Jiun-Wei Lai

Bio: Jiun-Wei Lai is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Braced frame & OpenSees. The author has an hindex of 7, co-authored 9 publications receiving 290 citations. Previous affiliations of Jiun-Wei Lai include National Center for Research on Earthquake Engineering & University of California.

Pseudo-dynamic tests of a full-scale CFT/BRB frame-Part I: Specimen design, experiment and analysis

Abstract: A series of pseudo-dynamic tests (PDTs) of a full-scale 3-story 3-bay buckling-restrained braced frame (BRBF) using concrete-filled tube columns was tested in the Taiwan National Center for Research on Earthquake Engineering using networked PDT techniques in October 2003. During the tests, real-time experimental responses and video were webcasted to Internet viewers. The input ground motions adopted for the PDTs were chosen from the 1999 Chi-Chi and the 1989 Loma Prieta earthquakes and scaled to represent three seismic hazard levels. This paper is in two parts, focusing on the investigations of the overall structure and the local members. This paper constitutes Part I and discusses the design, analytical investigations, and key experimental results of the specimen frame, such as the buckling of the brace-to-gusset joints. Part II of the paper, the companion paper, describes the gusset stiffening schemes and detailed experimental behavior of the BRBs and their connections. Experimental peak inter-story drifts of 0.019 and 0.023 radians, prescribed for the design basis and the maximum credible earthquakes, respectively, are within the target design limits of 0.020 and 0.025 radians. These tests confirmed that the PISA3D and OpenSees nonlinear structural analysis computer programs can simulate the experimental peak shears and floor displacements well. Copyright © 2008 John Wiley & Sons, Ltd.

Strongback System: A Way to Reduce Damage Concentration in Steel-Braced Frames

Abstract: This paper examines a newly developed seismic force-resisting system: the strongback system (SBS). To achieve improved seismic performance, this system combines aspects of a traditional concentric braced frame with a mast to form a hybrid system. The mast acts like a strong back to help resist the tendency of concentric braced frames to concentrate damage in one or a few stories during severe seismic excitations. The purpose of the strongback system is to promote uniform story drifts over the height of a structure. Three SBS prototypes were designed and analyzed considering a variety of earthquake excitations. Computed responses are compared with responses for three other braced frame systems. Results of quasi-static inelastic analyses, both monotonic and cyclic, are presented to demonstrate differences in the fundamental hysteretic behavior of the braced frame systems considered. A series of nonlinear dynamic response history analyses were then performed to compare the global and local dynamic re...

Experimental and Analytical Studies on the Seismic Behavior of Conventional and Hybrid Braced Frames

Abstract: This dissertation summarizes both experimental and analytical studies on the seismic response of conventional steel concentrically braced frame systems of the type widely used in North America, and preliminary studies of an innovative hybrid braced frame system: the Strong-Back System. The research work is part of NEES small group project entitled International Hybrid Simulation of Tomorrow's Braced Frames. In the experimental phase, a total of four full-scale, one-bay, two-story conventional braced frame specimens with different bracing member section shapes and gusset plate-to-beam connection details were designed and tested at the NEES@Berkeley Laboratory. Three braced frame specimens were tested quasi-statically using the same predefined loading protocol to investigate the inelastic cyclic behavior of code-compliant braced frames at both the global and local level. The last braced frame specimen was nearly identical to one of those tested quasi-statically. However, it was tested using hybrid simulation techniques to examine the sensitivity of inelastic behavior on loading sequence and to relate the behavior observed to different levels of seismic hazard. Computer models of the test specimens were developed using two different computer software programs. In the software framework OpenSees fiber-based line elements were used to simulate global buckling of members and yielding and low-cycle fatigue failure at sections. The LS-DYNA analysis program was also used to model individual struts and the test specimens using shell elements with adaptive meshing and element erosion features. This program provided enhanced ability to simulate section local buckling, strain concentrations and crack development. The numerical results were compared with test results to assess and refine and the ability of the models to predict braced frame behavior. A series of OpenSees numerical cyclic component simulations were then conducted using the validated modeling approach. Two hundred and forty pin-ended struts with square hollow structural section shape were simulated under cyclic loading to examine the effect of width-to-thickness ratios and member slenderness ratios on the deformation capacity and energy dissipation characteristics of brace members. The concept of a hybrid system, consisting of a vertical elastic truss or strong-back, and a braced frame that responds inelastically, is proposed herein to mitigate the tendency of weak-story mechanisms to form in conventional steel braced frames. A simple design strategy about member sizing of the proposed Strong-Back System is provided in this study. To assess the ability of the new Strong-Back System to perform well under seismic loading, a series of inelastic analyses were performed considering three six-story hybrid braced frames having different bracing elements, and three six-story conventional brace frames having different brace configurations. Monotonic and cyclic quasi-static inelastic analyses and inelastic time history analyses were carried out. The braced frame system behavior, bracing member force-displacement hysteresis loops, and system residual drifts were the primary response quantities examined. These indicated that the new hybrid system was able to achieve its design goals. Experimental results show for the same loading history that the braced frame specimen using round hollow structural sections as brace members has the largest deformation capacity among the three types of bracing elements studied. Beams connected to gusset plates at the column formed plastic hinges adjacent to the gusset plate. The gusset plates tend to amplify the rotation demands at these locations and stress concentrations tended to result in early fractures of the plastic hinges that form. To remedy this problem, pinned connection details used in the last two specimens; these proved to prevent failures at these locations under both quasi-static and pseudo-dynamic tests. Failure modes observed near the column to base plate connections in all of the specimens suggest the need for further study. Both OpenSees and LS-DYNA models developed in this study predict the global braced frame behavior with acceptable accuracy. In both models, low-cycle fatigue damage models were needed to achieve an acceptable level of fidelity. Shell element models were able to predict local behavior and the mode of failures with greater but not perfect confidence. OpenSees analysis results show that the proposed hybrid braced frames would perform better than conventional braced frames and that the story deformations are more uniform. Finally, future research targets are briefly discussed at the end of this dissertation.

Pseudo Dynamic Tests of a Full Scale CFT/BRB Composite Frame

Abstract: A full scale 3-story 3-bay concrete filled steel tube (CFT) column and buckling restrained braced (BRB) composite frame was tested using pseudo dynamic testing procedures and four earthquake accelerations. The key features of the structural system include using three different types of beam-to-CFT column moment connections as well as three different types of BRB . The CFT/BRB frame was designed using the displacement based seismic design procedure with a target inter-story drift limits of 0.02 and 0.025 radians for two hazard levels, 10% and 2% chances of exceedance in 50 years, respecti vely. The pseudo dynamic and analytical predicted responses of the specimen observed during the application of the earthquake load effects are in good agreement. This experimental program illustrates that the experimental response data and the video images of the specimen can be effectively disseminated through the Internet during and after the test s.

Seismic assessment of existing tall buildings: A case study of a 35-story steel building with pre-Northridge connection

Abstract: The Tall Buildings Initiative (TBI) program of Pacific Earthquake Engineering Research Center has been expanded to consider the seismic performance of existing tall buildings. This paper selects a 35-story steel moment resisting frame (SMRF), designed in 1968 with construction details representative of that period, for detailed seismic evaluation in the framework of Performance Based Earthquake Engineering (PBEE). A three-dimensional numerical model capturing the mechanical properties of the most critical structural elements was generated using the program: Open System for Earthquake Engineering Simulation (OpenSees). Systematic nonlinear response history analysis (NRHA) under two basic safety earthquake (BSE) hazard levels for existing buildings were performed following ASCE 41-13 guideline. Probabilistic checks on the confidence levels of the building to achieve collapse prevention (CP) and immediate occupancy (IO) at different hazard levels were conducted based on FEMA 351. In addition, damage and loss analysis was carried out using FEMA P-58 PBEE methodology. Analysis results following different procedures all predicted that the case-study building failed to meet the recommended performance objectives and had a variety of seismic vulnerabilities, and possible retrofits were needed.

Subassemblage tests and finite element analyses of sandwiched buckling-restrained braces

Abstract: This study presents the experimental and finite element analysis results of a proposed steel buckling-restrained brace (BRB) The proposed BRB has two components: (1) a steel core plate that carries all axial forces during tension and compression, and (2) two identical restraining members that sandwich the core plate with fully tensioned high-strength A490 bolts to prevent core buckling Instead of using unbonded material, a small air gap is provided between the core plate and the restraining members to allow for lateral expansion of the core plate under compression Since two restraining members can be disassembled easily by removing the bolts, a damaged steel core can be replaced after a large earthquake Thus, manufacturing new restraining members is not required Four BRB subassemblages were tested to investigate the inelastic deformation capabilities and verify the stability predictions for the braces Test results indicate that three BRBs with sufficient flexural rigidity of the restraining member develop (1) stable hysteretic responses up to core axial strains of 21%–26%, (2) maximum compressive loads of 1724–1951 kN (14–16 times the actual yield load), and (3) a cumulative plastic ductility that is much higher than that specified in AISC seismic provisions (2005) One BRB, intentionally designed with inadequate flexural rigidity of the restraining member, experienced global buckling as predicted Nonlinear finite element analysis was conducted for each BRB for a correlation study The objective of the analysis was to conduct a parametric study for different BRBs to further verify the effects of restraining member size, number of bolts, core plate length and cross-sectional area on buckling load evaluation The design procedure for the sandwiched BRB was provided based on test and finite element analysis results

Design Concepts for Controlled Rocking of Self-Centering Steel-Braced Frames

Abstract: The self-centering rocking steel-braced frame is a high-performance system that can prevent major structural damage and minimize residual drifts during large earthquakes. It consists of braced steel frames that are designed to remain elastic and allowed to rock off their foundation. Overturning resistance is provided by elastic post-tensioning, which provides a reliable self-centering restoring force, and replaceable structural fuses that dissipate energy. The design concepts of this system are examined and contrasted with other conventional and self-centering seismic force resisting systems. Equations to predict the load-deformation behavior of the rocking system are developed. Key limit states are investigated including desired sequence of limit states and methods to help ensure reliable performance. Generalized design methods for controlling the limit states are developed. The design concepts are then applied to a six-story prototype structure to illustrate application of the rocking steel fram...

BRBF response modification factor

Abstract: In this paper, overstrength, ductility and response modification factor of Buckling Restrained Braced frames were evaluated. To do so, buildings with various stories and different bracing configuration including diagonal, split X , chevron ( V and Inverted V ) bracings were considered. Static pushover analysis, nonlinear incremental dynamic analysis and linear dynamic analysis have been performed using Opensees software. The effects of some parameters influencing response modification factor, including the height of the building and the type of bracing system, were investigated. In this article seismic response modification factor for each of bracing systems has been determined separately and tentative values of 8.35 and 12 has been suggested for ultimate limit state and allowable stress design methods.

Local buckling restraint condition for core plates in buckling restrained braces

Abstract: Buckling Restrained Braces (BRBs) are commonly used as bracing elements in seismic zones. A key limit state governing BRB design is to prevent flexural buckling. However, when the wall thickness of the steel tube restrainer is relatively small compared to the cross-section of the core plate, the restraint conditions against the local buckling of the core plate can be critical for the stability and strength of the BRB. In this study, cyclic loading tests and numerical analyses of BRBs were carried out using various tube restrainer configurations to investigate the influence of local buckling of the restrainer on BRB strength and ductility.

Pseudo‐dynamic test of a full‐scale CFT/BRB frame—Part II: Seismic performance of buckling‐restrained braces and connections

Abstract: This paper is Part II of a two-part paper describing a full-scale 3-story 3-bay concrete-filled tube (CFT)/buckling-restrained braced frame (BRBF) specimen tested using psuedo-dynamic testing procedures. The first paper described the specimen design, experiment, and simulation, whereas this paper focuses on the experimental responses of BRBs and BRB-to-gusset connections. This paper first evaluates the design of the gusset connections and the effects of the added edge stiffeners in improving the seismic performance of gusset connections. Test results suggest that an effective length factor of 2.0 should be considered for the design of the gusset plate without edge stiffeners. Tests also confirm that the cumulative plastic deformation (CPD) capacity of the BRBs adopted in the CFT/BRBF was lower than that found in typical component tests. The tests performed suggest that the reduction in the BRB CPD capacities observed in this full-scale frame specimen could be due to the significant rotational demands imposed on the BRB-to-gusset joints. A simple method of computing such rotational demands from the frame inter-story drift response demand is proposed. This paper also discusses other key experimental responses of the BRBs, such as effective stiffness, energy dissipation, and ductility demands. Copyright © 2008 John Wiley & Sons, Ltd.