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.

防屈曲支撑框架抗震性能影响因素的模拟分析

本文旨在研究防屈曲支撑(BRB)平面布置方式和抗侧刚度比两种综合因素对防屈曲支撑钢筋混凝土框架(BRBF)的抗震性能影响,以及框架中BRB的合理布置方案。通过采用OpenSees软件建立了4、8和12层的BRBF空间模型,并进行双向地震作用下的结构弹塑性时程分析,对比分析3种BRB布置方式与5种抗侧刚度比的组合工况下结构抗震性能,基于分析结果综合判定BRB最优布置方案。结果表明:在外框架、内框架和全框架的3种支撑布置方式中地震损伤程度的控制效果分别可达30%、40%和60%,且效果随抗侧刚度比的增加而递增;BRB的设置显著增加了底柱的最大轴力,其中全框架布置方式在3种支撑方式中表现出最优的降低结构竖向反应的效果。BRB最优布置方案组合为内框架布置与刚度比1.0和全框架布置与刚度比2.5。研究成果可为每层采用相同BRB布置方式的规则钢筋混凝土框架结构的加固设计提供参考。

BRB不同布置高层钢框架BRBF静动力性能分析

以高层钢框架BRBF建筑为研究对象,以BRB不同布置形式为主要研究参量,对高层BRBF建筑进行二阶非线性分析,探讨高层BRBF建筑静动力性能,研究高层BRBF建筑中BRB最优设计方法。(1)从高层BRBF的静态持有性能来看,各高层BRBF建筑满足BRB减震及“强柱弱梁”的抗震设计要求,P-Δ效应对高层BRBF的弹塑性极限承载力和层刚度均有递减作用,忽略P-Δ效应影响会造成高层钢框架设计不安全;高层钢框架增设BRB,可改善P-Δ效应的影响;BRB交错布置的交错型建筑的稳定性好、弹塑性极限承载力值和层刚度值最大。(2)从高层BRBF的动态需求性能来看,高层钢框架增设BRB后各层的层间位移角和层间残余变形明显减小,BRB的塑性耗能占比基本在90%以上,说明BRB作为整体结构的第一道抗震防线,具备良好的耗能减震作用;同时,交错型建筑的减震、耗能效果显著。综上分析,高层钢框架BRBF结构中BRB交错布置方式最优。

三种中心支撑不同布置对平面钢框架抗侧性能影响分析

本文通过分析不同中心支撑布置对结构单元抗弯、抗剪刚度的影响,针对支撑布置水平夹角对结构单元抗剪刚度的影响,提出支撑与水平方向的夹角宜控制在30°~50°;针对三种不同中心支撑类型不同布置对结构抗侧性能的影响进行对比分析,结合SAP2000结构分析软件,计算了三种中心支撑不同布置位置下的结构最大侧移,研究了中心支撑布置对钢框架结构抗侧刚度的影响,为钢框架结构的设计提供参考。

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.

框剪结构黏滞阻尼器优化布置实用方法

高烈度地区结构设计常遇到通常会遇到竖向构件抗剪不满足、超筋、层间位移角大于规范限值等问题。近年,增设消能阻尼器的减震技术常被用来解决该问题,如通过将阻尼器在各层各向上均匀布置。阻尼器的空间分布对减震效率的影响暂未受到足够的关注,即使少数研究者已采用遗传算法等智能算法进行尝试,并得到了针对某种参数最优的布置方案。但可供优化的目标众多,且多目标优化的实现暂不成熟;另,算法的落地还需要将结构进行简化,这将建筑脱离出其应具备的功能属性。这些因素都对搜索阻尼器的空间最优布置提出了挑战。合理的阻尼器空间布置能充分发挥阻尼器的作用,最大化减震效率亟待探索。基于此,本文提出框剪结构黏滞阻尼器可按各层各向水平地震作用下的层间位移角量值进行数量分配,宜远离或贴近结构平面形心并于柱跨内1/2处布置,该法实践性强且极大程度发挥减震效率。

罕遇地震作用下BRB加固对结构抗震性能的影响

屈曲约束支撑是一种新型耗能减震构件,质量轻、施工速度快、对原结构影响小,能够保护主体结构在罕遇地震作用下不屈服或者不严重破坏,在结构加固中应用广泛。为了探讨罕遇地震作用下不同BRB布置对屈曲约束支撑结构抗震性能影响,对5种不同BRB布置方式的框架进行了罕遇地震下的动力时程分析,得到了结构的层间位移角、顶层监测点位移、顶层监测点加速度、基底剪力和屈曲约束支撑滞回耗能。研究结果表明:在结构端部和中部设置屈曲约束支撑的受力性能和抗震性能优于原结构和其他BRB布置方式的结构;在罕遇地震作用下,既有混凝土框架结构中的屈曲约束支撑处于塑性阶段,进入屈服耗能时期,并向结构提供侧向刚度。

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.

岚山油气管廊竖井结构受力分析及空间优化

竖井支护在岩石地层中通常使用锚杆喷射混凝土,在土层中使用“围护结构+内支撑”方式。依托烟台岚山管廊竖井工程,分析“锚喷网+拱架+注浆”的围护结构下双拼型钢横撑、无横撑、圆管横撑三种工况的复合地层矩形竖井围护结构受力和变形等参数;提出地表沉降、侧壁收敛、安全性、施工难易程度、竖井洞口有效面积和经济性六指标的竖井支护结构性能综合评价方法,并优选出“锚喷网+拱架+注浆+圆管横撑”支护结构。竖井支护结构的轴向应力、Mises应力,圆管横撑的压杆稳定性和挠度,均满足规范要求,结构安全可靠。

支撑布置对火灾下钢框架结构抗连续性倒塌性能影响研究

研究了支撑布置对火灾下钢框架结构抗连续性倒塌性能的影响。考虑水平和竖向两种支撑布置形式,前者沿水平向布置在结构顶部,后者布置在结构各楼层,分别考虑其布置在结构边缘或者内部的情况。应用OpenSees软件对某7跨8层平面钢框架结构模型进行了数值分析。结果表明,火灾下结构倒塌始于受火柱的屈曲,该局部破坏向周边传播,导致整体结构下沉式倒塌或侧向倾覆式倒塌。竖向支撑可有效抑制结构的侧向倒塌,但容易导致底层柱的依次屈曲,不建议单独使用;通过将竖向支撑布置在结构内部,可有效阻止破坏向周边结构传播;水平支撑通过平均分配各柱轴压力,有效抑制周边柱的压曲,但容易造成结构局部或是整体侧向倒塌;同时应用两种支撑可有效提高结构抗倒塌性能。

大跨中承式钢管混凝土拱桥的横撑布置

对大跨中承式钢管混凝土拱桥常用的X撑及米字撑的合理间距进行了探讨。鉴于结构自振频率、振型及弹性稳定系数的求解均可转化为源自结构动力平衡方程的特征值问题,先以结构基频及弹性稳定系数为衡量标准,对一座360 m跨的中承式钢管混凝土拱桥可能的米字撑及X撑布置方案进行了分析对比,并据此提出了X撑及米字撑的合理间距。按照布撑区域相等、横撑体系材料用量相近的原则,提出4个布撑方案,并分别进行了极限承载能力分析。结果表明,结合模态分析及弹性稳定性分析确定结构参数的方法可行;对于本文的360 m跨的中承式钢管混凝土拱桥,采用X撑即可满足刚度要求,且能获得更为轻巧的视觉效果。

高强钢组合K形偏心支撑框架抗震性能影响参数分析(Ⅱ)

为研究不同参数对高强钢组合K形偏心支撑框架结构在罕遇地震作用下的受力和变形能力的影响,以耗能梁段腹板高厚比、结构高跨比以及支撑布置形式为参数,对高强钢组合K形偏心支撑框架结构(耗能梁段为Q345钢,框架梁柱及支撑为Q460钢)进行了动力时程分析。分别研究了各参数对结构周期、层间位移角和楼层剪力、耗能梁段受力及变形的影响。研究表明:耗能梁段腹板高厚比取为34.5~52.0较为合理;高跨比H/L1取为0.5~0.7并适当增大无支撑跨高跨比H/L2,可减小结构层间位移角和耗能梁段变形,使各框架柱受力更均匀;支撑沿竖向连续布置时,结构内力比支撑沿竖向错列、均匀布置时更均匀,支撑宜尽量沿同跨竖向连续布置,且各支撑跨水平方向不宜太分散。

轻型钢屋盖结构支撑系统与结构安全性

轻型钢屋架破坏塌落事故常是忽略支撑系统而造成。为此分析轻型钢屋盖结构支撑系统的作用、设置以及支撑的结构形式，对屋架施工时保证安装和使用的安全性作了介绍。