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.

Deterministic and probabilistic analysis of great-depth braced excavations:A 32 m excavation case study in Paris

The Fort d’Issy-Vanves-Clamart(FIVC)braced excavation in France is analyzed to provide insights into the geotechnical serviceability assessment of excavations at great depth within deterministic and probabilistic frameworks.The FIVC excavation is excavated at 32 m below the ground surface in Parisian sedimentary basin and a plane-strain finite element analysis is implemented to examine the wall deflections and ground surface settlements.A stochastic finite element method based on the polynomial chaos Kriging metamodel(MSFEM)is then proposed for the probabilistic analyses.Comparisons with field measurements and former studies are carried out.Several academic cases are then conducted to investigate the great-depth excavation stability regarding the maximum horizontal wall deflection and maximum ground surface settlement.The results indicate that the proposed MSFEM is effective for probabilistic analyses and can provide useful insights for the excavation design and construction.A sensitivity analysis for seven considered random parameters is then implemented.The soil friction angle at the excavation bottom layer is the most significant one for design.The soil-wall interaction effects on the excavation stability are also given.

Effect of Variable Concrete Strength on the Behavior of Buckling Restrained Braced Frames under Blast Load

BRBF(buckling restrained braced frame)is a relatively new lateral force resisting system for building structures.BRBFs are used mostly to resist seismic force due to their high ductility after yielding and the ability to absorb higher strain energy.ASCE(American Society of Civil Engineers)first permits the use of BRBFs as a single seismic force-resisting system by quantifying the seismic parameters such as response modification coefficient(R),over-strength factor(Ωo)and deflection amplification factor(Cd)for a structure built with BRBF,in their 2010 code(ASCE 7-10).But it has not been investigated how a structure built with BRBF,which is primarily designed to resist seismic force,and behaves under sudden occurrence of a blast load.This research investigates the performance of a BRBF subjected to blast loading.In other words,this paper focuses on the effect of blast loading on BRBF.The architype for this investigation is a chevron type braced frame.The frame is subjected to a short duration blast load that lasts only 21.7 mili-second(ms).Blast loading effects on the braced frame are assessed by identifying the weakest plane of failure,deformation characteristics and out of plane bending.The research investigates how the properties of the surrounding concrete,especially compressive strength,affect the overall strength of the BRBF on resisting blast loading.It is observed that the compressive strength of the surrounding concrete plays a significant role in reducing the deformation characteristics,both in-plane and out-of-plane.

关节镜下改良双束修复前交叉韧带Sherman Ⅰ型损伤

背景:前交叉韧带重建术后产生的腘绳肌肌力减弱、髌前区疼痛、重建后较高的骨关节炎发生率及本体感觉丧失等问题导致术后功能恢复不佳;前交叉韧带关节镜下修补结合动态或静态增强修补,保留了原有韧带结构,并取得了良好的近期疗效。目的:观察关节镜下改良双束修复前交叉韧带Sherman Ⅰ型损伤的功能疗效及影像学表现。方法:选择2020年1月至2022年9月在佛山市中医院运动医学科就诊的前交叉韧带损伤患者60例,根据治疗方案分为2组,每组30例,修复组采用双束修补结合内支架固定,重建组采用自体腘绳肌单束解剖重建。术后所有病例随访12个月,分别于术后3,6,12个月评估两组患者国际膝关节评分委员会评分、Lysholm评分及KT-1000患侧-健侧差值。结果与结论:①术后3个月,两组患者的国际膝关节评分委员会评分、Lysholm评分及KT-1000患侧-健侧差值比较差异均有显著性意义(P<0.05),术后功能修复组优于重建组;②术后6,12个月,两组患者的国际膝关节评分委员会评分、Lysholm评分及KT-1000患侧-健侧差值比较差异无显著性意义(P>0.05);③结果表明,前交叉韧带修复保留了原有韧带结构,避免了因重建钻取较大的骨髓道及自体肌腱取出,可减少对原有正常结构的损伤,并发症少,术后早期疗效优于前交叉韧带保残重建,但术后6个月之后两组疗效无明显差异。

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.

Damping characteristics of friction damped braced frame and its effectiveness in the mega-sub controlled structure system

Based on energy dissipation and structural control principle, a new structural configuration, called the megasub controlled structure （MSCS） with friction damped braces （FDBs）, is first presented. Meanwhile, to calculate the damping coefficient in the slipping state a new analytical method is proposed. The damping characteristics of one-storey friction damped braced frame （FDBF） are investigated, and the influence of the structural parameters on the energy dissipation and the practical engineering design are discussed. The nonlinear dynamic equations and the analytical model of the MSCS with FDBs are established. Three building structures with different structural configurations, which were designed with reference to the conventional mega-sub structures such as used in Tokyo City Hall, are comparatively investigated. The results illustrate that the structure presented in the paper has excellent dynamic properties and satisfactory control effectiveness.

Mathematical modeling and full-scale shaking table tests for multi-curve buckling restrained braces

Buckling restrained braces （BRBs） have been widely applied in seismic mitigation since they were introduced in the 1970s. However, traditional BRBs have several disadvantages caused by using a steel tube to envelope the mortar to prevent the core plate from buckling, such as： complex interfaces between the materials used, uncertain precision, and time consumption during the manufacturing processes. In this study, a new device called the multi-curve buckling restrained brace （MC-BRB） is proposed to overcome these disadvantages. The new device consists of a core plate with multiple neck portions assembled to form multiple energy dissipation segments, and the enlarged segment, lateral support elements and constraining elements to prevent the BRB from buckling. The enlarged segment located in the middle of the core plate can be welded to the lateral support and constraining elements to increase buckling resistance and to prevent them from sliding during earthquakes. Component tests and a series of shaking table tests on a full-scale steel structure equipped with MC-BRBs were carried out to investigate the behavior and capability of this new BRB design for seismic mitigation. The experimental results illustrate that the MC-BRB possesses a stable mechanical behavior under cyclic loadings and provides good protection to structures during earthquakes. Also, a mathematical model has been developed to simulate the mechanical characteristics of BRBs.

Shaking table test and numerical analysis of a 1:12 scale model of a special concentrically braced steel frame with pinned connections

This paper describes shaking table tests of a 1：12 scale model of a special concentrically braced steel frame with pinned connections, which was fabricated according to a one-bay braced frame selected from a typical main factory building of a large thermal power plant. In order to investigate the seismic performance of this type of structure, several ground motion accelerations with different levels for seismic intensity Ⅷ, based on the Chinese Code for Seismic Design of Buildings, were selected to excite the model. The results show that the design methods of the members and the connections are adequate and that the structural system will perform well in regions of high seismicity. In addition to the tests, numerical simulations were also conducted and the results showed good agreement with the test results. Thus, the numerical model is shown to be accurate and the beam element can be used to model this structural system.

Experimental study on the seismic response of braced reinforced concrete frame with irregular columns

A 15-storey K-braced reinforced concrete model frame with irregular columns, i.e., T-shaped, L-shaped, as well as ＋-shaped columns, was constructed and tested on the six-degree-of-freedom shaking table at the State Key Laboratory for Disaster Reduction in Civil Engineering in Tongji, China. Two types of earthquake records, El-Centro wave （south-north direction） and Shanghai artificial wave （SHAW） with various peak accelerations and principal-secondary sequences, were input and experimentally studied. Based on the shaking table tests and theoretical analysis, several observations can be made. The failure sequence of the model structure is brace→beam→column→joints, so that the design philosophy for several lines of defense has been achieved. Earthquake waves with different spectrums not only influence the magnitude and distribution of the earthquake force and the storey shear force, but also obviously affect the magnitude of the displacement response. The aftershock seismic response of previously damaged reinforced concrete braced frames with irregular columns possesses the equivalent elastic performance characteristic. Generally speaking, from the aspects of failure features and drift ratio, this type of reinforced concrete structure provides adequate earthquake resistance and can be promoted for use in China.

Effects of bottom bracings on torsional dynamic characteristics of horizontally curved twin I-girder bridges with different curvatures

Curved twin I-girder bridges (CTIGBs) have low torsional stiffness that makes them vulnerable to dynamic loads. This study investigates the effects of bottom bracings on the torsional dynamic characteristics of CTIGBs. Five types of bottom bracings are designed to investigate their effects on the dynamic characteristics of CTIGBs with different curvatures under free and forced vibrations. To perform numerical investigations, three-dimensional (3-D) finite element (FE) bridge and vehicle models are established using commercial ANSYS code, and then a vehicle-bridge interaction analysis approach is proposed. Road roughness profiles generated from power spectral density and cross spectral functions are also taken into account in the analyses. The numerical results show that torsional frequencies increase significantly after providing bottom bracings, and the increasing rate depends on the type of bottom bracings and their locations of installation. Bottom bracings can act as load transmitting members from one main girder to the others. Large negative bearing forces that have occurred in bridges with small radii of curvatures can be remarkably reduced by providing bottom bracing systems. It is found that the performances of several bottom bracing systems are effective in improving the torsional dynamic characteristics of the bridges in this study.

全镜下增强的Internal Brace与肌腱重建距腓前韧带治疗慢性踝关节不稳患者的临床评价

目的评价全镜下Internal Brace(IB)增强术和肌腱重建距腓前韧带(anterior talofibular ligament,ATFL)两种术式的临床疗效。方法2017年11月至2020年12月,本研究纳入57例慢性踝关节不稳患者,26例使用增强的IB,设为增强的IB组,31例使用ATFL肌腱重建,设为ATFL肌腱重建组,术后随访1年。采用视觉模拟评分法(visual analogue scale,VAS)评分、美国足踝外科协会(American Orthopaedic Foot and Ankle Society,AOFAS)评分、Karlsson评分、前抽屉试验、患者满意度及相关并发症来评估临床疗效。结果VAS评分(P<0.001)和AOFAS评分(P<0.001)在增强的IB组和ATFL肌腱重建组之间差异有统计学意义。而两组患者的Karlsson评分(P=0.602)、前抽屉试验(P=0.933)、患者满意度(P=0.869)之间比较差异无统计学意义。增强的IB组出现1例皮肤并发症,而ATFL肌腱重建组出现2例肌肉萎缩症状。结论与ATFL肌腱重建组相比,增强的IB组在缓解疼痛方面有更好的临床疗效。而ATFL肌腱重建组在踝关节功能恢复方面表现较好,且AOFAS评分显著高于增强的IB组。在其他方面,两组均取得了比较满意的结果。

Do ankle braces provide similar effects on ankle biomechanical variables in subjects with and without chronic ankle instability during landing?

Purpose:The purpose of this study was to examine effects of a sport version of a semi-rigid ankle brace (ElementTM) and a soft ankle brace (ASO) on ankle biomechanics and ground reaction forces (GRFs) during a drop landing activity in subjects with chronic ankle instability (CAI)compared to healthy subjects with no history of CAI.Methods:Ten healthy subjects and 10 subjects who had multiple ankle sprains participated in the study as the control and unstable subjects,respectively.The CAI subjects were age,body mass index and gender matched with the control subjects.The arch index and ankle functions of the subjects were measured in a subject screening session.During the biomechanical test session,participants performed five trials of drop landing from 0.6 m,wearing no brace ( NB),ElementTM brace and ASO brace.Simultaneous recording of three-dimensional kinematic (240 Hz)and GRF (1200 Hz) data were performed.Results:The CAI subjects had lower ankle functional survey scores.The arch index and deformity results showed greater arch deformity of ElementTM against a static load than in NB and ASO due to greater initial arch position held by the brace.CAI participants had greater eversion velocity than healthy coutrols.The ASO brace reduced the first peak vertical GRF whereas ElementTM increased 2nd peak vertical GRF.ElementTM brace reduced eversion range of motion (ROM) and peak eversion velocity compared to NB and ASO.In addition,ElementTM reduced dorsiflexion ROM and increased peak plantarflexion moment compared to NB and ASO.Conclusion:Results of static arch measurements and dynamic ankle motion suggest that the restrictions offered by both braces are in part due to more dorsiflexed ankle positions at contact,and higher initial arch position and stiffer ankle for ElementTM.

Behavior of braced excavation in sand under a seismic condition: experimental and numerical studies

The behavior of braced excavation in dry sand under a seismic condition is investigated in this paper.A series of shake table tests on a reduced scale model of a retaining wall with one level of bracing were conducted to study the effect of different design parameters such as excavation depth,acceleration amplitude and wall stiffness.Numerical analyses using FLAC 2D were also performed considering one level of bracing.The strut forces,lateral displacements and bending moments in the wall at the end of earthquake motion were compared with experimental results.The study showed that in a post-seismic condition,when other factors were constant,lateral displacement,bending moment,strut forces and maximum ground surface displacement increased with excavation depth and the amplitude of base acceleration.The study also showed that as wall stiffness decreased,the lateral displacement of the wall and ground surface displacement increased,but the bending moment of the wall and strut forces decreased.The net earth pressure behind the walls was influenced by excavation depth and the peak acceleration amplitude,but did not change significantly with wall stiffness.Strut force was the least affected parameter when compared with others under a seismic condition.

Mechanical compression release device in steel bracing system for retrofitting RC frames

The development of an innovative structural system with satisfactory seismic performance of braced systems is an important and challenging area of interest in structural engineering. In this paper, a device that can release the compressive force in the bracing members is developed, and its performance is evaluated. For comparison, four steel braced RC frames were constructed and tested under reverse cyclic loads. Two of them had different amounts of bracing and the other two had the same amount of bracing but incorporated different type of device, called compression release device, which is developed and described in this paper. It can be concluded from the test results that the newly developed device can effectively be used in steel braced systems to prevent buckling failure of the bracing members. Therefore, the device enhances the ductility of brace-framed systems by allowing an adequate capacity for energy dissipation.

Strength Analysis on Brace Structure for Semi-Submersible in Consideration of Wave Slamming

Slamming on bracings of column stabilized units shall be considered as a possible limiting criterion under transit condition based on the requirements in DNV-OS-C103. However, the wave slamming loads under survival condition were ignored for the strength analysis of the brace structures in many semi-submersible projects. In this paper, a method of strength analysis of brace structure is proposed based on the reconstruction and extrapolation of numerical model. The full-scale mooring system, the wind, wave and current loads can be considered simultaneously. Firstly,the model tests of the semi-submersible platform in wind tunnel and wave tanker have been carried out. Secondly,the numerical models of the platform are reconstructed and extrapolated based on the results of model tests. Then, a nonlinear numerical analysis has been conducted to study the wave slamming load on brace in semi-submersible platform through the reconstructed and extrapolated numerical model. For the randomness of wave load, ten subcases under each condition have been carried out. The value of the 90% Gumble distribution values of the ten subcases are used. Finally, the strength on brace structure has been analyzed considering the wave slamming. The wave slamming loads have been compared between the survival condition and transit condition with the method. The results indicate that wave slamming under survival condition is more critical than that under transit condition.Meanwhile, the wave slamming is significant to the structural strength of the brace. It should be overall considered in the strength analysis of the brace structure.

Analysis on column-bracing forces considering random initial imperfections

A large number of column-bracing systems were modeled and analyzed by second-order analysis using finite element program ANSYS,in which the random combination of the initial imperfections between columns and horizontal braces was well considered by Monte Carlo Method.According to the analysis results,four kinds of instability modes of column-bracing systems were found,the probability density function of the mid-height horizontal bracing forces was established based on probability statistics,and the design bracing forces were also obtained.The results indicated that the above design bracing forces are smaller than that proposed by the "Code for design of steel structures"(GB50017-2003) when the top axial compressions on the braced columns are equal,and much smaller than the latter when the top axial compressions on the braced columns are unequal.The results also indicated that the random combination of the initial imperfections between columns and horizontal braces leads to the randomness of the mid-height horizontal bracing forces in compression or in tension,so that the design bracing forces can be reduced.

An intelligent procedure for updating deformation prediction of braced excavation in clay using gated recurrent unit neural networks

This paper aims to establish an intelligent procedure that combines the observational method with the existing deep learning technique for updating deformation of braced excavation in clay.The gated recurrent unit(GRU) neural network is adopted to formulate the forecast model and learn the potential rules in the field observations using the Nesterov-accelerated Adam(Nadam) algorithm.In the proposed procedure,the GRU-based forecast model is first trained based on the field data of previous and current stages.Then,the field data of the current stage are used as input to predict the deformation response of the next stage via the previously trained GRU-based forecast model.This updating process will loop up till the end of the excavation.This procedure has the advantage of directly predicting the deformation response of unexcavated stages based on the monitoring data.The proposed intelligent procedure is verified on two well-documented cases in terms of accuracy and reliability.The results indicate that both wall deflection and ground settlement are accurately predicted as the excavation proceeds.Furthermore,the advantages of the proposed intelligent procedure compared with the Bayesian/o ptimization updating are illustrated.

Seismic retrofitting of reinforced concrete frame structures using GFRP-tube-confined-concrete composite braces

This paper presents a new type of structural bracing intended for seismic retrofitting use in framed structures. This special composite brace, termed glass-fiber-reinforced-polymer （GFRP）-tube-confined-concrete composite brace, is comprised of concrete confined by a GFRP tube and an inner steel core for energy dissipation. Together with a contribution from the GFRP-tube confined concrete, the composite brace shows a substantially increased stiffness to control story drift, which is often a preferred feature in seismic retrofitting. An analysis model is established and implemented in a general finite element analysis program - OpenSees, for simulating the load-displacement behavior of the composite brace. Using this model, a parametric study of the hysteretic behavior （energy dissipation, stiffness, ductility and strength） of the composite brace was conducted under static cyclic loading and it was found that the area ratio of steel core to concrete has the greatest influence among all the parameters considered. To demonstrate the application of the composite brace in seismic retrofitting, a three-story nonductile reinforced concrete （RC） frame structure was retrofitted with the composite braces. Pushover analysis and nonlinear time-history analyses of the retrofitted RC frame structure was performed by employing a suite of 20 strong ground motion earthquake records. The analysis results show that the composite braces can effectively reduce the peak seismic responses of the RC frame structure without significantly increasing the base shear demand.

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.

Peak displacement patterns for the performance-based seismic design of steel eccentrically braced frames

Performance-based seismic design(PBSD) aims to assess structures at different damage states. Since damage can be directly associated to displacements, seismic design with consideration of displacement seems to be logical. In this study, simple formulae to estimate the peak floor displacement patterns of eccentrically braced frames(EBFs) at different performance levels subjected to earthquake ground motions are proposed. These formulae are applicable in a PBSD and especially in direct displacement-based design(DDBD). Parametric study is conducted on a group of 30 EBFs under a set of 15 far field and near field accelerograms which they scaled to different amplitudes to adapt various performance levels. The results of thousands of nonlinear dynamic analyses of EBFs have been post-processed by nonlinear regression analysis in order to recognize the major parameters that influence the peak displacement pattern of these frames. Results show that suggested displacement patterns have relatively good agreement with those acquired by an exact nonlinear dynamic analysis.