Performance Of Buckling-restrained Braces Research Articles

Shaking Table Test for Evaluating the Seismic Performance of Steel Frame Retrofitted by Buckling-Restrained Braces

To investigate the seismic performance of buckling-restrained braces under the earthquake action, the shaking table test with a two-story 1/4 scale model is carried out for the ordinary pure steel frame and the buckling-restrained bracing steel frame with low-yield-point steel as the core plate. The failure modes, dynamic characteristics, acceleration response, interstory drift ratio, strain, shear force, and other mechanical properties of those two comparative structures subjected to different levels of seismic waves are mainly evaluated by the experiment. The test results show that under the action of seismic waves with different intensities, the apparent observations of damage occur in the pure frame structure, while no obvious or serious damage in the steel members of BRB structure is observed. With the increase in loading peak acceleration for the earthquake waves, the natural frequency of both structures gradually decreases and the damping ratio gradually increases. At the end of the test, the stiffness degradation rate of the pure frame structure is 11.2%, while that of the buckling-restrained bracing steel frame structure is only 5.4%. The acceleration response of the buckling-restrained bracing steel frame is smaller than that of the pure steel frame, and the acceleration amplification factor at the second story is larger than that at the first story for both structures. The average interstory drift ratios are, respectively, 1/847 and 1/238 for the pure steel frame under the frequent earthquake and rare earthquake and are 1/3000 and 1/314 for the buckling-restrained bracing steel frame, which reveals that the reduction rate of lateral displacement reaches a maximum of 71.71% after the installation of buckling-restrained brace in the pure steel frame. The strain values at each measuring point of the structural beam and column gradually increase with the increase of the peak seismic acceleration, but the strain values of the pure steel frame are significantly larger than those of the buckling-restrained bracing steel frame, which indicates that the buckling-restrained brace as the first seismic line of defense in the structure can dramatically protect the significant structural members. The maximum shear force at each floor of the structure decreases with the increase in height, and the shear response of the pure frame is apparently higher than that of the buckling-restrained bracing structure.

Experimental study on two-level yielding buckling-restrained braces

Buckling-restrained braces (BRBs) are widely used in engineering especially for braced frames as load-bearing as well as excellent energy dissipating members. However, the conventional BRBs are designed in elastic state under frequent earthquakes for avoiding low-cycle fatigue, and cannot contribute to seismic energy dissipation. In order to overcome this defect, conventional BRBs are recommended to combine with metal tube dampers to form a novel type of two-level yielding buckling-restrained braces (TYBRBs). The tube damper in TYBRBs yields under frequent earthquakes to consume seismic energy. The core plate of TYBRBs yields under rare earthquakes to further dissipate energy and reduce seismic effect. In this paper, the configuration and mechanical models of TYBRBs are first presented. Two TYBRB specimens are designed and cyclic loading tests are carried out. It is found that: (1) the low-cycle fatigue resistance of tube dampers can meet the requirements of TYBRBs for frequent earthquake resistance; (2) The functions of TYBRBs can be realized with rational design; (3) The performance of BRBs in TYBRBs does not degrade with combination of tube dampers.

Quantifying Inelastic Force and Deformation Demands on Buckling Restrained Braces and Structural System Response

Buckling-restrained braced frames (BRBF) have become a very popular lateral-resisting system due to their balanced, full hysteresis and the ability to tailor stiffness, within limits, and strength to meet specific design requirements. This paper reports the results of an analytical investigation on the performance of buckling-restrained braces (BRB) and the global performance of BRBF with a focus on the ductility and overstrength demands on the braces. Nonlinear analytical models of various three- and six-story steel frames were subjected to a suite of earthquake records to determine the demands on the BRB elements and the overall frame response. The structure variations include the location (i.e., seismic hazard), seismic importance factor, Ie, brace configuration (chevron versus single diagonal), and BRB yielding core length. The analysis also investigates the ratio of the impact of an error in the approximation of the BRB elastic stiffness on the system performance.

Finite Element Modelling of Buckling Restrained Braces under Combined In-Plane and Out-of-Plane Loading

During earthquakes, buckling restrained braces (BRBs) are likely subjected to both in-plane (INP) and out-of-plane (OOP) loadings simultaneously, therefore, BRBs are required to act robustly under combined INP and OOP loading. It is believed that the OOP loading will reduce the energy dissipation ability of BRBs. The intent of this study is to numerically investigate the performance of BRBs under combined INP and OOP loading with a finite element model of BRB with circular cross-section. Restraining concrete within the BRB is modeled as connector elements in the model and is proven to be an effective way. Simulation results show that the performance of BRBs under combined INP and OOP loading is not as good as that under the INP loading only and the energy dissipation ability is decreased by about 15% when the magnitude of OOP loading is equal to that of INP loading. Furthermore, the results give a deeper insight into the behaviour of BRBs under different combined OOP and INP loading histories.

Effect of the unbonding materials on the mechanic behavior of all-steel buckling-restrained braces

Seven all-steel buckling-restrained braces (BRBs) were tested under cyclic loading to investigate the effect of the unbonding materials on the performance of BRBs, via the employment of a layer of 1-mm thick butyl rubber or pure air gap between the core plate and the restraining system. Test results indicate that all the BRBs exhibited rather well energy dissipation capacities and sustained cumulative plastic deformations over 1000 times the yield strain. However, significantly higher compression strength adjustment factor β was developed for the specimens without the unbonding materials, due to the gradually increasing friction force and the jamming between the end of the core plate and the restraining members. Moreover, the gradually increasing friction force and the jamming also induced the considerably nonuniform residual deformation for the specimens without the unbonding materials both observed in the test and the finite element models. Especially, for high performance BRBs with relatively long yielding segment and thin core plate, the unbonding materials are recommended to apply.

A numerical analysis on the performance of buckling restrained braces at fire-study of the gap filler effect

Buckling Restrained Braces (BRB) have been widely used in the construction industry as they utilize the most desirable properties of both constituent materials, i.e., steel and concrete. They present excellent structural qualities such as high load bearing capacity, ductility, energy-absorption capability and good structural fire behaviour. The effects of size and type of filler material in the existed gap at the steel core-concrete interface as well as the element's cross sectional shape, on BRB's fire resistance capacity was investigated in this paper. A nonlinear sequentially-coupled thermal-stress three-dimensional model was presented and validated by experimental results. Variation of the samples was described by three groups containing, the steel cores with the same cross section areas and equal yield strength but different materials (metal and concrete) and sizes for the gap. Responses in terms of temperature distribution, critical temperature, heating elapsed time and contraction level of BRB element were examined. The study showed that the superior fire performance of BRB was obtained by altering the filler material in the gap from metal to concrete as well as by increasing the size of the gap. Also, cylindrical BRB performed better under fire conditions compared to the rectangular cross section.

Numerical Study of the Seismic Efficiency of Buckling-Restrained Braces for Near and Far-Fault Inputs

Buckling-restrained braces are commonly installed in building structures as concentric diagonal or chevron braces to protect the main construction from seismic actions. These elements have shown repeatedly their usefulness for reducing the seismic response, both from theoretical and experimental studies; and a number of practical applications have been reported. However, seismic records with near-fault effects possess special characteristics that might impair the performance of these devices, similarly as what occurs in base isolation; about energy issues, in such records (containing strong velocity pulses) the energy is delivered in a short time interval, thus being difficult to be absorbed. This work presents a numerical study regarding the performance of buckling-restrained braces under three types of seismic records: cortical far-field, subductive far-field and near-field (i.e. containing velocity pulses). The study is carried out on a symmetric 4-story steel moment-resisting unbraced frame that was tested at the E-defense laboratory, Japan; the dynamic response of such unbraced bare frame is numerically simulated, obtaining a satisfactory agreement. The same numerical model is used to describe the 2-D dynamic behavior of the steel frame equipped with buckling-restrained braces. The inputs are three series of ten ground motion records; each of these series belongs to one of the three aforementioned types. The average responses for each of the three types of inputs are compared; the obtained results show that the buckling-restrained braces are able to reduce the dynamic response of the frame and that no significant differences can be observed among the efficiency for far-fault and near- fault records.

Influence of design parameters of buckling-restrained brace on its performance

The contact interaction between the core and external restraining members is a vital factor that affects the overall performance of buckling-restrained braces (BRBs) significantly. In this study, refined finite element (FE) model was employed to evaluate the contact force between the core and external restraining members and to investigate the BRB performance. Moreover, the influences of strength and stiffness of external restraining member, core length, and other geometric parameters on the BRB performance were also studied. On the basis of numerical results obtained in the study, the recommended values of core width-to-thickness ratio, core thickness and gap were proposed. By considering the core plasticity and the lateral restraint of external restraining member, some formulas for predicting the half-wave length of core buckling as well as contact force were also proposed while the core buckled in multi-wave form. These formulas were numerically verified and could be references in the design of BRB.

鋼モルタル板を用いた座屈拘束ブレースの有限要素法による弾塑性大変形の解析

According to the author's past experimental studies, performance of buckling-restrained braces using mortar planks depends on the index that Euler's buckling load of a restraining part is divided by the yield load of a core plate. When the index of restraining effect is more than 1.0, the strength of the buckling-restrained brace does not decline until one percent strain equivalent to 1/100 story drift angle. This is the experimental result, but, the meaning of the index of restraining effect is not confirmed by an analytical approach. And the core plates shape into high-order buckling mode at the maximum load-bearing capacity of the buckling-restrained braces. It is also necessary to analyze how such buckling behavior proceeds. Referring to knowledge of the past experimental results, the authors conducted finite element analysis considering elasto-plasticity and large deformation. The index of restraining effect indicated the same meaning for not only the experimental results but the analytical results. And also the analytical results confirmed that when load increased, first, the core plate shaped into first-order mode, then, it changed to high-order mode gradually.