Investigation of the seismic response of three-story special concentrically braced frames

Special concentrically braced frames (SCBF) are stiff, strong and economical lateral-load resisting systems, which can sustain large inelastic deformation if properly detailed. Historically, experimental research on the seismic response of braced frame research has focused on the cyclic and monotonic responses of isolated components, such as braces or gusset plate connections. However, these components do not work in isolation, and recent research shows that accurate evaluation of their seismic performance requires consideration of the complete system. A collaborative research program with investigators from National Center for Research on Earthquake Engineering (NCREE) in Taiwan, and the Universities of Washington (UW), California, and Minnesota was undertaken to investigate the full system response of SCBFs. The research results presented herein focus on two three-story SCBFs that were tested at the NCREE laboratory. The specimens evaluated a new design approach for midspan gusset plate connections. The two specimens had HSS or wide-flange braces in combination with framing members and connections typical of those used in a three-story building in regions of high seismicity. Composite, concrete slabs were placed on each story. The tests were designed using a recently proposed design method to balance the desired yield mechanisms and form yield hierarchy. The results demonstrate that multi-story SCBFs exhibit good inelastic seismic performance with proper design detailing. Together with prior test results, the test specimens advanced design recommendations for SCBFs, which result in thinner, more compact corner gusset plate connections, a rational method of dimensioning mid-span gusset plates, and a balanced-design procedure for enhanced ductility.     

A balanced design procedure for special concentrically braced frame connections

Concentrically braced frames (CBFs) are stiff, strong structures that are suitable for resisting large lateral loads. Special CBFs (SCBF) are used for seismic design and are designed and detailed to sustain relatively large inelastic deformations without significant deterioration in resistance. Current AISC Seismic Design Provisions aim to ensure the brace sustains the required inelastic action, but recent research showed that current SCBF design requirements lead to variable seismic performance, unintended failure modes, and limited deformation capacity. To improve the seismic response of SCBFs, a balanced design procedure was proposed. The premise of the design methodology is to balance the primary yield mechanism, brace buckling and yielding, with other, complementary ductile yielding mechanisms, such as gusset plate yielding. This balance process maximizes ductile yielding in the frame thereby maximizing the drift capacity of the frame. Further, the undesirable failure modes are balanced with the yield mechanisms and the preferred failure mode, brace fracture, to ensure that the frame fails in the desired manner. To achieve the objectives of the design methodology namely maximum drift capacity, and adherence to a desired yield and failure hierarchy, rational resistance checks and appropriate balance factors (β factors) are used to balance each yield mechanism and failure mode. These factors were developed, validated, and refined using the measured results from an extensive test program. An SCBF connection design example to illustrate the application of the balanced design method and to demonstrate differences from the current AISC design method is presented in an appendix.     

Improved analytical model for special concentrically braced frames

Special Concentrically Braced Frames are commonly used as the seismic resisting system in buildings. Their inherent strength and stiffness assure serviceable performance during smaller, more frequent earthquakes. Inelastic tensile yield and post-buckling compressive deformations of the brace dominate performance during large seismic events. However, inelastic deformations of the brace place secondary yet significant inelastic deformation demands on beams, columns, and connections, which significantly affect the seismic performance. These response modes must be included in an analytical model of the system to capture the response. However, conventional practice uses beam–column elements for the brace, to simulate brace buckling, with pin-ended or rigid end connections; these computer models cannot capture the full range of SCBF behaviors. The research presented in this paper was undertaken to develop a modeling approach for SCBFs to more accurately predict their seismic performance. Beam–column elements are used for the braces, beams and columns and these elements include nonlinear geometric effects to simulate brace buckling. A new connection model is proposed to simulate the behavior of the gusset plate. The model parameters are based upon the member sizes, properties and connection designs. Simulated results are compared with experimental results and predictions from approaches more commonly used in practice. Although a step beyond models currently used in design practice, the proposed model remains simple in its implementation and is suitable for a wide range of practical applications. The proposed model provides accurate simulation of global behavior, while retaining simplicity and providing reasonable predictions for many local behaviors.     

Numerical analysis of the nonlinear performance of concentrically braced frames under cyclic loading

Concentrically braced frames (CBFs) are widely used as lateral-load resisting system in steel structures. This study examines the effects of different parameters especially those associated with connections, on the behavior of CBFs. A single bay, singlestory frame is used to evaluate the interaction between structural members. Nonlinear analyses using a detailed inelastic finiteelement model (FEM) are carried out to study the behavior of frames subjected to cyclic loading. Models are designed based on seismic codes and analyzed to evaluate the performance of both SCBFs and OCBFs. The equivalent plastic strain concept is used to determine the ductility capacity and to predict fracture and failure in these models. Results show that the seismic performance of CBFs, which are designed according to current provisions can be improved by configuring the details of gusset plate connections in a way that inelastic demands are balanced in middle of brace and gusset plate corners.     

工字截面倒V型支撑及其节点板在循环荷载作用下的性能研究

倒V型支撑及其中心节点板连接在特殊中心支撑框架中备受青睐。为了提高其抗震性能和在抗震设计中的应用,对9个非弹性往复荷载作用下的工字截面倒V型支撑及其节点板的滞回性能进行试验和数值模拟。主要参数为:节点板上支撑端部间隙、支撑杆件节点位置、自由端长度与节点板厚的比值。在支撑低周疲劳破坏前,框架的极限承载力并没有降低,但有一个长期的低承载稳定阶段。虽然有直线间隙的试件抗震性能良好,但只要节点板不先于支撑破坏,也可以不设间隙。中等偏心的支撑节点性能很好且尺寸也很经济,但节点板上的支撑杆件位置可能引起板面外变形并降低结构延性。基于试验结果,提出节点板自由边长度与厚度比值限值。     

Cyclic behavior studies on I-section inverted V-braces and their gusset plate connections

Inverted V-braces and their central gusset plate connections are popular patterns of brace arrangements for special concentrically braced frames (SCBF). To improve the understanding of their seismic performances and promote their applications in seismic designs, the hysteretic behavior of nine I-section inverted V-braces and their gusset plate connections subject to inelastic cyclic loading is examined through experiments and analytical simulations. It is found that the clearance at the brace end on the gusset plate, the locations of the intersection point of bracing members, and the ratio of the free edge length to the gusset plate thickness are the key parameters. The loading capacities of braced frames show no decrease before the brace low-cycle fatigue fracture, but a longer plateau at a lower load level exists in the hysteretic loops. Although specimens with a linear clearance exhibit better seismic behaviors, a negative clearance is also acceptable as long as the gusset plate does not fracture prior to the braces. A brace intersection point with moderate eccentricity is preferable for its better behavior and its economical dimension of the gusset plate, but the brace point location in the gusset plate could induce out-of-plane deformations in the gusset plate and cause the system ductility to deteriorate. Based upon test results, a suggested limitation of the ratio of the free edge length to thickness for the gusset plates is presented.     

Simulated behavior of multi-story X-braced frames

Steel braced frames are a commonly used seismic resisting system and thus, multi-story X-braced frames are frequently used. However, research into the behavior of these systems with midspan gusset plates, as used in practice, is limited. As a result, their seismic performance and the influence of connection design on this performance are not well understood. A comprehensive series of inelastic analyses were undertaken to better understand the nonlinear, cyclic behavior of multi-story X-braced frames and their gusset plate connections. Finite element (FE) analyses were conducted and the FE model was developed and verified by comparing the simulated results with cyclic tests and nonlinear analyses of single story systems, conducted at the University of Washington. The verified analytical model and associated failure estimation procedures were used to predict all yield mechanisms and failure modes, frame deformation capacity, and initial cracking and fracture of critical elements within the frame. A parametric study was performed to examine the influence of the gusset plate, framing members and other structural elements on the seismic performance of multi-story X-braced frames. The results show that the design and detailing of the gusset plate has a significant impact on the seismic performance of the frame. Connections designed with proposed end-rotational clearance models, and with strength and stiffness values balanced to the buckling and tensile yield capacities of the brace provided the best ductility and deformation capacity. In addition, the results suggest that floor slabs, gusset plate stiffeners and framing member sizes affect the frame performance and must be considered in the analysis and design of the system.     

Compressive behavior of central gusset plate connections for a buckling-restrained braced frame

Buckling-restrained braced frames (BRBFs) are used as lateral-load resisting systems in seismic design. The braces in BRBFs are connected to beams and columns by gusset plate connections, and can yield in both tension and compression instead of buckling. Although tests of buckling-restrained braces (BRBs) have demonstrated their ability to withstand significant inelastic axial deformation, large-scale BRBF tests have exhibited central gusset plate buckling before BRBs develop the ultimate compressive strength. To extend and better understand the experimental work, this paper presents an analytical study of the compressive behavior for BRBF central gusset plate connections using the finite element computer program ABAQUS. A model of a previously tested BRBF is conducted to predict experimental buckling load of the central gusset plate and verify the accuracy of a simple model of a central gusset plate connection including a beam and part of the BRB. The out-of-plane deformation of the central gusset plate resembles the buckled shape of a gusset plate with low bending rigidity provided by the BRB end. The experimental buckling load of the central gusset plate cannot be predicted based on the AISC-LRFD approach with an effective column length factor of 1.2. Therefore, a parametric study on the compressive strength of BRBF central gusset plate connections is conducted with various gusset plate dimensions and free-edge stiffeners. An inelastic plate buckling equation together with coefficient charts is proposed to predict ultimate load. For gusset plates with sufficient free-edge stiffener rigidity, the yield load can be developed and increased to the post-yield strength level. A required free-edge stiffener size is also recommended for BRBF central gusset plates to develop compressive yield load.     

Numerical simulation of gusset plate connections with rectangular hollow section shape brace under quasi-static cyclic loading

In concentrically braced frames, gusset plate connections to rectangular hollow section braces are fabricated using welds to connect the gusset plate to both brace and flanges of the beam and of the column framing into the brace. The beam-to-column connection at the gusset plate is either welded or bolted. However, past experimental studies have indicated that undesirable failure modes could occur in the gusset plate even when using a linear clearance rule in the design of the gusset plate, especially when connecting hollow rectangular shapes.For these reasons, this study investigates through numerical analyses the local seismic performance of gusset plate connections with fully restrained beam-to-column connections as well as partially-restrained bolted connections. The latter are provided at the outside corner of the gusset plate, away from the face of the column, in order to facilitate the beam rotation at the bolted connection upon continued lateral deformation. The main goal of the study of the local performance of gusset plate connections is to validate the design procedure presented in this paper; to compare the various clearance rules proposed in the literature and to propose an alternative clearance rule to the linear clearance rule.The local performance is examined through detailed finite element models of a braced bay located at the ground floor of a four storey concentrically braced frame using the MIDAS finite element program. Finally, local performance of the models is compared in terms of strain concentrations in gusset plates, beams and columns.     

钢板装配式屈曲约束支撑RC框架平面#br# 外力学性能试验研究

框架平面外方向的层间侧移在一定程度上影响框架中屈曲约束支撑抗震性能的发挥。为了研究框架平面外的层间侧移作用下,节点板加边肋和屈曲约束支撑端部加强对带屈曲约束支撑钢筋混凝土框架平面外方向力学性能的影响,对3榀安装了钢板装配式屈曲约束支撑的钢筋混凝土框架试件在平面外方向进行拟静力试验。研究了带钢板装配式屈曲约束支撑的钢筋混凝土框架平面外力学性能,节点板加边肋和BRB端部加强对其自身在平面外方向的受力与变形特征。结果表明：钢板装配式屈曲约束支撑对钢筋混凝土框架平面外方向的承载能力影响很小;框架平面外变形引起屈曲约束支撑轴向变形非常微小;在框架平面外层间位侧移作用下,带节点板的BRB呈“C”形的变形模式,节点板加边肋与屈曲约束支撑端部加强的设计能减小BRB轴力对其在平面外方向变形的影响。     

Cyclic behavior of concentrically braced frames with built-up braces composed of channel sections

Concentrically braced frames are earthquake resistant systems commonly used in buildings. Seismic behavior of this type of structures is affected by their configurations, brace properties, and brace to gusset plate connections. In this paper, the results of three experiments conducted to investigate the cyclic behavior of concentrically braced frames with braces built-up of double channels are reported. Significant damage was observed in beam to column connections. Large out of plane deformation of braces caused some cracks in the connector welds; however they did not result in fracture. Although large drift was applied to the frames, no brace fracture was observed. Furthermore, experiments showed that the majority of compressive strength in post-buckling state and a noticeable portion of tensile strength originated from frame action. By choosing connector spacing as the main parameter and using finite element models, a parametric study was performed to investigate the effect of this parameter on this type of frames with two different details of brace to gusset plate connections. It is observed that reducing the connector spacing increases the inelastic strain demand in braces and decreases it in gusset plates. However, gusset plates, which accommodate 2t linear clearance, are less dependent on connector spacing, compared to those accommodating 6t elliptical clearance. It seems that the limitations of slenderness ratio of individual section, stipulated in current seismic provisions, need further study.     

Effect of gusset connection configurations on frame–gusset interaction in steel buckling‐restrained braced frame

Although buckling restrained braces (BRBs) are commonly applied in seismic buildings to mitigate structural damage, their performance was often limited by rupture of the corner gusset connections due to additional frame action. This issue may be resolved by alternative gusset connections to mitigate the frame–gusset interaction. In this study, commonly used procedures for design of the traditional gusset connection are reviewed, followed by a case study on the effect of frame action on the structural behavior of these gusset connections in steel frames with BRBs. Inspired by these analysis, two different strategies, aiming at releasing frame–gusset shear interaction using sliding gusset connection or reducing normal interaction using dual gusset plates, are tried to mitigate the frame action effects. Finite element analysis is conducted on steel frame subassemblages with/without BRBs to examine the effect of different gusset connections on the structural behavior of these framing systems. It shows that the sliding gusset connection shows beneficial effect in reducing the frame action, having much smaller stress responses on the gusset interfaces, as well as smaller shear force and plastic responses on the framing system. Thus, it becomes a promising gusset connection for improved seismic performance of the steel framing system with brace‐type dampers.     

Behavior and design of gusset plate connections in compression

A total of thirteen full-scale tests were conducted to investigate the compressive behavior and strength of gusset plate connections. The test parameters included gusset plate thickness, size, and brace angle. In addition, the effects of frame action on the compressive behavior of gusset plate connections were also investigated. In general, the gusset plate specimens were failed by sway buckling of the connection since no out-of-plane restraint was provided from the bracing member. The test results indicated that, in general, significant yielding of the gusset plate specimens occurred prior to reaching the ultimate load. However, only limited yielding was observed for the thin specimens with a plate thickness of 6.5 mm. The ultimate load of the specimens increased almost linearly proportional to the gusset plate thickness and decreased with increasing plate size. A slight decrease in the ultimate load of the specimens was observed when a 30° brace angle was used instead of a 45° one. The beam and column moment had only negligible effects on the ultimate load of the specimens; however, yielding of the specimens was detected at a load level significantly lower than that had no framing moment. The numerical simulation provided good agreement with the test results.     

Ultimate strength of gusset plate connections with fillet welds

Braced frames are commonly used as lateral-load resisting systems in seismic design. The braces are connected to the beams and columns by gusset plate connections. Fillet welds are commonly used to connect the gusset plates to the beams and columns. And the fracture of the interface welds were observed in the past research and earthquakes. This paper focused on the ultimate strength of interface weld connection between gusset plate and frame elements when the brace is in tension. Pilot experimental study was conducted with four specimens and proved that the evaluation recommended by AIJ works well. A verified finite element analysis model was developed to conduct a parametric study. The studied parameters are the brace angle, gusset plate size, and eccentricity of brace. From the parameter study, it is confirmed that the tensile brace axial force is primarily transferred to the interface weld within an extension Whitmore region, which is named as the effective region in the AIJ evaluation. And the extension Whitmore region is affected by the gusset plate geometrical constraint. A revised extension Whitmore region is suggested by considering the aforementioned parameters. The AIJ evaluation using the revised extension Whitmore region is also compared with the UFM, and showed better evaluation for the rectangular shape gusset plate.     

Influence of masonry infill on the seismic performance of concentrically braced frames

This paper presents an experimental and analytical study to investigate the effect of masonry infill on the seismic performance of special Concentrically Braced Frames (CBFs). Cyclic lateral load tests are conducted on three half-scale specimens including two special CBFs with and without masonry infill and a moment resisting steel frame with masonry infill for comparison purposes. Companion analyses are performed to study the influence of masonry infill on the potential rupture of gusset plates and top-seat angle connections by using detailed FE models validated with experimental results. It is shown that the presence of masonry infill could increase the lateral stiffness and load carrying capacity of the special CBF by 33% and 41%, respectively. However, the interaction between masonry infill and the frame significantly increased the strain demands and failure potential of the connections. The results of the experimental tests and analytical simulations indicate that ignoring the influence of masonry infill in the seismic design process of CBFs results in a premature fracture of the connection weld lines and a significant reduction in the deformation capacity and ductility of the frame. This can adversely influence the seismic performance of the structure under strong earthquakes. The results of this study compare well with the damage observations after the 2003 earthquake in Bam, Iran.     

Concentric X-braced frames with HSS bracing

Concentrically braced frames are stiff, strong systems frequently used to resist wind and seismic loading; in regions of high seismicity in the US special concentrically braced frames (SCBFs) are used. CBF configurations vary, but in low rise or other structures with modest levels of demands single-story, X-configured braced frames (X-braced) are commonly used. The brace sections used also vary but hollow structural sections (HSS) are the most common in the U.S. Although important, in part because low-rise structures sustain large demands, few research programs have focused on the single-story X-brace configuration. A large research program was undertaken to understand and improve the response of SCBFs with selected testing on single-story X braced SCBFs. The test matrix consisted of two, full-scale planar X-braced frame experiments and one nearly-full-scale three-dimensional X-braced frame. The tests were designed using a new design and detailing philosophy, called the Balanced Design Method. In this paper, application of this design method to the frame is investigated, with a focus on the center-splice connection. The results show that the ultimate inelastic deformation capacity of the system is less dependent on the specific design detail at this splice. Additionally, the bi-directional load testing indicated that the out-of-plane demands did not impact the system performance.     

防屈曲支撑角部节点板与钢框架的相互作用效应

防屈曲支撑是一种高效稳定的耗能减震装置,其与框架结构一般通过焊接节点板形式连接。目前节点板连接设计方法仅考虑支撑轴力的影响,并没有直接考虑框架开合效应(梁柱在水平地震力下产生的张开/闭合变形)的不利作用,导致焊接节点板在连接处提前发生开裂。通过有限元模拟的方法,同时考虑开合效应和支撑轴力的共同影响,对防屈曲支撑钢框架与角部节点板连接的相互作用进行研究。有限元模型共5组,主要参数包括节点板尺寸、节点板与框架的连接形式以及节点板是否设置自由边加劲肋。在连接形式方面,提出了一种可减小开合效应不利影响的新型可滑移螺栓连接节点板,并与传统焊接节点板的受力性能进行比较。分析结果表明,平面尺寸较小的焊接节点板对结构的抗侧刚度影响最小,可减小设置防屈曲支撑的子框架所分担的地震剪力,相应的节点板受力性能也优于平面尺寸较大的焊接节点板|在焊接节点板上设置自由边加劲肋并不能明显改善其受力性能|所提出的新型可滑移螺栓连接节点板可有效减小节点板对结构刚度的影响,以及框架开合效应对节点板的不利影响,是一种在消能钢框架支撑体系中具有应用前景的新型节点板连接。     

低屈服点钢扣板连接的试验研究

同心支撑框架被广泛用于钢结构房屋的抗震设计中。在地震激励下,同心支撑框架的支撑会承受循环拉压荷载。由于支撑的屈曲,其抗压强度通常低于抗拉强度,这可能会降低支撑框架的抗震性能。该文对采用弱扣板强支撑的设计理念进行了验证。扣板选用低屈服点钢(LYP),从而使设计的扣板在支撑屈曲前发生屈服。低屈服点钢的屈服强度很低,但其延性很好。通过一系列试验验证循环荷载作用下低屈服点钢扣板的性能。研究发现,在低屈服点钢扣板上增加槽型约束(STR)可以大大提高其抗震性能。在拉压荷载作用下,有槽型约束的低屈服点钢扣板可以提供类似大小的强度。扣板的耗能能力同样得到提高。基于此研究成果,给出低屈服点钢扣板的一些设计建议。     

Experimental study of low yield point steel gusset plate connections

Concentrically braced frames have been used widely in the seismic-resistant design of steel building structures. During earthquake excitation, the braces of the concentrically braced frame are subjected to recursive tensile and compressive forces. The compressive strength of the brace is usually less than its tensile strength because of the buckling of the brace, and this may degenerate the seismic resistance capacity of the braced frame. In this reported research, an alternative design concept that adopts the weak gusset plate-strong brace is examined. The gusset plate is designed to yield prior to the buckling of the brace. Low yield point (LYP) steel is selected for the gusset plate. The LYP steel possesses low yield strength and high elongation capacity. A series of experimental studies was carried out to examine the LYP steel gusset plates under cyclic loads. It is found that adding slot-type restrainers (STR) to the LYP steel gusset plate greatly enhances the seismic resistance of the gusset plate. The proposed LYP steel gusset plate with an STR is able to provide similar strengths under tensile and compressive loads. The energy dissipation capacity of the gusset plate is also increased substantially. Based on this study, suggestions are made for the design of LYP gusset plates.     

Cyclic flexural analysis and behavior of beam-column connections with gusset plates

This research investigates the cyclic flexural behavior of double-angle concentrically braced frame beam-column connections using three-dimensional nonlinear finite element analysis. Prior experimental research demonstrated that such connections possess appreciable flexural stiffness, strength, and ductility. The reserve capacity provided by these connections plays a significant role in the seismic behavior of low-ductility concentrically braced frames, so knowledge about the impact of connection parameters on local limit states and global connection performance is needed for employing reserve capacity to design and assess concentrically braced frames. Finite element models were developed and validated against prior experiments with focus on the limit states of failure of the fillet weld between the gusset plate and beam, low-cycle fatigue fracture of the steel angles joining the beam and gusset plate to the column, and bolt fracture. The models were used to evaluate the flexural stiffness, strength, and ductility of braced frame connections with primary attention on the effects of beam depth, angle thickness, and a supplemental seat angle. The finite element analysis demonstrated that increasing beam depth and angle thickness and adding a supplemental seat angle all increased the stiffness and strength of the connection while maintaining deformation capacity. A procedure to estimate the flexural behavior of beam-column connections with gusset plates was developed based on the results of the numerical simulations.