Experimental and numerical study of buckling-restrained braces configured with out-of-plane eccentricity under cyclic loading

This study focuses on variations in the hysteretic behavior of buckling-restrained braces(BRBs)configured with or without out-of-plane eccentricity under cyclic loading.Quasi-static experiments and numerical simulations were carried out on concentrically and eccentrically loaded BRB specimens to investigate the mechanical properties,energy dissipation performance,stress distribution,and high-order deformation pattern.The experimental and numerical results showed that compared to the concentrically loaded BRBs,the stiffness,yield force,cumulated plastic ductility(CPD)coefficient,equivalent viscous damping coefficient and energy dissipation decreased,and the yield displacement and compression strength adjustment factor increased for the eccentrically loaded BRBs.With the existence of the out-of-plane eccentricity,the initial yield position changes from the yield segment to the junction between the yield segment and transition segment under a tensile load,while the initial high-order buckling pattern changes from a first-order C-shape to a secondorder S-shape under a compressive load.

Ductility demands on buckling-restrained braced frames under earthquake loading

Accurate estimates of ductility demands on buckling-restrained braced frames(BRBFs)are crucial to performance-based design of BRBFs.An analytical study on the seismic behavior of BRBFs has been conducted at the ATLSS Center,Lehigh University to prepare for an upcoming experimental program.The analysis program DRAIN-2DX was used to model a one-bay,four-story prototype BRBF including material and geometric nonlinearities.The buckling- restrained brace(BRB)model incorporates both isotropic and kinematic hardening.Nonlinear static pushover and time- history analyses were performed on the prototype BRBF.Performance objectives for the BRBs were defined and used to evaluate the time-history analysis results.Particular emphasis was placed on global ductility demands and ductility demands on the BRBs.These demands were compared with anticipated ductility capacities.The analysis results,along with results from similar previous studies,are used to evaluate the BRBF design provisions that have been recommended for codification in the United States.The results show that BRB maximum ductility demands can be as high as 20 to 25.These demands significantly exceed those anticipated by the BRBF recommended provisions.Results from the static pushover and time- history analyses are used to demonstrate why the ductility demands exceed those anticipated by the recommended provisions. The BRB qualification testing protocol contained in the BRBF recommended provisions is shown to be inadequate because it requires only a maximum ductility demand of at most 7.5.Modifications to the testing protocol are recommended.

Development and subassemblage cyclic testing of hybrid buckling-restrained steel braces

Buckling-restrained braced frames(BRBFs) are vulnerable to relatively higher post-earthquake residual drifts under high intensity ground shakings. This is primarily due to the low axial elastic and post-elastic stiffness of bucklingrestrained braces(BRBs) satisfying the design force demand requirements. In the present study, a hybrid buckling restrained bracing system consisting of a short yielding core length BRB component and a conventional buckling-type brace component connected in series has been developed with an aim to increase the axial stiffness of braces. This study is focused on the experimental investigation of six hybrid bucking restrained braces(HBRBs) to investigate their overall behavior, loadresisting capacity, strength-adjustment factors and energy dissipation potential. The main parameters varied are the crosssectional area, the yielding length of core elements as well as the detailing of buckling-restraining system of short yielding core length BRBs. Test results showed that the HBRBs with yielding core length in the range of 30% of work-point to workpoint lengths withstood an axial strain of 6% without any instability and can deliver stable and balanced hysteretic response and excellent energy dissipation under reversed cyclic loading conditions.

Application of buckling-restrained braces in the seismic control of suspension bridges

Buckling-restrained braces(BRBs)are widely used to improve the seismic performance of buildings.This paper aims to introduce BRBs to suspension bridges and assess the seismic performance of bridges with BRBs.Taking the Dadu River Bridge as a case study,an FEA model of the bridge is established,and different seismic measures(BRBs between the deck and the tower,BRBs at the middle of the span to replace the inclined suspenders to connect the deck and the main cables,fluid viscous dampers(FVDs)between the deck and the tower,the combination of BRBs to replace the inclined suspenders as well as FVDs between the deck and the tower)are applied to the suspension bridge.The influence of the parameters of BRBs on the seismic response of the suspension bridge is studied,and the performance of the bridge with BRBs is compared with that of the bridge with FVDs.The results indicate that the use of BRBs in place of the inclined suspenders is beneficial to reduce the displacement of the deck and limit the shear force and bending moment of the tower.The seismic performance of the suspension bridge with BRBs and FVDs is better than that of the bridge with BRBs or FVDs.Therefore,the application of BRBs is a feasible method to improve the seismic performance of the suspension bridge.

Assessment of buckling-restrained braced frame reliability using an experimental limit-state model and stochastic dynamic analysis

Buckling-restrained braces （BRBs） have recently become popular in the United States for use as primary members of seismic lateral-force-resisting systems. A BRB is a steel brace that does not buckle in compression but instead yields in both tension and compression. Although design guidelines for BRB applications have been developed, systematic procedures for assessing performance and quantifying reliability are still needed. This paper presents an analytical framework for assessing buckling-restrained braced frame （BRBF） reliability when subjected to seismic loads. This framework efficiently quantifies the risk of BRB failure due to low-cycle fatigue fracture of the BRB core. The procedure includes a series of components that： （1） quantify BRB demand in terms of BRB core deformation histories generated through stochastic dynamic analyses; （2） quantify the limit-state of a BRB in terms of its remaining cumulative plastic ductility capacity based on an experimental database; and （3） evaluate the probability of BRB failure, given the quantified demand and capacity, through structural reliability analyses. Parametric studies were conducted to investigate the effects of the seismic load, and characteristics of the BRB and BRBF on the probability of brace failure. In addition, fragility curves （i.e., conditional probabilities of brace failure given ground shaking intensity parameters） were created by the proposed framework. While the framework presented in this paper is applied to the assessment of BRBFs, the modular nature of the framework components allows for application to other structural components and systems.

Stability Performance of Buckling-Restrained Brace with Brace Joints

The stability and ductility of four buckling-restrained braces (BRBs) with brace joints were studied. The load-carrying element of BRB was fabricated with steel (Chinese Q235), and a layer of colloidal silica sheet (0.5 mm in thickness) or four layers of plastic film (0.2 mm in thickness) were used as unbonding materials to provide space to prevent the buckling of inner core in higher modes and facilitate its lateral expansion in case of compression. Based on the equation of BRBs with brace joints of different restrained stiffnesses, the buckling load is calculated considering the initial geometric imperfections and residual stress, and the theoretical values agree well with the experiment results. It is concluded that the buckling load and ductility of BRBs are influenced greatly by the restrained stiffness of brace joints. If the restrained stiffness is deficient, the unstrained segment of BRBs with less stiffness will buckle firstly. As a result, the ultimate load of BRBs decreases, and the maximum compression load is reduced to about 65% of the maximum tension load; the stiffness also degenerates, and there is a long decreasing stage on the back-bone curve in compression phase; the ductility decreases, i.e., the ultimate tension ductility and ultimate compression ductility are approximately 15 and 1.3 respectively, and the cumulative plastic ductility is only approximately 200. If the restrained stiffness of joint is large enough, the stability will be improved as follows: the yielding strength and ultimate strength of BRBs are nearly the same, and there is an obvious strain intensification in both tension and compression phases; the ductility of brace also increases obviously, i.e., the ultimate tension ductility and ultimate compression ductility are both approximately 14, and the cumulative plastic ductility reaches 782.

Experimental study on slender buckling-restrained knee braces with round steel bar cores

This study aimed to investigate a novel slender buckling-restrained knee brace damper (BRKB) for welded and weld-free steel framing systems. The proposed BRKB adopts steel bar cores connected by a central coupler and restrained by tube buckling restrainers with a cover tube supporter. The advantages of the proposed damper include easy assembly compared to conventional buckling restrained braces, and high architectural flexibility for the retrofitting of large-span weld-free or welded steel moment-resisting systems. Specifically, by increasing the number of contraction allowances, undesirable failure mechanisms that are global instability and local buckling of the restrainer ends can be effectively suppressed because the more uniform plastic deformation of the core bar can be achieved longitudinally. In this study, displacement-controlled compression and cyclic loading tests were carried out to investigate the deformation capacities of the proposed BRKBs. Structural performance metrics associated with both loading tests, such as strength capacities, strains at the cover tubes and buckling restrainers, and hysteretic behaviors of the proposed damper under cyclic loads, were measured and discussed. Test results revealed that the geometrical characteristics of the cover tubes and adopted contraction allowances at the dampers play essential roles in their load-bearing capacities.

Seismic performance of a reinforced concrete building retrofitted with self-centering shape memory alloy braces

Self-centering earthquake-resistant structures have received increased attention due to their ability to reduce post-earthquake residual deformations and,thus,repair time and cost.This stimulated the development of recentering shape memory alloy(SMA)dampers that use superelastic nitinol wires to dissipate energy and self-center the structure.However,there are still a few case studies applications on full-scale RC buildings in the literature.Moreover,general guidelines or even simplified approaches for the practical design of SMA damped braces are still lacking.This paper focuses on evaluating the effect of using self-centering shape memory alloy dampers for buckling-restrained braces applied for the seismic retrofit of a complex RC building structure.A design method originally proposed for elastoplastic dampers was implemented to size the SMA dampers to be placed on selected spans and stories of a building.The effectiveness of the design procedure was demonstrated by nonlinear time-history analyses under different sets of earthquake strong ground motions.The analysis results show that the recentering shape memory alloy bracing system is effective in limiting the maximum transient inter-story drifts and reducing the residual inter-story drifts after strong seismic events,due to its excellent recentering behavior together with its not negligible energy dissipation capacity.

Inelastic displacement ratio of low-to mid-rise BRBFs designed under variable levels of seismicity

Buckling-restrained braces(BRBs)have shown their capability to provide building structures with stiffness,strength,and ductility.Estimating the seismic drifts of buckling-restrained braced frames(BRBFs)is an important design step to control structural and non-structural damage.In current practice of seismic design,the estimation of seismic drifts of BRBFs is performed by using empirical calculations that are independent upon either the type of the structural system or the design level of seismicity.In these empirical calculations,the seismic drifts are estimated by amplifying the reduced elastic drifts obtained under design lateral loading with a displacement amplification factor(DAF).The value of DAF is considered equal to the product of the response modification factor R and the inelastic displacement ratioρ.The goal of the current research is to assess the value ofρfor low-to mid-rise BRBFs designed under low and high levels of seismicity.This goal has been achieved by conducting a series of elastic and inelastic time-history analyses pertaining to an ensemble of earthquake records on 3-,6-and 9-story BRBFs.The results indicate that theρ-ratio increases with an increase in design seismic intensity and an increase in experienced inelasticity.The range ofρfor low seismicity designs ranges from 0.63 to 0.9,while for high seismicity designs this range stretches from 0.83 to 1.29.It has been found that the consideration of a generalρ-ratio of 1.0 is a reasonable estimation for the design of the BRBFs considered in this study.

Experimental study of the pinned double rectangular tube assembled buckling-restrained brace

In this study, seven pinned double-rectangular tube assembled buckling-restrained brace （DRT-ABRB） specimens were experimentally charaeterised by means of an axial cyclic test. The core member of the specimens was a single flat-plate. Two rectangular tubes were assembled using high strength bolts to form an external restraining member. Each rectangular tube was composed of an external steel channel and a cover plate. A gap or thin rubber filler was set between the core and the external restraining member to form an unbonded layer. The influence of several design parameters on the failure mode and energy dissipation capacity of the ABRB was investigated, including the height of the core wing plate, thickness of the external cover plate, and height of the external channel flange. This experimental study demonstrated that a local pressure-bearing failure at the end of the external member arises when the external cover plate is too thin or if the end construction detail is unreasonable. When the end rotations of the DRT-ABRB were restricted, the hysteretic performance was shown to be superior to that of a pure pinned DRT-ABRB. Finally, all the tested DRT-ABRBs exhibited excellent energy dissipation performance which amply satisfied existing regulation requirements.

基坑工程装配式钢管混凝土内支撑体系设计方法

为了解决常用基坑内支撑刚度不足、拆装不便等问题,针对明挖地铁车站基坑工程特点提出装配式钢管混凝土内支撑体系,包括标准直撑、短接直撑及连接节点等.根据该支撑体系受力和结构特点,推导了各组成部分的内力计算公式;依据钢管混凝土规范和压杆稳定欧拉公式,建立了钢管混凝土支撑、围檩等部件的承载力计算方法;通过混凝土材料的配合比设计及工作、物理、力学性能检测试验,配制出了具有轻质、微膨胀和自密实性能的CL50高强混凝土作为钢管混凝土内支撑的填充材料;以北京地铁某车站基坑工程设计资料为依据,对其标准段进行了详细的支撑体系设计,并对基坑变形、稳定性和支撑受力进行了验算.研究结果表明:所提出的内支撑体系设计计算方法合理可靠,所设计的装配式钢管混凝土内支撑体系能满足该基坑工程内支撑体系受力和基坑稳定的要求.

开孔钢板装配式屈曲约束支撑设计方法研究

文章结合开孔钢板装配式屈曲约束支撑研究成果和相关规范,提出开孔钢板装配式屈曲约束支撑的设计方法,给出其设计流程,并通过4个开孔钢板装配式屈曲约束支撑的拟静力试验对设计方法与流程的合理性和有效性进行验证。试验结果表明提出的开孔钢板装配式屈曲约束支撑设计方法与流程合理有效,可供实际工程使用;根据设计方法与流程得到的开孔钢板装配式屈曲约束支撑力学性能实测值与设计值吻合较好、整体与局部稳定性好,滞回性能稳定、滞回曲线饱满、耗能能力强、承载力对称性和塑性变形能力良好、抗疲劳性能稳定。

新型装配式屈曲约束支撑试验研究

为了克服普通灌浆型屈曲约束支撑加工周期较长、钢结构加工和填充料加工交叉作业等缺点,提出了一种新型装配式屈曲约束支撑;该支撑采用填充料分段预制,然后与支撑芯板及外套筒组合拼装的加工方式,使得钢结构部分和灌浆料部分同时进行加工,简化了支撑加工工序,缩短了加工周期,降低了支撑制造过程中的人工费用。并对该新型屈曲约束支撑进行了往复加载试验,对试验数据进行分析处理。结果表明:此新型装配式屈曲约束支撑具有很好的滞回特征和耗能性能,各项指标均符合《建筑抗震设计规范》对位移相关型消能器的性能指标要求,是一种十分有效的耗能构件。

骨科多功能床头牵引架的制作与应用

目的 制作一种能满足骨科临床牵引需要的床头牵引器械。方法 参考骨科常见牵引中所要求的牵引力大小、牵引调节等多方面因素 ,结合临床中简便易用的原则 ,创作出一种由“H”字形支架、套芯支架和移动滑轮三部分组成的骨科多功能床头牵引架 ,它由不锈钢材料制成 ,既能满足大重量的牵引需要 ,同时又能对牵引方向进行上下和左右水平方向的大范围的调整。结果 经临床应用显示 ,该多功能床头牵引架简便实用 ,能基本满足骨科临床牵引需要 ,同时为创伤急救及危重患者的全方位治疗提供了保障。结论 该多功能床头牵引架是一种经济实用、简便。

北京冬季奥运村人才公租房项目结构抗震性能分析

为研究采用钢框架-防屈曲钢板墙结构的北京冬季奥运村人才公租房项目的抗震性能,建立了反映两边连接防屈曲钢板墙受力特点的简化模型——等效交叉支撑模型,介绍了等效交叉支撑模型中初始刚度、单根支撑的等效截面面积、支撑屈服强度等关键参数的计算方法。选取典型户型结构,建立钢框架-防屈曲钢板墙结构整体精细化有限元计算模型,研究该双重抗侧力体系的变形特点、构件失效顺序、塑性发展模式、内力分配关系以及结构刚度退化过程;对防屈曲钢板墙在结构中的消能减震能力进行评价。通过该类结构抗震性能的研究分析表明,结构满足“大震不倒”的抗震设防目标。

装配式基坑支护结构在广州地区的应用

装配式基坑支护结构是将基坑支护的围护结构部分在工厂预制成型,再在现场与撑或锚装配组成的支护结构。工程实践表明,此项技术可有效节省工程投资、缩短工期。

三维织物复合材料在运动护具上的应用前景

以三维织物为基材的增强复合材料在航天、军事等领域发挥着重大作用,但是,它在体育运动领域的应用研究和发展相对滞后。从三维织物复合材料结构的合理性、织造技术的多样性和复合固化技术的先进性,说明三维织物复合材料具有力学性能优异、抗冲击性好、强度高和重量轻等特点,在织物结构和高性能纤维运用及织造制备方法上具有很大的优势;介绍三维织物复合材料在运动型护具、技击性运动护具和抗冲击性运动服装上的应用前景,新型运动护具在运动中能更好地保护运动员,随着三维织物复合材料在运动上的广泛应用,将会对体育运动的发展产生深远影响。

圆支管端部压扁的节点制作与设计

钢管桁架结构中采用圆支管端部压扁与主管连接的节点,具有制造简单、安装方便的特点,非常适用于小型或临时建筑结构。本文综述了楔形压扁、全压扁和部分压扁等几种常用的支管端部压扁的形式、特点和其制作工艺,介绍了目前国外对支管端部压扁的N型、K型、T型和X型节点强度的研究工作和相关的计算方法,可供结构工程师设计时参考,以期促进支管端部压扁节点的钢管桁架结构在我国的应用。

基于边缘屈服准则装配式屈曲约束支撑分析与设计

基于芯材边缘屈服准则和弯曲屈曲理论,建立装配式屈曲约束支撑极限应力、支撑间隙及芯材截面的计算公式。以十字型芯材支撑为例,分析芯材屈曲半波数对支撑极限应力,支撑间隙和芯材截面的影响。研究结果表明,基于芯材边缘屈服准则和弯曲屈曲理论的分析理论可以合理解释试验和有限元分析结果 ,所建立的相关计算公式可用于装配式屈曲约束支撑的设计中。

屈曲约束支撑的缺陷影响分析

对屈曲约束支撑在施工过程中容易产生的套管与核心部分间空隙尺寸偏差和核心板厚偏差进行研究。借助ANSYS有限元软件,分析这两类施工缺陷对屈曲约束支撑临界荷载的影响,得出了缺陷影响的曲线和图表,结果表明空隙尺寸偏差和板厚偏差对支撑的弹性临界荷载影响明显,设计和施工过程应该严格控制。另外,文中提供的曲线图表可应用于设计中预估屈曲约束支撑临界荷载,应用有限元方法可拓展屈曲约束支撑的试验研究,减少试验成本。