms of BRBs include pinned, bolted and welded connections [22]. Wigle and Fahnestock [23] simulated BRBFs with pinned, bolted and welded BRBs, and the results showed that the BRB connections have little influence on the BRB hysteresis response and structural inter-story stiffness. Zhao et al. [24] noted that the change in the BRB connections will influence the stability of the BRBs in the BRBF. Moreover, AISC 341–10 [25] indicated that the design method of BRBFs with different BRB connections requires further research. Takeuchi et al. [26] proposed a design suggestion of a pinned and welded BRB to prevent the instability of the BRB ends [19]. Wang et al. [27,28] experimentally studied the influence of BRB connection forms on the performances of a single BRB and a BRBF.

In this paper, four pseudostatic tests were conducted on RSFBs with different BRB and support connection forms, namely, S1: pinned BRB and pinned support, S2: bolted BRB and pinned support, S3: pinned BRB and bolted support, and S4: pinned BRB and welded support. However, the structural performance cannot be fully used due to the failure of the out-of-plane restraints. As a complementary approach, finite element analyses of the test specimens with full out-of-plane restraints were conducted. Moreover, an additional RSFB with welded BRBs and a pinned support (S5) was numerically investigated. Specimens S1, S2 and S5 and specimens S1, S3 and S4 were used to investigate the influence of the BRB and support connection forms on the cyclic performance of RSFBs, respectively.