Equivalent Viscous Damping Ratio Research Articles

Cyclic and uniaxial tensile properties of superelastic niti shape memory alloy cables

This study investigates the impact of various factors, including annealing duration, strain amplitude, cyclic loading, loading rate, and pre-training, on the mechanical properties of Nickel–Titanium shape memory alloy (SMA) cable. The primary focus is on evaluating their recovery ability and energy dissipation capabilities. The tested SMA cable has an outer diameter of 9 mm and a 7 × 7 configuration. The variation of strength, stiffness, residual strain, hysteretic energy, and equivalent viscous damping ratio of SMA cable with the loading cycle is analyzed. Furthermore, the impact of various annealing durations on the tensile strength and elongation of both SMA cables and wires was examined through monotonic tensile tests. The results indicate that the annealing duration considerably affects the superelastic behavior of SMA cables by shifting the stress-strain loops down and widening them. The recovery ability of SMA cable degrades more progressively with increasing loading amplitude and the number of loading cycles. The mechanical properties gradually stabilized after 20 times of constant strain amplitude loading and unloading training. The strain selection for cyclic training should not make the SMA cable in the martensite hardening stage. The recovery ability and peak stress of SMA cable can be significantly improved by pre-training. With the increase of annealing duration, the tensile strength of the SMA cable decreases gradually. Compared with SMA wire, SMA cable has better ductility and robustness and provides sufficient restoring force under large deformation.

A double-stage buckling restrained brace with progressive failure pattern: Experimental verification and numerical calibration

Buckling-restrained brace (BRB) is a widely used energy dissipator by the yielding of core steel plate. Single-core BRB (SBRB) only has one yielding point and a single failure pattern. Consequently, the SBRB has a risk of abrupt failure under extremely rare earthquakes, which may compromise the seismic safety and collapse mode of the building with SBRB. A new double-stage buckling-restrained brace (DSBRB) characterized by a progressive failure pattern is proposed. The progressive failure pattern of DSBRB is realized by setting up multiple core plates with different cross-sectional areas and lengths in the yielding part. A quasi-static test was conducted to investigate the difference in cyclic behavior and failure pattern between the SBRB and DSBRB. The test results showed that the yielding displacement and yielding force of the tested DSBRB were 8.74 % and 5.64 % smaller than those of SBRB respectively, resulting in a greater equivalent damping ratio for DSBRB under the small deformation. SBRB showed an abrupt failure pattern while the DSBRB was a progressive double-stage failure pattern at the cost of a reduced equivalent viscous damping ratio under the extreme deformation. The theory equations about the elastic stiffness, yielding force, and yielding displacement of DSBRB were derived and justified by the test results, i.e., differences between test results and theoretical derivation were all within ± 8.51 %. Two numerical models were established in ABAQUS and calibrated by the test results. The concept of double-stage BRB with a progressive failure pattern was verified.

Seismic performance of light wood shear wall infilled timber frame structures with openings

To study the seismic performance of wood frame structures filled with light wood shear walls, three full-size single-layer single span wooden frame structures with infill walls were designed and manufactured. The beams and columns were connected by mortise-tenon joints, and quasi-static tests were conducted on the specimens under reversed cyclic load. The failure modes and load-displacement hysteresis performance of structures with door opening infill wall, window opening infill wall, and solid infill wall were investigated. The seismic performance was analyzed using indicators such as strength, ductility, and equivalent viscous damping ratio. The failure modes of light wood frame filling walls were the tearing of sheathing panel and the failure of nail connections. The filled wall with the opening initially exhibited inclined cracks at the corner of the opening, and then they extended to the periphery. Compared with the solid filled wall, the positive and negative bearing capacity of the structure with door opening decreased, and that of the structure with window opening decreased also. Because the specimens with opening in the filled wall were more conducive to the deformation of the structure when the bearing capacity was not significantly reduced, the ductility of the specimen with door opening was the highest.

Design and Hysteretic Performance Analysis of a Novel Multi-Layer Self-Centering Damper with Shape Memory Alloy

This paper presented the development process of a novel multi-layer self-centering damper utilizing a NiTi shape memory alloy (SMA) with remarkable superelastic properties. The construction and operating principles of the novel damping device were introduced. A model for calculating the restoring force–displacement hysteretic curve of the novel damper was established, and based on this theoretical model, a parameter analysis of the damper’s hysteresis performance was conducted. The effect of SMA pre-strain, SMA diameter, number of layers in the damper, and number of SMA wires per layer on the damper’s stiffness, the unit cycle energy dissipation, and the equivalent viscous damping ratio were investigated, respectively. The results showed that the restoring force–displacement hysteretic curve of the novel SMA damper exhibits a full spindle shape, demonstrating the damper’s excellent energy dissipation capacity, self-centering capability, significant stroke, and unique variable stiffness characteristics (i.e., appropriate initial stiffness, minimal isolation stiffness, and significant limit stiffness). The results also indicated that the SMA pre-strain has a minor impact on the damper’s stiffness but a significant influence on unit cyclic energy dissipation and equivalent damping ratio. As the SMA pre-strain increased from 0.03 to 0.04, 0.05, and 0.06, the maximum stroke of the damper continuously decreases, while the unit cyclic energy dissipation initially increases and then decreases, with the optimal energy dissipation achieved at a pre-strain of 0.04. Increasing the SMA diameter results in a higher damper stiffness and energy dissipation capacity, with no significant change in maximum stroke and equivalent damping ratio. Increasing the number of damper layers leads to an increase in maximum stroke and unit-cycle energy dissipation, accompanied by a decrease in stiffness and almost constant equivalent damping ratio. As the number of SMA wires per layer increased from 8 to 16 and 32, the maximum stroke and equivalent damping ratio presented little variation, but the damper’s stiffness and unit cyclic energy dissipation continuously increased.

Experimental investigation of self-centering beam-to-column connections with different ribbed top and seat angles

This study experimentally investigated the seismic behavior of a novel self-centering beam-to-column connection with the different ribbed top and seat angles. The connections were composed of square concrete filled steel tubular (CFST) column, H-shaped steel beam, angles with or without stiffening rib, and post-tensioned (PT) high-strength steel tendons. Eight half-scale specimens were designed and tested under cyclic loading. The influences of stiffening rib type including plane rib and multi-wave rib, PT tendon distance, and with or without PT tendons upon the performance of the connection were evaluated experimentally. The results show that the connections installed PT tendons exhibited the double linear hysteretic feature with a slight narrow hysteretic loop due to the friction of the components, the connections assembled the angles or ribbed angles took on the fuller and more stable hysteretic loop, and the connections installed the PT tendons and ribbed angles exhibited the double-flag shaped hysteretic curve. The employment of plane or multi-wave ribbed angles significantly increased the initial rotational stiffness and ultimate moment. For the connection combined the PT tendons and ribbed angles, the average residual rotation angle ratio and the maximum equivalent viscous damping ratio reached about 0.095 and 0.07 at the drift ratio of 2%, respectively. The distance of PT tendons did not affect the hysteretic behavior of the connection. The PT tendons could provide the excellent self-centering behavior of the connection.

Experimental investigation of mechanical properties of NiTi superelastic shape memory alloy cables

As a self-centering element in a recoverable functional structure, shape memory alloys (SMAs) usually require a large diameter to provide sufficient bearing capacity, and pre-strain is applied to obtain good self-centering performance and initial stiffness. This study aims to explore the mechanical properties of large-diameter NiTi SMA cables and their potential applications in recoverable functional structures. Through the uniaxial tensile test of 11 SMA cables with 7 × 7 × 1.0 mm, the mechanical properties of SMA cables under different strain amplitudes, cyclic loading effects, loading rates, and different pre-strain modes were evaluated. The results indicate that the energy dissipation capacity of SMA cable increases with the increase of strain amplitude, but at the same time, it will reduce its self-centering capabilities. The equivalent viscous damping ratio decreases after the SMA cable enters the martensitic hardening stage. The residual strain of SMA cable depends on the combined effect of strain amplitude and number of cycles. The larger the strain amplitude and the number of cycles, the more significant the degradation of self-centering performance. After 20 times of constant amplitude loading and unloading, the mechanical properties of SMA cable are relatively stable. The reverse phase transformation stress of the pre-strained SMA cable gradually decreases with the increase of the tensile strain amplitude. The pre-strained SMA cable exhibits a stable restoring force strength in the sub-cycle, and the residual strain is zero. The test results support experimental data for applying large-diameter SMA cables in recoverable functional structures.

Development and optimization of a novel response‐amplified friction damper with different friction pairs

AbstractIn order to improve the efficiency of friction dampers, a shear‐type response‐amplified friction damper (RAFD) was proposed, which utilized a lever mechanism to amplify the shear displacement and increase the sliding distance of friction pairs. Cyclic loading tests were conducted on 12 RAFD specimens to evaluate the effects of machining processes, friction materials, loading protocols, and bolt pretension on the mechanical performance of the specimens. The results showed that the RAFD specimens with NAO and brass friction pads exhibited plump hysteresis loops, and the bearing capacity, effective stiffness, energy dissipation, and equivalent viscous damping ratio all met the stability requirements of FEMA 356 for the friction damper. Reducing the clearance between pins and holes and strengthening the hole edges shortened the plateau near zero resistances of the hysteresis loops by 75%–90%. Furthermore, combined with rotational analysis of the lever, a design methodology for the RAFD specimen was developed. Based on the response amplification factor R, key mechanical indices such as bearing capacity and equivalent friction coefficient were derived, and a theoretical hysteresis model for the RAFD was constructed, which matched well with experimental results.

Performance assessment of steel frame buildings with hybrid self-centering braces under extremely rare far-field earthquakes

This paper investigates the seismic performance enhancement of steel frame buildings using a novel hybrid self-centering braces (HSBs) under extremely rare earthquake events. The hybrid self-centering brace consists of shape memory alloy (SMA) cables and viscoelastic (VE) dampers. A prototype bracing system is designed and fabricated to explore its basic mechanical behavior and working mechanism under cyclic loading, with a focus on its failure modes under large deformation loading condition. A multi-material mechanical model is developed to capture the mechanical behavior and failure of the HSB. Furthermore, five steel frame buildings with different parameterized HSBs are designed and modeled in OpenSees. Nonlinear dynamic analyses and incremental dynamic analyses are conducted on the five case-study frames using 44 far-field ground motions. The risk-based seismic performances of steel buildings with HSB are evaluated to assess the performance of HSB during extremely rare seismic events. The results show that the hybrid self-centering brace exhibits excellent self-centering and energy dissipation capabilities with the maximum equivalent viscous damping ratio reaching 9.4 %. Even under large deformations, VE dampers continue to work effectively after the failure of SMA cables, demonstrating remarkable redundancy. Numerical simulations further reveal that the redundancy of HSB can improve the structural seismic resilience in terms of inter-story drift ratio, residual drift, and floor absolute acceleration. The higher the redundancy of HSB in the case-study frames, the smaller the seismic response and mean annual frequency of exceedance of the engineering demand parameters, thereby indicating a significant improvement in seismic performance.

Mechanical properties and analytical model of a novel dissipative angle bracket for CLT structures

The connections are essential in providing the strength, stiffness, ductility, and energy-dissipating capacity for CLT structures. In this paper, a novel dissipative angle bracket for CLT structures was proposed, which combines the benefits of soft-steel brackets and high-damping rubber to offer great energy-dissipating capacity and high ductility. Both quasi-static monotonic and reversed cyclic loading tests were conducted to evaluate the failure mechanisms and mechanical properties of this connection. Failure modes, including the rupture of steel ribs, fracture of the CLT wall, and debonding of the CLT slab, were observed during the tests. The rupture of steel ribs is the controlled failure mode, but the connection can still work as a whole to resist applied load after the steel ribs’ failures thanks to the rubber that is effectively bonded to the steel bracket. The tested connections’ load-displacement curves exhibit a bi-linear form, with the yielding point resulting from the yielding of steel ribs. Almost all the tested specimens have a ductility larger than 9.0 and an equivalent viscous damping ratio of around 10%–22%, indicating high ductility and energy-dissipating capacity. Furthermore, a simplified analytical model of the novel dissipative angle bracket was established. Comparison with test results demonstrates that the proposed model can reasonably estimate the initial stiffness, post-yield stiffness, and yield force. The outcomes of this study are expected to provide valuable references for future research and potential applications of the novel dissipative angle bracket for CLT structures.

Analysis and design of a novel segmented energy absorbing steel plate shear wall system

A steel plate shear wall (SPSW) system capable of providing strength and stiffness under normal load conditions and dissipating energy under extreme load conditions is proposed. The infill plate of this proposed Segmented Energy Absorbing SPSW (SEA-SPSW) system consists of two trapezoidal plate segments. For each plate segment, one edge is connected to the beam while the opposite edge is joined together by steel strips. The steel strips are held together using high strength bolts with long-slotted holes. Under normal load conditions, the SEA-SPSW behaves like a conventional SPSW. However, under extreme load conditions, the plate segments will slide between the steel strips and dissipate energy through friction. Because the proposed system does not rely on yielding or buckling to dissipate energy, it can be reset and reused after a severe load event. The behavior of this SEA-SPSW is studied using finite element and compared to an all steel conventional SPSW system. It is found that the proposed system not only helps reduce inter-story displacement, it also offers a desirable equivalent viscous damping ratio as well as a rather favorable energy dissipation to mass ratio.

Numerical Investigation of Cyclic Behavior of Angled U-shaped Yielding Damper on Steel Frames

Yielding dampers are often selected as a cost-effective solution for improving steel structures compared to other energy dissipation systems. The objective of this research study was to investigate the cyclic behavior of the angled U-shaped yielding damper (AUSYD) on a steel frame using numerical method. The numerical model was first verified using two experimental samples. Next, the influence of the number of dampers on the cyclic behavior of the steel frame was examined. The parametric model outcomes included energy dissipation, elastic stiffness, strength, and equivalent viscous damping ratio (EVDR). Additionally, an analytical equation was proposed for calculating the ultimate strength of the AUSYD, which correlated well with the experimentally obtained results. The study findings revealed that the increase in elastic stiffness and strength of the frame equipped with the AUSYD was nearly equivalent to the sum of the elastic stiffness and strength of the bare frame and its supplementary dampers. Furthermore, the results showed that models with 8 to 12 dampers had comparable energy dissipation and EVDR. Adding 8 dampers to the frame increased the energy dissipation and damping coefficient of the frame by 42% and 67%, respectively.

Experimental and analytical studies on a novel double-stage coupling damper

A series of experimental and numerical studies on the novel double-stage coupling damper (DSCD) are presented in this paper. The effects of the damper configuration, friction-yield ratio (Rfy), and loading protocol on the hysteresis performance of the DSCD are investigated via quasi-static tests on seven specimens, and the variation in the failure mode, compression-strength adjustment factor, energy dissipation capacity, and equivalent viscous damping ratio of the DSCD is discussed. The test results demonstrate that the arrangement of ribs in the DSCD increased the average energy dissipation per loading cycle, compared to the device without these elements. The damper exhibits double-stage energy dissipation characteristics for several friction-yield ratios, however, relatively low values of this parameter (Rfy = 0.5) leads to reduced energy dissipation capacity, due to significant variations in the friction force associated with creep, ploughing, and preload effect. Furthermore, a detailed finite element model of the DSCD is conducted based on the experimental data, focusing on exploring the effectiveness of the device. Numerical analysis revealed that the cumulative energy dissipation and peak force of the DSCD reduced with increasing the length of the friction mechanism (Lf). A large clearance between the yield segment and the restraining system (Cb) reduces the stability of the equivalent viscous damping ratio of the damper under compression, but Cb has no effect in the cumulation energy dissipation of the damper, which has important implications in the design of these devices for the seismic control of structures. Based on these results design recommendations for the DSCD are provided.

Experimental and numerical study on the mechanical properties of NiTi-SMA bars

This paper presents an experimental investigation that aims to study the mechanical properties of NiTi shape memory alloy (SMA) bars, with a particular emphasis on comprehensively evaluating their superelastic properties and energy dissipation capabilities. The study examines the influence of thermal treatment processes, loading rates, pre-training, and loading amplitudes on crucial parameters such as residual strain, energy dissipation capacity, initial stiffness, and equivalent viscous damping ratio of SMA bars. Additionally, the microstructure of the SMA bars' cross-sections, both prior to and following thermal treatment, is examined using scanning electron microscopy. The existing Graesser constitutive model is improved, and its validity is verified through numerical fitting. The findings demonstrate that SMA bars with a direct working section of 10 mm exhibit favorable superelastic properties after undergoing heat treatment at 350 °C for 35 min. It is recommended that the loading rate of SMA should not exceed 0.15 % per second, and appropriate pre-training measures should be implemented in practical engineering applications. Microstructural analysis reveals distinct dimples and intergranular fracture marks in the cross-sections of heat-treated SMA bars, indicating a high level of material toughness. Moreover, the improved Graesser constitutive model accurately calculates the mechanical characteristics of SMA bars and effectively describes their residual strain behavior. The research outcomes of this study provide valuable theoretical and technical insights for the practical application of shape memory alloy bars in engineering contexts.

Experimental and numerical study of a hollow laminated viscoelastomer-filled steel tube damper for seismic application

This study proposed a hollow laminated viscoelastomer-filled steel tube damper (HLVSTD), which consists of a hollow laminated viscoelastomer, a steel tube, and two connection end plates. Three HLVSTD specimens and two solid laminated viscoelastomer-filled steel tube dampers (SLVSTD) were designed based on the fabrication processes and the laminated viscoelastomer. Cyclic loading tests were carried out to investigate their energy dissipation performance, mechanical behavior, failure mode, and cyclic loading behavior. The fabrication processes more conducive to damper energy dissipation performance was given. Then, numerical models were established in ABAQUS and verified by the experimental test. Further numerical analysis was conducted to gain insight into the working mechanism and stress distribution of the HLVSTD. Finally, the bilinear model and Bouc-Wen model were compared to better capture the hysteretic response of the damper. The results indicate that HLVSTD has stable hysteresis behavior, the maximum equivalent viscous damping ratio is 44.31%. Hollow laminated viscoelastomer and solid laminated viscoelastomer can effectively prevent steel tube buckling. Compared to the SLVSTD, the HLVSTD has a higher energy dissipation capacity and saves 16% raw materials. The steel tube of cambered surface achieved full cross-section yielding, which is beneficial to improve energy dissipation capacity and ensure reliable end connection. The calculation results of the initial stiffness and the yield force agreed well with the test results. The Bouc-Wen model is suggested for capturing the mechanical behavior of the proposed HLVSTD in the real seismic application.

Experimental and analytical modelling on a novel self-centering viscous damper

Self-centering (SC) supplemental dampers offer a feasible approach to realize low-damage structures. A novel self-centering viscous damper (SCVD) that incorporates an SC part and a viscous damper (VD) part in parallel, and then series with a length adjustable (LA) part. The working mechanism and hysteretic model were both analyzed for the SCVDs. Dynamic reversed sinusoidal displacement inputs were used in experimental proof-of-concept validation studies. Each component was examined separately to describe and distinguish their distinct contributions to the whole device. Individual results in summing are consistent with hybrid device outcomes for the same input. The SCVDs showed excellent self-centering capability with satisfactory energy dissipation. A maximum equivalent viscous damping ratio (EVDR) of 99.37% was observed. A theoretical model and a numerical model were built and examined for the proposed SCVDs. The inevitable gaps from installation have a noticeable effect on the VD part and the SCVD, especially for small strokes. Except for small strokes, comparisons of test findings and analytical models reveal similar hysteretic reactions, for which the coefficients of determination are greater than 0.9, thereby validating the performance of the theoretical and analytical models to simulate the behavior of the tested SCVDs. In comparison to existing SCVDs, the suggested SCVDs have a more controllable SC ability, length adjustability, and energy dissipation capacity based on the demands of the designers. Furthermore, five distinct lateral resisting components were used in nonlinear response history analysis to compare the seismic and resilient performance amongst the moment resisting frame (MRF), buckling restrained brace (BRB), SC, VD and SCVD systems. Finally, the addition of SCVDs and VD parts can both be effective in controlling the maximum and residual story drift ratios on average. Overall, our findings provide innovative, simple implementable choices for developing low-damage structures.

Full-scale cyclic testing of an innovative energy dissipating device for seismic resiliency

Metallic dissipaters, which dissipate seismic energy by plastic deformation of the metal, have been most widely used in building construction to achieve seismic resilience. In this research, an innovative metallic dissipater called multiple U-shaped flexural plates (UFP) dissipater (MUD), was discussed. MUD can be used as a bracing member in eccentrically or concentrically braced frames. It has three main components: internal core member (usually steel universal column), external casing member (usually steel hollow section) and UFPs. UFPs are bolted between the internal and the external members in pairs. The internal (movable) and external (fixed) members can move relative to each other by rolling motion of the UFPs under axial compression and tension. This paper presents the experimental findings of four quasi-static cyclic tests of the MUD with varied displacement amplitudes and loading rates. Deformation behavior, hysteretic response, equivalent viscous damping ratio, energy dissipation capacity and other structural performance are investigated. The experimental results show that the MUD is able to achieve stable and repeatable hysteresis performance. All specimens can meet the cumulative inelastic deformation criteria in ANSI/AISC 341-16. The last specimen was tested until the capacity decreased by 30% to study the failure behaviour. The results show that the MUD has a promising ability to maintain strength after initiation of fracture and to prevent sudden failure of the structure.

Experimental study on bamboo-shaped buckling-retrained energy dissipater with different grades of steel

Buckling-restrained energy dissipaters are miniature energy dissipaters that constrain an inner core through an outer tube and have been widely utilized to improve the seismic performance of fabricated structures. Low yield point steel is often used to produce energy dissipaters, but the mechanical behavior of low yield point steel energy dissipater is rarely investigated. In this paper, a quasi-static test of bamboo-shaped energy dissipaters with different grades of steel was carried out. The influence of the material and configuration of the core on the fatigue performance and failure mode was evaluated. Test results showed that all specimens demonstrated stable hysteretic performance. The equivalent viscous damping ratio of low yield point steel energy dissipaters was relatively large at small strain amplitudes. At large strain amplitudes, both had a similar energy dissipation capacity. The compressive strength adjustment factors and strain hardening factors of low yield point steel energy dissipaters were higher because of their lower post-yield stiffness and significant isotropic hardening. The fracture mode on the low yield point steel energy dissipaters was similar to flat rupture, and significant necking was observed here. However, the normal strength steel energy dissipaters exhibited slant fracture, and no evident necking occurred. Two deformation modes were proposed. A calculation method for the buckling wavelength of the energy dissipater was proposed. When the effective length of the yielding region was smaller than the buckling wavelength, the fatigue performance of low yield point steel energy dissipater was better than that of normal strength steel energy dissipater, and the low yield point steel energy dissipater had a better reserve performance.

Influence of uncertainties on the seismic performance assessment of chevron braced frames

In the framework of the probabilistic seismic response assessment of buildings, a key aspect is the evaluation of the influence of uncertainties on the mean annual frequency of exceedance of assigned limit states. In this regard, this study is focused on the seismic performance of concentrically braced frames in the chevron configuration. Out of the possible sources of uncertainties related to the numerical model (e.g. modelling of infills, soil-structure interaction), the effects of modelling of connections at the ends of braces are investigated by considering three different models: a model with pinned braces, a model with braces connected to beams and columns by means of rigid links simulating the gusset plates and a model with braces connected to beams and columns by means of elements with axial and flexural stiffnesses based on the real size of the gusset plates. In the two latter models out of plane buckling of braces is allowed. All the considered models are first validated against an experimental test and then used to estimate the fragility curves and the mean annual frequency of exceedance of Damage Limitation, Significant Damage and Near Collapse limit states of four case-study buildings. In addition to the uncertainty in the main characteristics of the seismic input, the uncertainties in the yield strength of material, equivalent viscous damping ratio, permanent and variable loads are also considered. The effects of these additional sources of uncertainty on the structural response are determined by comparing results provided by numerical models with random values of the uncertain variables with results provided by a single model with median values of the same variables. Finally, a simplified approach is proposed to simulate the effects of the additional uncertain variables in analyses of models with only variability in the seismic input.

Numerical and analytical investigation of U-shape dampers and its effect on steel frames

Yielding dampers are one of the structural systems which dissipate a portion of the seismic force via premature yielding. The numerical and analytical methods were used in this study to investigate the behavior of the U-Shape Damper (USD) and its effect on steel frames. For this purpose at first, extensive parametric studies were performed using calibrated USD model. The parametric studies involved investigating the effect of USD dimensions on its elastic stiffness, strength, effective stiffness, and equivalent viscous damping ratio. Finite element models were analyzed under cyclic loading using nonlinear static analysis. An analytical equation was then presented to estimate seismic parameters of the USD. In the second part, the effect of this damper on the steel frame was investigated using push over analysis. The seismic parameters of the frame, including elastic stiffness, strength, ductility, and energy dissipation, were investigated through 56 numerical models to determine the effects of the number of USDs and the axial force of the column. The results indicated that reducing the radius and increasing the thickness of the damper led to an increase in effective stiffness, energy dissipation, and strength, while reducing the equivalent viscous damping ratio. Furthermore, the results obtained from the steel frame equipped with a USDs revealed that the addition of the damper mitigates the adverse effects of axial force on the frame.

Cyclic behavior of ellipse and peanut-shaped perforated buckling-restrained braces

Auxetic (effect of negative Poisson’s ratio) metamaterials have increasingly attracted attention recently. Buckling-restrained braces (BRB) are commonly used as structural components to resist lateral loads and dissipate earthquake input energy. This study introduced the concept of auxetics to develop novel types of BRB. Two forms of perforated auxetic structures, including ellipse and peanut-shaped structures, were used in this study to develop new types of BRB core plates. Both experimental and numerical studies were conducted to evaluate the hysteretic performance of the two newly developed types of perforated BRBs. The test program included two ellipse-shaped BRB and three peanut-shaped BRB specimens. Each specimen had a fracture that occurred at the ligament between neighboring holes. Out-of-plane deformation was insignificant in each specimen. Test results showed that the perforated BRBs had a stable hysteretic response and good energy dissipation capacity. Finite element modeling illustrated that plastic strain was localized at the ligaments between neighboring holes. Parametric studies were also conducted on the effects of two key parameters: the unit cell number and the porosity. With a given dimension of the yielding segment, an increase in the unit cell number increased the BRB load-carrying capacity. With a given unit cell size and number, an increase in the porosity increased the equivalent viscous damping ratio, and decreased the load-carrying capacity and effective Poisson’s ratio. For either ellipse or peanut-shaped structures, the effective Poisson’s ratio may become negative as the porosity larger than 0.4. A negative Poisson’s ratio may be beneficial for BRB energy dissipation capacity. The limitations of the numerical models and future research directions were also discussed in this paper.