Seismic Design and Performance of Buckling Restrained Braced Frames with Eccentric Brace Configurations Part 1: Design Procedure and Case Studies

Background: Different bracing systems of steel Eccentric Braces (EBs) and steel-concrete Buckling-Restrained Braces (BRBs) can be used in steel frames in order to make the frames stronger in resisting lateral loads. These steel frames with EBs or BRBs are generally called Eccentrically Braced Frames (EBFs) or Buckling-Restrained Braced Frames (BRBFs), respectively. Objective: This study aims to investigate steel frames with bracing systems of steel EBs and steel-concrete BRBs having moment link. Methods: The EBFs and BRBFs are nonlinearly analysed employing the finite element software ABAQUS. Experimental tests of the EBF and BRB are utilised for the validation of their modelling. The modelling is validated by comparing the modelling results with experimental tests results. Then, an EBF and a BRBF are designed having moment link. The extreme earthquake records of Tabas, Chi-Chi, and Northridge are selected for the dynamic analyses of the EBF and BRBF. The validated modelling method is applied to analyse the designed EBF and BRBF under the selected earthquake records. Results: The achieved results from the analyses are lateral displacements, base shears, and energy dissipations of the EBF and BRBF and moment link rotations. These results are compared and discussed. Conclusion: It is concluded that the hierarchy of the lateral displacements of the analysed EBF and BRBF, having moment link, is related to the Tabas, Chi-Chi, and Northridge records because the lateral displacements of the frames are directly proportional to the peak ground accelerations of the records, and there is the same hierarchy for the records in terms of their peak ground accelerations. Lower lateral displacements are witnessed for the BRBF than the EBF subjected to the Tabas and Chi-Chi records. However, larger lateral displacement is observed for the BRBF than the EBF under the Northridge record. The same procedure as the lateral displacements is also revealed for the effectiveness of the BRBF with regard to its link rotations compared with the EBF. Moreover, the BRBF improves the base shear capacities and energy dissipations of the frame compared with the EBF. Consequently, the BRBF is generally demonstrated to be superior to the EBF from the structural performance point of view. Thus, the BRBF can be used more efficiently in structures subjected to large lateral loads compared with the EBF.

The present paper studies eccentric braces (EBs) and buckling-restrained braces (BRBs) used in steel frames. The eccentrically braced frames (EBFs) and buckling-restrained braced frames (BRBFs) that have respectively employed the EBs and BRBs are considered with different types of links as shear, moment-shear and moment links depending on their link length which is an important factor in the design of the EBFs and BRBFs. The BRB consists of a steel core and its surrounding steel tube filled with concrete. The concrete confinement prevents buckling of the steel core. The analysed EBFs and BRBFs by the finite element software ABAQUS under earthquake records are taken into account. Effects of the links of the EBFs and BRBFs on the performance of the frames are discussed. The results uncover that most of the lateral displacements of the EBFs and BRBFs having the shear link are smaller than their counterparts having the moment-shear and moment links, whilst, all the base shear capacities of the former are the greatest. However, majority of the EBFs and BRBFs with the moment link dissipate less energy than their counterparts with the shear and moment-shear links, whereas, most of the link rotations of the former are smaller than the latter. In addition, the BRBFs generally demonstrate better performance than their EBF counterparts.

Buckling-restrained braced frame (BRBF) systems exhibit robust performance when subjected to seismic loading. However, the low post-yield stiffness of buckling-restrained braces (BRBs) may cause BRBFs to develop large maximum and residual drifts and allow the formation of soft stories. Thus, reserve strength provided by other elements in the structural system is critical to improving seismic performance of BRBFs. Reserve strength can be provided in two primary ways: (1) moment-resisting connections within the BRBF and (2) a steel special momentresisting frame (SMRF) in parallel with the BRBF to create a dual system. These two approaches to providing reserve strength can be used together or separately, leading to a variety of potential configurations. In addition, special attention must be given to beam-column connections within the BRBF since moment-resisting connections have been observed experimentally to limit drift capacity due to undesirable connectionrelated failure modes. This paper presents nonlinear dynamic analysis results and evaluates performance of BRBF and BRBF-SMRF systems using moment-resisting and non-moment-resisting beam-column connections within the BRBF. Reserve strength is shown to play a critical role in seismic behavior and performance of BRBFs.

Due to the low postyield stiffness of buckling‐restrained braces (BRBs), multistory buckling‐restrained braced frames (BRBFs) subjected to earthquakes are prone to lateral deformations and damage concentrations at certain stories, which is deemed a damage concentration effect (DCE). A series of nonlinear pushover analyses and response history analyses are conducted to investigate the key factors affecting the DCE of BRBFs. Two comparisons of the DCE are performed for different types of structures and different beam‐to‐column connections in the main frame (MF). These comparisons show that BRBFs equipped with BRBs as the main earthquake resistance system have a more serious DCE than the traditional moment‐resisting frame or conventional braced frame and that the MF stiffness significantly affects the structural residual displacement and DCE. Then, parametric analyses are performed to investigate the influence of two stiffness distribution parameters (in the horizontal and vertical directions) on the DCE of a 6‐story BRBF dual system designed according to the Chinese seismic code. The results show that increasing the MF stiffness and avoiding abrupt changes in the BRB stiffness between stories can effectively mitigate the DCE of BRBFs. Finally, the correlations between various damage performance indices are analyzed. A low statistical correlation between the peak and residual drift responses can be observed in BRBFs. Therefore, it is recommended that the DCE be considered in BRBF design.

The design focus for a buckling-restrained braced frame (BRBF) is that the buckling-restrained braces (BRBs) dissipate most of the seismic energy while the main frame retains a degree of elastic stiffness under a major earthquake. An elastic displacement spectrum based design method is presented in this article, which can directly determine the sectional area of the BRBs. The yield displacement in the roof of the main frame is taken as the target displacement under a major earthquake. An elastic displacement design spectrum is used to solve the target period of the BRBF. To validate this method, a six-story buckling-restrained braced steel frame is designed using the proposed method, and a series of nonlinear response history analyses (RHAs) are performed to verify the design result. The example shows that the required BRB area can be simply and accurately determined by the proposed method. The error between the given target displacement and the RHA results is 4.0% and 21.3% for BRBFs designed with BRB yield strength of 235 Mpa and 100 Mpa, respectively.

Elastic displacement spectrum-based design of damage-controlling BRBFs with rocking walls

Numerical evaluation of the hysteretic behavior of concentrically braced frames and buckling restrained brace frame systems

This study involves the application of timber‐based bracings elements. For this purpose, seismic analyses are performed on special portal steel frames without the brace and diagonally braced with Glued Laminated Timber (glulam) and Timber‐Steel Buckling Restrained Brace (TS‐BRB), and the results are compared with the same configuration using steel Hollow Structural Sections (HSS) bracing, using OpenSees structural analyzer. First, to verify the accuracy of the modeling, the numerical results are compared with experimental measurements on several types of elements: (a) diagonally braced frame with steel Hollow Structural Sections with a concentrically steel braced frame which was tested by the quasi‐static method under cyclic loading protocol by previous researchers, (b) diagonally glulam braced frame with results of shake table tests on single‐story timber braced frames, and (c) Timber‐Steel Buckling Restrained Brace (TS‐BRB) frame with experimental results of Heavy Timber Buckling‐Restrained Braced Frame (HT‐BRB). In the second step, the aforementioned timber base bracing alternatives (glulam, TS‐BRB) are applied in the special portal steel frame, then the seismic performance of the frame is investigated under pushover, cyclic, time history, and incremental dynamic analysis (IDA), and then the results are compared with the behavior of similar portal frame in two conditions without the brace and diagonally braced with the steel‐HSS brace. Results showed that steel‐HSS, glulam, and timber‐steel buckling restrained braces have significant roles in energy dissipation, increasing shear capacity, decreasing interstory drift, and decreasing weight and cost of estimation of the structure.

SummaryThe damped‐outrigger system has been proposed to improve the performance of conventional outrigger systems in controlling the structural seismic response by increasing the damping and stiffness. The purpose of this study is to demonstrate the effectiveness of using damped‐outrigger systems in midrise buildings and provide engineers with a comparison between conventional structural systems such as moment resisting frame (MRF) and buckling‐restrained braced frame (BRBF) in proposing the most suitable structural system. In this study, the buckling‐restrained brace and viscous damper are adopted as the energy dissipation devices in the damped‐outrigger system. A total of 48 midrise numerical models with various building heights and structural systems are analyzed using nonlinear response history analysis and incremental dynamic analyses. The analysis results show that the midrise buildings equipped with a damped‐outrigger system with either viscous damper or buckling‐restrained brace (BRB) can reach similar and even better seismic performance when compared with the BRBF; it also reduces the structural responses by around 30% for the maximum roof drift and acceleration responses when compared with MRF. The analysis results could provide a reference for structural engineers when selecting suitable lateral force resisting systems for midrise buildings.

The current paper compares the seismic performance of buckling-restrained braced frames (BRBFs) and ordinary concentrically braced frames (CBFs) through nonlinear static and time history analysis. Two groups of BRBFs and CBFs including 5, 10 and 15 story frames are modeled and nonlinear pushover analysis is carried out. To further investigate the performance of frames, a selection of near-fault and far-field ground motion records are used for time history analysis. The results indicate the acceptable seismic performance and conformity of low-rise CBFs with BRB frames under far-field records. However, BRB frames present more satisfying results with regards to inter-story drifts, residual drift responses and energy dissipation in case of near-fault seismic excitations. Moreover, the implementation of BRBs provides better performance against the Basic Safety Earthquake 1 (BSE-1).

SummaryA gusset plate is subjected to forces induced from a buckling‐restrained brace (BRB) and frame action. In this study, a performance‐based design method of the gusset connections incorporating a BRB and frame actions is investigated. The force demands resulting from the BRB axial force are computed from the generalized uniform force method. The force demands induced from the frame action effects primarily result from beam shear. A conservative method, which considers the beam axial force effect and the thereafter reduced beam flexural capacity possibly developed at the gusset tips, is adopted in estimating the maximum beam shear. An improved equivalent strut model is used to represent the gusset plate subjected to the frame action effect. The total force demands of the gusset connection are combined from the BRB force and the frame actions. For design purposes, the stress distributions on the gusset interfaces are linearized. The maximum von Mises stress combining the normal and shear stresses is considered as the demand for the gusset plate design. In order to verify the effectiveness of the proposed design method, experiments on a two‐story full‐scale buckling‐restrained braced frame (BRBF) were performed. The chevron and single diagonal brace configurations were arranged in the second and the first stories, respectively. Two different corner gusset connection configurations including one single corner gusset and one coupled corner gusset connection, where two braces in adjacent stories joined at the same beam‐to‐column joint, were tested. The BRBF specimen was subjected to cyclically increasing lateral displacements with a maximum frame drift of 0.04 rad. The maximum story drifts reached 0.035 and 0.061 rad. in the first and the second stories, respectively. At the end of the tests, no fractures were observed on any of the gusset interfaces. Along the gusset interfaces, the normal and shear stress distributions computed from the proposed design procedures and the FEM analysis correlated well with the experimental results. This paper concludes with the procedure and recommendations for the performance‐based design of gusset connections. Copyright © 2015 John Wiley & Sons, Ltd.

As the use of buckling-restrained braced frames (BRBFs) has increased in the United States, the need has grown for knowledge about member and system behavior under seismic loads and for implementing this knowledge into design provisions. In particular, methods for designing BRBFs and predicting seismic response require validation. To address this need, along with the need for experiments demonstrating system-level BRBF performance, a research program composed of numerical and large-scale experimental simulations was initiated at the ATLSS Center, Lehigh University. This paper describes the nonlinear dynamic analyses that were conducted as part of this research program. Numerical simulations of BRBF response were conducted using ground motion records scaled to two seismic hazard levels. The performance of the prototype BRBF was acceptable and performance objectives were met in terms of structural damage. It is shown that the currently accepted deflection amplification factor underestimates mean inelastic lateral displacements under design-level earthquakes and the system overstrength factor may be unconservative. The current method for predicting BRB maximum ductility demands is also shown to be unconservative and a more rigorous method for predicting BRB maximum ductility demands is provided.

Evaluation of buckling-restrained braced frame seismic performance considering reserve strength

Response modification factors of buckling-restrained braced frames designed according to the Egyptian code

Braced frames are often used to resist lateral earthquake loads in steel buildings, but braces can interfere with architectural features. Eccentrically braced frames (EBFs) will accommodate windows, doors, and halls, but have performance limitations when link-to-column connections are required. An alternative to EBFs may be buckling-restrained braced frames with eccentric configurations (BRBF-Es). This paper introduces the concept of BRBF-Es and highlights design considerations. An analytical study was conducted that compares the performance and economy of BRBF-Es with EBFs. Results from non-linear time history analyses indicate that BRBF-Es will have greater residual drifts than comparable EBFs, but are less susceptible to failures at link-to-column connections. BRBF-Es require more steel than EBFs, but savings in design, fabrication, and erection may offset higher material costs.