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.     

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.     

Plastic design of CB-frames with reduced section solution for bracing members

The seismic behaviour of concentrically braced frames (CBFs) designed according to the current European provisions is unsatisfactory due to the premature out-of-plane buckling of columns. For this reason, a new design methodology, based on a rigorous application of “capacity design” criteria has been recently proposed. In addition, aiming at a reduction of the plastic out of plane deformations of gusset plates due to brace buckling and at the prevention of sudden impact load affecting connections at the end of the straightening phase, Eurocode 8 requires the limitation of the brace slenderness. This limitation leads to the oversizing of diagonals and, consequently, of beams and columns. Therefore, to avoid this problem a new design strategy for bracing members is suggested: the Reduced Section Solution (RSS). It allows the calibration of the diagonal yielding resistance, leaving the brace slenderness practically unchanged.The results of dynamic inelastic analyses carried out with reference to braced frames designed according to the proposed procedure, both with and without RSS, are compared with those obtained with reference to the same structural schemes designed according to Eurocode 8. The obtained results show that the proposed design approaches are able to assure a significant improvement of the seismic performance.     

端部配置延性铸造件的装配式防屈曲支撑抗震性能试验研究

中心支撑钢框架结构(CBFs)中支撑与框架梁柱连接时,采用传统焊接连接方式容易发生脆性破坏,使支撑不能发挥应有的作用.为此,设计了一种可与框架结构采用螺栓连接的端部配置延性铸造件的装配式防屈曲支撑,并对6个支撑完成了低周往复加载试验,研究了其破坏模式、应力分布、滞回性能等.结果 表明,端部配置延性铸造件的装配式防屈曲支...     

Seismic design of concentric braced frames

In this paper some modifications to the design procedure, currently implemented in the modern European seismic code for ductile cross concentric braced frames (X-CBFs), are proposed. The code procedure is aimed to obtain a ductile and dissipative ultimate behaviour by imposing that the yielding of diagonal members occurs before the damage and premature failure of beams, columns and connections (capacity design); this approach, involving overstrength requirements and diagonal slenderness limitations, strongly affects the design of CBFs and generally leads to oversized structural solutions, thus suggesting a high weight premium related to the capacity design. The approach proposed by the authors in this paper consists of some modifications to the current design provisions of the European seismic codes, with the major aim of controlling the overstrength requirements to the non-dissipative members of braced frames, thus reducing the associated structural weight premium while preserving a satisfactory inelastic behaviour. In order to assess the reliability of the proposed approach, the results of non-linear FE analyses are presented in the paper with reference to three, and six‐ and nine‐storey buildings, for which different structural solutions are designed according to the current and the proposed approaches.     

人字形延性中心支撑钢框架的抗震设计与分析

讨论美国钢结构抗震设计规程中人字形延性中心支撑钢框架的抗震设计和分析。在强震作用下的基本设计理念为:钢支撑框架中的塑性变形集中于支撑上,通过支撑受拉屈服和受压屈曲来消耗能量,其他结构构件需保持弹性。结合1栋9层支撑钢框架,阐述美国钢结构抗震设计规程的相关具体规定,并得到设计建议:支撑截面的选择受到局部宽厚比的限制;框架梁截面的尺寸由强度控制,主要考虑由于支撑受拉屈服和受压屈曲所形成的竖向不平衡力的作用;框架柱截面的尺寸由考虑所有受压支撑移去时的结构模型控制。     

防屈曲支撑-钢筋混凝土框架结构基于能量平衡的抗震塑性设计

为确定防屈曲支撑在结构中的布置方式以使结构抗震性能充分发挥,提出了基于能量平衡的防屈曲支撑 钢筋混凝土框架结构抗震塑性设计方法。构建了结构的“强柱弱梁”整体屈服机制,采用侧力比将总结构体系离散为防屈曲支撑体系和纯框架体系,并建立了结构的双线性能力曲线。基于能量平衡方法计算结构的设计基底剪力并分别得到支撑体系和框架体系的设计侧向力,进而完成支撑的截面设计。按照塑性内力分配机制和考虑支撑屈服后性能,计算梁柱构件内力需求。以一幢5层结构为例,分别设计了不同侧力比的14个结构模型,对比了基底剪力、防屈曲支撑面积和梁柱钢筋用量等。通过22条地震波下的弹塑性时程分析,研究了不同侧力比结构的最大层间位移角、屈服机制、楼层剪力比、支撑最大位移延性、累积位移延性和结构残余层间位移角。分析结果表明：所提出的方法能实现结构的预期失效模式,并满足结构的抗震性能要求,并建议设计侧力比选取在0.3~0.5之间。     

轻质组装墙板内置钢板支撑滞回性能试验研究

为改善墙板内置钢板支撑的延性,避免钢筋混凝土墙板局部冲切破坏,便于检修内置支撑和减小墙板自重,提出了轻质组装墙板。通过对6个组装墙板内置钢板支撑的试验研究,考察了支撑和墙板的厚度、支撑与墙板间的间隙等构造对支撑滞回性能的影响。试验表明,轻质组装墙板内置Q235钢板支撑具有良好的延性和耗能能力。总体上,墙板内置支撑破坏前骨架曲线呈双折线,支撑屈服后因钢材应变硬化以及支撑和墙板间摩擦等因素,支撑的承载力随侧移的增加而增大。达最大侧移角约1/25时,受拉承载力调整系数范围为1.36～1.61。侧移角在1/25以内时,受压承载力调整系数均小于13,支撑的轴向累积非弹性变形能力远大于200,均满足美国ANSI/AISC 341 16的要求。试件最终因内置支撑受拉断裂而破坏,破坏前滞回曲线饱满稳定。组装墙板保持完好,可重复利用。支撑与墙板间留置适宜间隙后,受压支撑在墙板孔壁内仅发生微幅多波弯曲变形,避免了墙板局部破坏。当仅考虑支撑附件的主钢管和开孔钢板简化计算墙板绕钢板支撑弱轴的欧拉临界力,墙板的欧拉临界力与内置支撑的最大轴向受压承载力之比(约束比)达1.15~2.42,墙板内置支撑不发生受压整体失稳。     

开孔钢板装配式屈曲约束支撑钢框架抗震性能试验研究

为研究开孔钢板装配式屈曲约束支撑（buckling-restrained brace,BRB）钢框架的抗震性能及框架平面外变形对其抗震性能的影响,对两个相同设计的单层单跨单斜式开孔钢板装配式BRB钢框架分别就是否考虑框架平面外变形情况下进行了拟静力试验,并对相同设计的开孔钢板装配式BRB构件进行了拟静力试验。结果表明：不考虑框架平面外变形和考虑框架平面外变形10mm的开孔钢板装配式BRB钢框架均表现出良好的滞回耗能性能,滞回曲线饱满且基本对称,满足GB 50011—2010《建筑抗震设计规范》最大弹塑性层间位移角1/50的限值要求;框架平面外变形10mm对开孔钢板装配式BRB钢框架平面内抗震性能影响很小,其弹性水平刚度、层间屈服剪力和层间最大剪力受框架平面外变形的影响略为降低,变化范围均在5%以内;框架平面外变形10mm对BRB轴向变形的影响很小,框架中开孔钢板装配式BRB和开孔钢板装配式BRB构件均具有良好的滞回性能,约在1/720层间位移角时先于钢框架进入屈服状态,发挥耗能作用,其滞回曲线饱满,延性良好,累积塑性变形能力系数均大于600,完全满足ANSI /AISC 341-10中要求的大于200的要求。     

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.     

Evaluating response modification factors of concentrically braced steel frames

Response modification factor is one of the seismic design parameters to consider nonlinear performance of building structures during strong earthquake. Relying on this, many seismic design codes led to reduce loads. The present paper tries to evaluate the response modification factors of conventional concentric braced frames (CBFs) as well as buckling restrained braced frames (BRBFs). Since, the response modification factor depends on ductility and overstrength, the static nonlinear analysis has been performed on building models including single and double bracing bays, multi-floors and different brace configurations (chevron V, invert V and X bracing). The CBFs and BRBFs values for factors such as ductility, overstrength, force reduction due to ductility and response modification have been assessed for all the buildings. The results showed that the response modification factors for BRBFs were higher than the CBFs one. It was found that the number of bracing bays and height of buildings have had greater effect on the response modification factors.     

低延性中心支撑钢框架结构振动台试验

中心支撑钢框架结构是一种典型的双重抗侧力体系，强震作用下支撑失效会引起结构承载力和刚度的折减，支撑失效后，结构剩余部分作为储备体系能够继续承担地震作用。为深入了解低延性中心支撑钢框架结构在地震作用下的非线性反应，研究在支撑失效后结构储备体系的抗震性能，开展了3层中心支撑钢框架结构模型的振动台试验。模型结构为3榀2跨结构，中间榀布置一跨低延性人字形中心支撑(截面宽厚比超出我国规范限值)，模型长度缩尺比为1∶6.5，分别采用硬土、软土场地的地震波单向激励，峰值加速度逐级增加。结构在7度罕遇地震作用下发生底层支撑失效，储备体系避开了硬土场地地震波的卓越周期，加载至超过9度罕遇地震后依然未出现明显损伤。储备体系对软土场地地震波(宁河波)更为敏感，在8度罕遇地震作用下濒临倒塌。结构2、3层支撑始终未发生屈曲和破坏。研究结果表明，当储备体系设计合理时，低延性中心支撑钢框架结构具有良好的抗震性能和抗倒塌能力。     

《钢结构设计规范》修订的建议

本文介绍了"对柱计算长度系数的一点认识","梁柱连接计算方法的改进","日本连接系数新规定","支撑设计方法的修订","忽略偏心弯矩的设计"和"节点域剪切变形计算",供修订钢结构设计规范参考.     

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.     

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。     

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.     

Capacity design for composite partially restrained steel frame‐reinforced concrete infill walls with concealed vertical slits

This paper presents an innovative capacity‐based design procedure that aims to achieve the ideal seismic performance for the composite partially restrained (PR) steel frame‐reinforced concrete (RC) infill wall with concealed vertical slits (PSRCW‐CVS). The proposed method adopts the direct capacity design principles and preselected preferred plastic mechanism such that the RC infill wall undergoes ductile failure prior to the other steel components in the event of a rare‐level earthquake (i.e., earthquake with a 2% probability of exceedance in 50 years). Based on the ultimate resisting capacity of RC infill walls, the free‐body diagrams and simplified design formulae for the surrounding steel components, including the vertical boundary element (VBE), horizontal boundary element (HBE), PR connection, and shear connectors, were proposed. To demonstrate the reasonability of the capacity‐based design procedure, a five‐story PSRCW‐CVS structure was designed according to the proposed design method, followed by a series of nonlinear time history analyses. The overall seismic response of this example was evaluated in terms of story displacement, interstory drift ratio, residual story displacement, and residual interstory drift ratio. The proposed method yielded a more uniform interstory drift ratio distribution along the height of the five‐story PSRCW‐CVS structure. Structural damage was controlled by achieving the preselected preferred plastic mechanism.     

Earthquake testing and response analysis of concentrically-braced sub-frames

The results of shake table tests on three concentrically-braced sub-frames are compared with a series of correlative inelastic analyses. Both transient time-history and nonlinear static (pushover) analyses are considered. The lateral resistance of the test frames is provided by a pair of cold-formed tubular steel members. The brace cross-section is varied between tests to investigate the influence of brace slenderness on the stiffness, resistance and ductility displayed by the frame under strong earthquake loading. Response simulations using a two-dimensional analytical model of the test frame are compared with the experimental results. Brace strengths and elastic test frame properties established in complementary static tensile tests and low-level shake table tests, respectively, are used to determine model input data. The transient acceleration and displacement response of the frames calculated in nonlinear time-history analyses are compared with corresponding experimental measurements. The base shear-frame drift relationship is calculated using pushover analyses that consider either the resistance of both frame braces or the tension brace alone. These are compared with the experimental hysteretic response of the test frame. On the whole, the analytical results are observed to agree well with the experimental results, especially with respect to brace tension forces and frame base shear. Cases where the experimental and analytical responses deviate are identified and the implications for earthquake response assessment and design are discussed.     

防屈曲支撑、普通和特殊中心支撑钢框架结构抗震性能分析

基于考虑人字形防屈曲支撑屈服后超强和几乎不再对被撑梁提供竖向支点作用这两个因素,本文提出了采用该种支撑的钢框架结构的设计方法,并分别对采用普通及特殊中心支撑和防屈曲支撑的框架结构的抗震性能进行了对比分析.结果表明,虽然防屈曲和特殊中心支撑框架结构的层间侧移总体上大于普通中心支撑框架结构,但前者的基底剪力却大大低于后者.罕遇地震下,三种结构中的柱子基本保持弹性,普通和特殊中心支撑出现了大幅的平面外失稳,而防屈曲支撑在拉压作用下均进入屈服耗能.三种结构中被撑梁的最大挠度在支撑屈服或失稳前后分别出现在撑点两侧和撑点位置.屈服后的防屈曲支撑几乎不产生对被撑梁竖直向下的不平衡剪力,而失稳后的普通和特殊中心支撑则对被撑梁产生较大的不平衡剪力.     

Experimental study on seismic performance of steel frame- K-shaped brace structure system

为实现钢结构住宅中不凸梁凸柱的建筑效果，采用小截面钢构件组成的钢框架-K形支撑结构体系，通过K形钢支撑的设置，有效减小了构件截面尺寸。为研究钢框架-K形支撑结构体系的抗震性能，选取其中典型的抗侧力单元，制作足尺模型进行拟静力试验，并采用ABAQUS软件对其抗震性能进行有限元分析。研究结果表明：该结构体系可实现多遇地震弹性、罕遇地震不倒的设计目标，当结构位移角达1/30时仍可保持承载能力，表现出良好的抗震性能；按照斜向支撑、竖杆、框架梁柱的顺序依次屈服，斜向支撑可有效保护主体框架；位移延性系数为10.0，滞回曲线饱满；当位移角为1/50时，等效阻尼比为18.9%，耗能能力优异。