基于滑移连接的防屈曲支撑钢框架节点抗震性能研究

防屈曲支撑（BRB）是一种拉压均可全截面屈服耗能而不屈曲的金属阻尼器,在建筑结构的抗震减震设计中得到广泛应用。然而,由于大变形下支撑框架节点存在显著开合效应,在罕遇地震作用下易出现节点板和相邻梁柱构件的提早断裂现象,限制了BRB抗震性能的充分发挥。为此,在总结BRB钢框架节点的现行设计方法及节点失效模式基础上,提出了可释放节点开合效应的滑移连接节点板,采用低摩擦材料减小接触面摩擦力。建立有限元模型,通过与传统焊接节点板对比,分析两种不同连接对节点板、梁柱和BRB受力性能的影响。以此为基础,设计该类节点足尺试验模型,对其进行拟静力试验,分析其在往复荷载下的抗震性能。研究结果表明：所提出的滑移连接可有效释放节点板与梁柱之间的切向约束和开合效应,显著降低了节点板的塑性损伤,实现了罕遇地震作用下节点板弹性的性能目标;梁塑性铰由节点板端部移至梁柱交界面处,降低了梁柱构件的剪力水平和塑性损伤;在层间位移角4%下各关键构件仍具有饱满稳定的滞回性能,显著提高了BRB钢框架的抗震性能。     

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。     

滑移连接的防屈曲支撑RC框架节点抗震性能试验研究

防屈曲支撑(BRB)与框架之间采用的传统焊接节点会约束梁柱开合变形,在强震作用下易使框架先于支撑发生破坏。为释放传统焊接节点对梁柱的刚性约束,提出滑移连接的BRB框架节点,通过对预埋入混凝土内部的连接钢板表面进行无黏结处理,以抗拔不抗剪的方式传递BRB轴力。在滑移节点的工作原理、设计方法和施工流程基础上,对2个基于滑移连接的足尺BRB-框架梁柱组合体开展拟静力试验研究,并与1个传统焊接节点的破坏模式、滞回性能和节点应力进行比较。试验结果表明:当层间位移角达到1/33时,滑移节点的层剪力与传统焊接节点相比降低了34%,有效避免了框架发生剪切破坏;滑移节点的应力水平显著焊接节点的显著降低;在滑移节点与框架梁端设置的附加纵筋可更有效地减小节点区的开合效应和发挥BRB的耗能作用;滑移节点中BRB的滞回性能与传统节点的滞回性能接近。     

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.     

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.     

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.     

Evolutionary structural optimization of steel gusset plates

Evolutionary structural optimization is applied to determine the optimum shape of steel gusset plates subjected to axial forces. A number of different gusset plates used in various types of connections is considered for this purpose. The evolutionary structural optimization approach is employed to find the optimum shapes of a gusset plate used in these connections. The first example considers a gusset plate having two holes which are utilized in the connection of double angle carrying a tensile force. Within this example the effect of certain parameters in the evolutionary structural optimization process such as material removal ratio, mesh size and modeling of holes on the final shape is investigated. The gusset plates having three, four and five holes are also designed for optimal shape. Furthermore design examples include two rows of multiple holes as well as staggered holes and connections with multiple members. The final shapes obtained in the single member bolted connections are generally similar to those used in practice. However, they are lighter. Those shapes obtained for the multiple member connections and welded double angle connections are unpredictable. Although the shapes obtained in all the examples are lighter than the ones used in practice, they might be more expensive to produce. It is shown that the evolutionary structural optimization method has a potential in determining the optimum shape of gusset plates.     

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.     

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.     

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.     

Instability of steel gusset plates in compression

As a result of the structural failure of the I-35W Bridge in Minneapolis in 2007, the Federal Highway Administration (FHWA) issued guidelines for the load rating of gusset plates and recommended that the capacity of these plates on non-redundant steel truss bridges be verified. The purpose of this study is to examine the buckling behaviour of steel gusset plates in greater detail, accounting for parameters that were not explicitly included in the guidelines, such as initial deformations of the gusset plate, stiffness of the framing members, load distribution from the framing members to the plate and load eccentricity. For this purpose, a finite-element model of a gusset plate was developed and verified against experimental measurements. Results show that, for in-plane compressive loads with no moment and no eccentricity, the FHWA guidelines for load rating are conservative and safe for initial out-of-plane deformations up to one plate thickness.     

Study on failure modes of steel truss bridge gusset plates related to tension and shear block failure

Tension and shear block failure is a limit state which occurs in the connection of tension steel members. The failure mechanism is a combination of tensile failure on one plane and shear failure on the perpendicular plane. The design strength equations for the tension and shear block failure are described in the well known design codes. However, they provide inconsistent levels of safety when they are used in different types of connections. In this paper, the authors survey the design strength equations in the various codes. Then, the pertinent strength equations for the gusset plates of steel truss bridges are selected. Moreover, the authors propose a pair of strength equations for compression and shear block failure for gusset plates subjected to compressive force. And to examine the applicability of proposed equations and to investigate the mechanical behavior of compression gusset plates, parametric analyses on the various thicknesses of gusset plates were conducted.     

Cyclic responses of three 2-story seismic concentrically braced frames

Three full scale two-story steel concentrically braced frames (CBFs) were tested at the National Center for Research on Earthquake Engineering (NCREE) in Taipei. The specimen is a single bay with the braces arranged in a two-story X-brace configuration. The main differences among the three tests are the brace types (hollow structural or wide-flange section) and the design criteria adopted for the gusset plate connections. Results of these three tests confirm that the two-story X-shape steel CBFs all have rather good energy dissipation characteristics up to a story drift of about 0.03 radians under the cyclically increasing lateral displacements. Severe brace local buckling and out-of-plane displacements were observed during each test. Tests confirm that both the 2t-linear and 8t-elliptical designs of the gusset plate connection provide satisfactory ductility for the steel CBF. Hollow structural section (HSS) braces fractured at a story drift smaller than that found using wide flange sections. The nonlinear fine element method (FEM) program ABAQUS was used to simulate the responses of the specimen. The base shear versus the story drift relationships obtained from the tests and the FEM analytical results are quite agreeable in various levels of lateral frame displacement. The analytical results confirm that the severe out-of-plane buckling of the braces can be accurately simulated. FEM analyses also illustrate that the steel moment resisting frame takes about 40% story shear when the inter-story drift is greater than 0.02 radians.     

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 responses of three 2-story seismic concentrically braced frames

Abstract Three full scale two-story steel concentrically braced frames (CBFs) were tested at the National Center for Research on Earthquake Engineering (NCREE) in Taipei. The specimen is a single bay with the braces arranged in a two-story X-brace configuration. The main differences among the three tests are the brace types (hollow structural or wide-flange section) and the design criteria adopted for the gusset plate connections. Results of these three tests confirm that the two-story X-shape steel CBFs all have rather good energy dissipation characteristics up to a story drift of about 0.03 radians under the cyclically increasing lateral displacements. Severe brace local buckling and out-of-plane displacements were observed during each test. Tests confirm that both the 2t-linear and 8t-elliptical designs of the gusset plate connection provide satisfactory ductility for the steel CBF. Hollow structural section (HSS) braces fractured at a story drift smaller than that found using wide flange sections. The nonlinear fine element method (FEM) program ABAQUS was used to simulate the responses of the specimen. The base shear versus the story drift relationships obtained from the tests and the FEM analytical results are quite agreeable in various levels of lateral frame displacement. The analytical results confirm that the severe out-of-plane buckling of the braces can be accurately simulated. FEM analyses also illustrate that the steel moment resisting frame takes about 40% story shear when the inter-story drift is greater than 0.02 radians.     

屈曲约束支撑与钢框架节点板连接的抗震性能#br# 试验与分析研究

屈曲约束支撑作为耗能减震构件,其与钢框架连接形成屈曲约束支撑钢框架结构体系。然而目前对于屈曲约束支撑与节点板不同连接形式的抗震性能和破坏模式尚缺乏研究。为了获悉不同连接形式对屈曲约束支撑钢框架结构抗震性能和破坏机理的影响,进行5榀屈曲约束支撑与钢框架节点板连接试件的水平低周往复荷载试验,观察试验现象和破坏特征,考察屈曲约束支撑与节点板两端采用销轴连接、螺栓连接、焊接连接和混合连接对钢结构抗震性能的影响,研究屈曲约束支撑与钢框架节点板连接试件的滞回曲线、骨架曲线、延性系数、刚度退化、耗能能力等抗震性能指标,探讨屈曲约束支撑与钢框架节点板转动变形和关键部位的应变规律,分析结构的破坏模式和各构件屈服顺序。结果表明：屈曲约束支撑的芯板先于梁、柱和节点板屈服,试件滞回曲线饱满,表现出良好的抗震性能和延性。文章研究成果以期为屈曲约束支撑钢框架结构设计和应用提供科学依据。     

Experimental study on seismic performance of concrete‐filled steel tabular frame‐shear wall structure with buckling‐resistant braces

Three specimens of concrete‐filled steel tubular (CFST) frame‐shear wall structures with a scaling ratio of 1:4 were designed and tested in the present study. Two of them were installed with triple‐steel tube buckling‐resistant braces (BRBs). The seismic performances of the specimens were evaluated by testing them under lateral cyclic loading with constant axially compressive load being applied on the tops of the columns and the shear wall. The structural performances, such as failure characteristics, hysteretic behaviour, skeleton curve, strength degradation, stiffness degradation, energy dissipation capacity and strains at different locations of the three specimens, were measured and analysed in detail. The results showed that the load‐bearing capacity, the deformation capacity and the energy dissipation of the CFST frame‐shear wall structure were significantly improved due to the dissipation capacity of the BRBs, with the strength and stiffness degradation being obviously reduced. The results also showed that the CFST frame‐shear wall structure with BRBs has preferable mechanical behaviour and more reasonable failure mode. It was verified that the BRB can be used to improve the seismic performance of the CFST frame‐shear wall structure. Copyright © 2014 John Wiley & Sons, Ltd.     

人字形支撑与横梁板式连接节点滞回试验的数值模拟

支撑板式连接节点是铰接钢架节点的重要连接形式,它直接影响着框架支撑结构体系的抗震性能。采用ANSYS有限元软件对9个工字型截面人字形支撑板式连接节点试件的滞回性能试验进行了数值模拟。在验证有限元结果与试验结果较好吻合的同时,研究了支撑轴线交点位置、支撑端部净距和节点板厚度等重要参数对支撑体系的承载力、滞回耗能性能和节点板等效塑性应变的影响规律,所得结论可为板式连接节点的设计与应用提供依据。     

Influence of connection design parameters on the seismic performance of braced frames

Special concentrically braced frames (SCBFs) are commonly used lateral-load resisting systems in seismic design. In SCBFs, the braces are connected to the beams and columns by gusset plate connections, and inelastic deformation is developed through tensile yielding and inelastic post-buckling deformation of the brace. Recent experimental research has indicated that the seismic performance of SCBFs can be improved by designing the SCBF gusset plate connections with direct consideration of the seismic deformation demands and by permitting yielding in the gusset plate at select performance levels.Experimental research provides important information needed to improve SCBF behavior, but the high cost of experiments limits this benefit. To extend and better understand the experimental work, a companion analytical study was conducted. In an earlier paper, the inelastic finite element model and analysis procedure were developed and verified through detailed comparison to experimental results. In this paper, the model and analytical procedure extend the experimental results. A parametric study was conducted to examine the influence of the gusset plate and framing elements on the seismic performance of SCBFs and to calibrate and develop improved design models. The impact of the frame details, including the beam-to-column connections, the brace angles, and their inelastic deformation demands, was also explored. The results suggest that proper detailing of the connections can result in a large improvement in the frame performance.     

屈曲约束支撑装配式混凝土框架结构抗震性能试验研究

消能减震技术能有效地提高装配式混凝土结构的整体抗震性能,改善其破坏模式。然而,目前对屈曲约束支撑装配式混凝土框架结构尚缺乏试验研究。为了获悉屈曲约束支撑装配式混凝土框架的抗震性能和破坏机理,文章进行3榀屈曲约束支撑装配式混凝土框架和1榀现浇混凝土框架的低周反复荷载试验。研究了地震作用下采用整体式和暗牛腿式预制混凝土梁柱节点的屈曲约束支撑装配式混凝土框架抗震性能,观察和记录试验现象和破坏特征,对各试件的滞回曲线、刚度退化、耗能能力、骨架曲线和延性系数等进行对比分析。试验结果表明：屈曲约束支撑装配式混凝土框架具有良好的耗能能力和延性,能量耗散系数E=1.464?1.759,延性系数?=3.02~3.61,屈曲约束支撑可以有效地提高装配式混凝土框架的抗震性能,改善其失效模式;研发的支撑与梁柱连接节点的连接构造可以有效地实现屈曲约束支撑与装配式混凝土框架在地震作用下的协同受力。文中研究成果将为屈曲约束支撑在多高层装配式混凝土结构中的设计和应用提供科学依据。