All-steel Buckling-restrained Braces Research Articles

Effect of loading sequence on the ultra-low-cycle fatigue performance of all-steel BRBs

Buckling-restrained braces (BRBs) are intended to protect the primary structure of buildings by absorbing most earthquake input energy through the plastic deformation of the steel cores. The ultra-low-cycle fatigue performance of BRBs is crucial for their seismic performance as energy dissipators. Despite the plenty of research on the ultra-low-cycle fatigue behavior of steel members, it remains susceptible if arbitrary loading paths have a major effect on the ultra-low-cycle fatigue life or cumulative plastic deformation (CPD) capacity of BRBs. This paper introduces an experimental campaign of nine supposedly identical BRB specimens subjected to three different loading protocols of similar average strain amplitudes. For each loading protocol, the test was repeated for three times to account for possible uncertainty. The results show that the effect of loading protocol is trivial given the similar average strain amplitudes throughout the loading. The Miner's assumption that the loading sequence has no effect on the fatigue life of BRBs is supported by the test results, however, the proper choice of the Coffin-Masson model's parameters is crucial. The skeleton ratio, which is derived from decomposing the hysteresis curve into a skeleton part and a Bauschinger part and has been used to account for the loading path effects, is shown to be insensitive to varied loading protocols by the test results.

Effect of Pre-tensioning Force on Behavior of Buckling Restrained Brace (BRB) Supported by Double Pre-tensioning System

The study explores the feasibility of a new all-steel buckling restrained brace (BRB) with the double pre-tensioning system as an alternative to conventional concentrically braced frames by using finite element method (ABAQUS program), And to verify modeling, use U-V1 specimen results and compare them with test results. The restraining element in an all-steel BRB is built from a rectangular plate with a double cross arm and cables, making it easy to fabricate, inspect, and replace after a severe earthquake. Numerical analyses were conducted on four groups: first group of specimens using dynamic loading to achieve the minimum thickness for stable hysteretic behavior and high efficiency for BRB, and three groups of specimens using monotonic loading to achieve the best pre-tensioning forces value in steel 37, 44 and 52 for greater efficient use of all steel BRB. The results showed a 66% improvement in bearing capacity, with the external restraining case thickness reduced to 20 mm, making it lightweight, economical, and easy to maintain. The specimens with a pre-tensioning force of 70 kN, 50 kN and 40 kN providing a 31%, 51% and 73.2 % for steel 37, 44 and 52 respectively over than the design specimen without pre-tensioning system.

A new Coplanar Dual Core Buckling-Restrained Brace

This study presents a new all-steel Buckling-Restrained Brace (BRB) named Coplanar Dual Core (CDC) BRB, that consists of a rectangular cross-sectional core divided into two coplanar yielding lateral bands, whose movements are constrained by an external restrainer (ER) and by an internal restrainer (IR). In contrast with the traditional BRB with dog-bone shaped core, the CDC-BRB has the advantage that the core can be easily extracted and inspected. The ER is composed of two rectangular tubes welded to a pair of rectangular solid calibrated bars. The core and IR are simultaneously laser cut from the same sheet of steel, which provides additional advantages: minimum material waste, minimum laser cut length, regular and well controlled gap, reduction of weight and manufacturing cost. Formulae to design the CDC-BRB, to determine the appropriate values of the gaps, and to predict its response and ultimate energy dissipation capacity are proposed. Four specimens representing the new CDC-BRBs with different gaps are tested under cyclic loadings to failure. The results of the tests show that the cyclic behavior of the new CDC-BRB is stable, highly dissipative, and its response and ultimate energy dissipation capacity can be well predicted insofar the gaps fit the proposed values.

Finite element analysis of an all-steel buckling-restrained brace

Finite element analysis of an all-steel buckling-restrained brace

The effect of design parameters on mechanical performance of all-steel buckling restrained brace with double steel tubes

To explore the effect of design parameters on the mechanical performance of the proposed all-steel buckling-restrained brace (BRB), the finite element software ABAQUS is adopted. The brace consists of an inner round steel tube and an outer square steel tube to restrain its inner core, designed with both the cruciform-section core and the H-section steel core. Quasi-static tests are performed on the all-steel BRB to explore its energy dissipation capacity. The cyclic behavior of the specimen models is evaluated. The validity of the numerical model is verified by comparing the predicted results with the experimental results. Next, the effects of three parameters on the mechanical properties of the brace—the width-thickness ratio of the inner core, the clearance between the inner core and the inner round steel tube c1, and the constraint ratio—are investigated by the validated numerical model. The width-thickness ratio varies from 6 to 20, the gap value ranges from 1 to 7 mm, and the constraint ratio ranges from 0.43 to 1.03. Results show that the hysteresis curves of the two braces are full, with stable hysteretic performance and good energy dissipation capacity, which justifies the construction. Based on the parameters analysis results, the optimum values of the width-thickness ratio, gap, and constraint ratio have been recommended for steel BRBs.

Cyclic Behavior of All-steel BRBs with Bolted Angle Restrainers: Testing and Numerical Analysis

ABSTRACT This study presents the sub-assemblage tests on six all-steel buckling-restrained braces (ABRBs) with bolted angle restrainers. The clearance between the core and restrainers, the core configurations, and the position of stoppers were varied in the experiments. ABRBs with stopper welded in the elastic segments and a higher clearance exhibited higher cumulative ductility. The specimen with perforated core plates showed a stable hysteretic response even for a smaller gap width. Numerical modeling was conducted to predict their hysteretic response and the core fracture of ABRBs. The proposed modeling technique successfully predicted the hysteretic response and the core fracture of braces.

Mechanical Property of Beam-to-Column Connection of Steel Structures With All-Steel Buckling-Restrained Braces

To avoid brittle fracture and plastic yielding of steel beam-to-column connections under earthquakes, a new beam-to-column connection of steel structures with all-steel buckling restrained braces (BRBs) is proposed. The all-steel BRB is connected to the steel beam and column members through pins to form a new connection system. Taking the T-shaped beam-to-column connection steel structure as the research object, two structural types with an all-steel BRB installed on one side (S-type) and two sides (D-type) are considered. Theoretical equations of the connection system’s initial stiffness and yield load are derived through the mechanical models. The yield load, main strain distribution, energy dissipation, and stiffness of the connection system are investigated through quasi-static tests to verify the connection system’s seismic performance. The tests revealed that the proposed new connection system is capable of achieving a stable hysteresis behavior. At the end of loading, the beam and column members are not damaged, and the plastic deformation is concentrated in the plastic energy dissipating replaceable BRB, and the beam and column basically remain elastic. The proposed equations approximately estimated the load response of the proposed connection system. The results show that the damage mode of this new connection system under seismic loading is BRB yielding, with an elastic response from the beam-column members.

Cyclic Load Test and Finite Element Analysis of NOVEL Buckling-Restrained Brace.

Compared to concrete or mortar-filled Buckling-Restrained Braces (BRBs), all-steel BRBs provide weight and fabrication time reductions. In particular, all-steel buckling braces with H-section cores are gaining attention in cases where large axial strength is required. In this paper, an all-steel BRB, called NOVEL (Noise, CO2 emission, Vibration, Energy dissipation and Labor), is presented. It comprises an H-section core encased in a square casing, and its behavior was studied through full-scale subassembly and brace tests, followed by a finite element parametric study. Two failure modes were observed: global buckling and flange buckling of the H-section core, which occurred in test specimens with Pcr/Py ratios of 1.68 and 4.91, respectively. Global buckling occurred when the maximum moment in the casing reached its yielding moment, although the test specimens had sufficient stiffness to prevent global buckling. Failure by core flange buckling occurred at a core strain of 1.2%. The finite element parametric study indicated that adjusting the width-to-thickness ratio of the core flange is more feasible than stiffening the flange or adjusting the unconstrained-length end stiffeners. The value of 5.06 was the minimum flange slenderness ratio that provided a stable hysteresis to the end of the loading protocol of the American Institute of Steel Construction standard.

Analytical and numerical studies on reducing lateral restraints in conventional & all steel Buckling Restrained Braces

A typical Buckling Restrained Brace (BRB) consists of a steel core and a mortar-filled tube to prevent buckling of the core. In BRBs, the mortar used in a casing considerably increases their weight. In another type of such braces, to reduce the weight, a steel casing is used to avoid buckling of the steel core. In this paper, discontinuous restraint is investigated analytically and numerically to manage buckling behavior of the core along the steel tube.In the analytical section, equations are proposed to determine a suitable distance between the restraints in rectangular and cruciform sections using inelastic buckling, tangent and double modulus theories and their combination with the Romberg-Osgood behavioral model. In the numerical section, first the cyclic behavior of 9 Conventional BRB models with continuous and discontinuous mortar casing is evaluated using the Abaqus software. Then, the appropriate distance between restraints is determined for the numerical model and compared to those from analytical equations. Results show that the tangent modulus theory is more compatible with the finite element model. Moreover, in cruciform cores, it is possible to reduce the amount of mortar up to 64% without any changes in the cyclic behavior of bracing. Finally, two all-steel BRB models are exposed to cyclic loading where steel plates prevent core bucking. Those results demonstrate that the cumulative ductility exceeds the minimum value defined by AISC.

Analytical study of the seismic performance of all-steel buckling-restrained braces with H-shaped cores

The seismic performance of an H-shaped core buckling-restrained brace (HBRB) is evaluated through detailed finite element analysis. The numerical model is validated against available experimental data and a parametric study is carried out to investigate the effect of influential parameters on the cyclic behavior and failure modes of HBRB. Several design parameters including the yielding length ratio, the constrained length, the presence of stiffeners and their thicknesses, the length of the transition region, and the gap size are considered in the parametric investigation. Based on the analysis results reducing the yielding length of HBRB increases its post-yield stiffness and the axial strain and stress of the H-section core. Since buckling of the unconstrained region was observed in HBRBs with insufficient stiffener thickness, an analytical method is proposed for evaluating the sufficiency of stiffeners by estimating the critical buckling load of HBRB as a three-segment stepped compression member.

Lateral thrust in all-steel buckling-restrained braces: Experimental and FEM results

The lateral thrust — the total contact force exerted by the buckled core of a buckling-restrained brace (BRB) against its restraining profiles — in bolted all-steel BRBs was studied mainly by means of laboratory tests. The primary purpose was to obtain a better understanding of the “predictability” of the lateral thrust for a given set of boundary conditions. Eleven nominally identical all-steel, reduced-scale BRB samples were subjected to the same cyclic loading, and data were recorded concerning (i) the value of the total lateral thrust, (ii) the relative slip between the buckled core and the restraining surfaces, and (iii) the longitudinal strain at opposite faces of the buckled core. A very high scatter among the peak values of the lateral thrust was observed. The experimental results concerning points (ii) and (iii) were integrated by the results of a refined 3D Finite Element analysis. Very little core-restraining profiles relative slip was observed in the tests. The elastic-plastic core buckling seems to be of the von Kármán type, i.e., with simultaneous loading and unloading at the opposite sides of the core.

Optimal design of steel buckling-restrained braces considering stiffness and strength requirements

This paper is concerned with the optimal (or least weight) design of steel buckling-restrained braces against global buckling. First, the global buckling prevention criterion of steel buckling-restrained braces (BRB) is reconstituted to take on a symmetric expression, i.e., (Pcr/Pu−1)[Mr/(v0Pu) −1] ≥ 1, so as to provide clarity on the equal importance of the stiffness and strength requirements of the restraining member, where Pcr is the elastic buckling load, Mr the yield moment resistance, and v0 the initial imperfection of the restraining member whereas Pu is the ultimate compressive load of the core under cyclic axial loads. It is shown herein that the optimal design of the restraining member satisfies (1) the boundary of the global buckling prevention criterion, and (2) the relationship between the stiffness term Pcr/Pu and the strength term Mr/(v0Pu) in view of the maximum feasible cross-sectional height-to-thickness ratio as specified by the adopted design code recommendation. The satisfaction of these two conditions furnishes a unique optimal design of the restraining member. The optimal design can be expressed as an explicit equation in terms of the second moment of area of the restraining member. A design example is presented to illustrate the material (or weight) efficiency of all-steel BRB designs based on the proposed design equation.

Study of a new-type of steel buckling-restrained brace

The rectangle core plate of all-steel buckling-restrained braces (BRBs) usually exhibit obvious local buckling, due to the lack of longitudinal restraint from the encasing tube. To eliminate the undesirable effects, a novel steel BRB is proposed. In this new-type steel BRB, two T-shaped steels are adopted as the minor restraint elements to restrain the core plate instead of infilled concrete or mortar. Meanwhile, the ingot-iron material with low yielding strength and high elongation is applied to the steel core to study the mechanical properties of steel BRBs. To validate the theoretical requirements for the width-to-thickness ratio of the steel core and the thickness of angle steel, quasi-static tests of eight specimens were conducted. The tests focused on the energy dissipation capacity and failure modes of the proposed steel BRBs. Nonlinear finite element analysis was also carried out to validate the experimental results. Both the aforementioned results imply that appropriately designed steel BRBs can meet the performance requirements for BRB components.

Experimental investigation on H-section buckling-restrained braces with partially restrained flange

For the development of buckling-restrained braces (BRBs) with high bearing capacity and light weight, a new type of all-steel buckling-restrained braces with H-section steel core (HBRBs) is proposed. The restraining member of the new HBRB mainly consists of the restraining plates and the steel channels. The outside of flange of the H-section steel core is restrained by the restraining plate, the edge parts of the inside of flange are restrained by the steel channel, and web of the H-section steel core is unrestrained. There is adequate operation space for welding stiffeners in the elastic segment of the H-section steel core, and the cross-section of the restraining member is uniform along the longitudinal direction. Thus, the processing complexity of the new HBRB is reduced. First, cyclic tests are performed on the new HBRBs and one conventional HBRB of which the inside of flange is unrestrained. Then, numerical simulations are carried out to further investigate the hysteretic performance and deformation mode of the new HBRB. Test and numerical simulations demonstrate that the new HBRB shows stable hysteretic performance, and the energy absorption capacity of the HBRB can be significantly improved when the inside of flange is partially restrained. Decreasing the restrained width of the inside of flange is capable of reducing the normal contact force between the H-section steel core and the restraining member, also reducing the compression strength adjustment factor of the new HBRB. There is no visible decrease in the compressive strength of web with the strain amplitude, demonstrating that web can be effectively restrained by the top and bottom flanges.

On the sensitivity of Finite Element results in the calculation of the lateral thrust for all-steel buckling-restrained braces

The paper discusses the numerical analysis of all-steel buckling-restrained braces. These devices are designed to work in an inherently unstable regime, and their behavior is acutely sensitive both to any configuration detail in reality, and to any adopted parameter in their analysis. Their dissipative axial behavior can be reasonably well predicted by any suitable FEM model, if a good constitutive law is available. Nevertheless, it is shown here that an apparently accurate FEM calculation of the lateral contact forces, generated by the buckled core when it contacts the external containment profiles, produces results so sensitive to modelling details that they are practically devoid of an acceptable degree of accuracy. These observations might raise some doubts about the advisability of employing this type of bracing in normal engineering practice. In our viewpoint, a similar conclusion could be reached for any structural element designed to work in an unstable regime, unless the imperfections – of any type, including possible modelling details – could be explicitly taken into account, either parametrically or in a statistical way.

Investigation of the Seismic Behavior of Brace Frames with New Corrugated All-Steel Buckling Restrained Brace

All-steel buckling restrained brace (BRB) is a type of relatively new common bracings considering its low weight and whilst it is not required curing of mortar at the core of braces. In this study, a new form of all-steel BRB is introduced with corrugated edges of the core and the external sheath, and it was analyzed by using of finite element method. Existence of corrugated and ribbed edges led to enhance of buckling resistance. Numerical model was validated with laboratory samples, and after receiving an acceptable compliance in model behavior, the numerical models were offered. In this analysis, key parameters were size of brace section and distance of gap between the internal and external sheaths. The most appropriate size for gap between the core and external tube was determined to be 10 mm. Upon specifying the non-linear modeling parameter, two cases of frame with ordinary concentrically brace and the proposed all-steel BRB were compared for three structures with 4, 8 and 12 floors using pushover and non-linear time history analysis. The results demonstrated that utilizing of the proposed BRB will lead to an increase in behavior coefficient and structure ductility as well as an alteration in performance level of tall structures from collapse prevention to life safety level.

Experimental Study on High-Performance Buckling-Restrained Braces with Perforated Core Plates

The compressive deformation is mainly contributed by axial compressive deformation and high-order in-plane and out-of-plane global buckling deformation for conventional buckling-restrained braces (BRBs). A novel type of all-steel BRBs with perforated core plates, termed as perforated BRBs (PBRBs), are proposed in this study, where shear deformation can occur in addition to the aforementioned deformations in a conventional BRB under compression. Experimental study was carried out using five specimens with different configurations of holes under cyclic loading. Stable hysteretic properties, high ductility, and energy dissipation capacity were obtained for the PBRBs. The effects of two parameters, i.e. the slenderness ratio of the chord and hole spacing factor defined as the ratio of the hole length to the hole spacing, on seismic performance of the specimens were investigated. The compressive deformation mechanisms of the PBRBs were further investigated through a numerical study. The compressive deformation was found to be composed of axial compressive deformation, flexural deformation owing to in-plane and out-of-plane global buckling, and in-plane shear deformation of the latticed core plate.

Effect of Loading History and Restraining Parameters on Cyclic Response of Steel BRBs

This study presents a numerical modelling and prediction of cyclic response of all-steel buckling-restrained braces (BRBs) under different loading histories using a finite element software ABAQUS. The numerical models are validated by comparing the predicted hysteretic response, core fracture, energy dissipation and displacement ductility with the results of a past experimental study. Later, the validated numerical model is used in a parametric study to investigate the influence of coefficient of friction and clearance between the steel core and restrainers, the length of yielding core segment and the position of stoppers on core segments on the cyclic response of steel BRBs. The coefficient of friction is varied in the range of 0–0.5, whereas the gap value is varied in the range of 0–5 mm. The main parameters studied are the hysteretic response, axial resistance, instance of core fracture and energy dissipation response. Based on the analysis results, the optimum values of friction coefficient, gap, length of yielding core and stopper orientation have been recommended for steel BRBs.

Subassemblage tests and design of steel channels assembled buckling-restrained braces

This study investigates the load-carrying behavior of a proposed all-steel buckling-restrained brace (BRB) under seismic loading. The proposed steel channels assembled BRB (SCA-BRB) consists of a steel core plate encased by four steel channels assembled by high-strength bolts. Self-weight of an SCA-BRB is relatively small since the use of concrete is eliminated, leading to easier transportation and erection of the BRB. Firstly, by simplifying the restraining system of an SCA-BRB into a 2-chord battened column, the reduction factor of the elastic buckling load of the restraining system considering the discrete connection of bolts was derived. On this basis, the formulas in predicting ultimate resistance of an SCA-BRB were deduced along with the formulas of the lower limit of restraining ratio, and the upper limit of bolt spacing. Then, subassemblage test results of four specimens were reported. All the test specimens maintained stable load-carrying capacity during the loading process, and satisfactory energy-dissipating ability was achieved. Finite element (FE) models were established, and they well predicted the hysteretic responses of the test specimens. A total of 25 additional FE models was analyzed to perform a parametric study, verifying the effect of the member length, number of bolts, steel grade, gap size and geometrical imperfection on the hysteretic response and ultimate resistance of BRB. Finally, a design procedure of the SCA-BRB was proposed based on theoretical formulas, which have been validated by tests and FE results.

An analytical model for estimating restrainer design forces in bolted buckling-restrained braces

This article presents an analytical model to predict the normal thrust force for the design of the restraining system of all-steel buckling-restrained brace (BRB) members. The proper design of the restraining mechanism is key to obtaining a stable inelastic response from BRBs. Compared with existing models, the presented model accounts for the cyclic strain hardening behaviour of the BRB's steel core, the flexibility of the restraining system, the flow of frictional forces, the longitudinal variation of the core axial load and strain, and Poisson's effects on the core cross-section properties. The buckled shape of the core and the resulting normal thrust and frictional forces are evaluated along the core length using an iterative approach to satisfy the force equilibrium and geometric compatibility. The model predicts the maximum compressive and tensile loads resisted by the brace, buckling wavelengths, and the magnitude and distribution of the normal thrust and the frictional force along the core. Using a simplified stability analysis, the required restraining stiffness to achieve a stable inelastic response and avoid excessive gap opening is provided. A method to estimate the available restraining stiffness of a typical bolted all-steel BRB is presented and the internal actions in the restraining system resulting from the local buckling of the core are obtained through a simple and practical analysis. The analytical model is validated against the results of physical tests and finite element analyses for two different BRB members. The model was found to provide sufficiently accurate results for typical earthquake engineering applications.