Brace Buckling Research Articles

Development and application of out-of-plane deformable X-shaped brace for energy dissipation and thermal stress mitigation: An experimental and numerical study

Exoskeleton systems are increasingly employed in both new and existing buildings to enhance seismic performance. Addressing the critical challenges of energy dissipation and thermal stress mitigation in these systems, an out-of-plane deformable X-shaped energy dissipation brace (OPD-XEDB), integrating an out-of-plane X-shaped brace with a shear-type metallic damper, was proposed in this study. A seismic design methodology for OPD-XEDB was firstly developed, ensuring effective energy dissipation of the damper before any potential brace buckling. Following this methodology, two specimens, exhibiting distinct failure modes of damper shear fracture and brace buckling, were designed and subjected to quasi-static loadings to explore their energy dissipation mechanisms. Upon these experimental results, a numerical model incorporating buckling and pinching behaviors was proposed and calibrated, effectively capturing the hysteretic responses of the specimens. This model was then employed in a comprehensive numerical analysis, validating the effectiveness of design methodology and determining the optimal specifications for the OPD-XEDB in a prototype building. Ultimately, thermal analysis and nonlinear time history analysis were conducted on the prototype building. The thermal analysis proved the effectiveness of the OPD-XEDB's out-of-plane configuration in mitigating thermal stress by out-of-plane deformation, significantly mitigating thermal stress in braces and interconnected moment frames by 96.59–96.66 %. The time history analysis revealed that the dampers in OPD-XEDBs dissipated 11.6–23.2 % of total seismic energy, effectively preventing substantial buckling in the X-shaped braces. This energy dissipation mechanism led to a remarkable 79.7 % reduction in steel consumption for post-earthquake retrofitting, thereby significantly enhancing the seismic resilience of the primary structure.

Performance-based seismic design of sliding gusset plate braced frames incorporating thermodynamic models

A sliding gusset plate braced frame (SGBF) is proposed to enhance the seismic resilience of steel concentrically braced frames (CBFs). The symmetric friction connection (SFC) is used at the brace-to-gusset plate connection to create a sliding gusset plate brace. The sliding gusset plate brace offers high initial stiffness to withstand wind and service level earthquake loads. It also absorbs energy through friction beyond service level earthquakes, which mitigates performance degradation caused by brace buckling. Besides, replaceable links are added to the beam ends where plastic hinges are anticipated to form, creating a dual system that allows for damage control and rapid repair. The paper first presents the design concepts and performance objectives of SGBFs. Subsequently, a thermodynamic model is established to reveal the energy–temperature–coefficient of friction (CoF) (E-T-μ) relationship based on quasi-static tests of SFCs. The model partially tackles and addresses the challenge of temperature-dependent variations in CoF during sliding. By incorporating the model into performance-based seismic design, the overstrength ratio of SFC due to temperature-induced hardening can be quantified, according to the energy absorption demand of a specific SGBF during an earthquake. In this manner, unexpected damage and failure of members adjacent to the SFC and sliding gusset plate braces can be avoided. To demonstrate the application of the thermodynamic model and verify the seismic performance of SGBF, a prototype is designed based on a practical four-story hospital and validated through an excessive array of nonlinear dynamic analyses. The results show that the proposed SGBF can achieve the targeted performance objectives under different seismic shaking intensities. In addition, the SGBF has a sufficient margin of safety against collapse. Hence, the proposed SGBF can be used as an efficient and resilient structural system in high seismic zones.

Effect of type of internal core on the behavior of diagrid tube systems under lateral seismic loads

Diagrid structural systems have emerged as innovative and adaptable structural systems for tall buildings. These structures contain diagonal truss elements throughout the height of the structure. The present study investigated the effect of an inner steel moment frame core and an inner concentrically-braced core on a diagrid tube system under lateral seismic loading. The effect of inner cores on ductility and failure of the structures was assessed. A 36-story composite diagrid structural system with three different arrangements of diagonal angles was considered and the response spectrum analysis and nonlinear static analysis were conducted. The diagrid structures with inner cores indicated that most of the transferring lateral force in all stories is tolerated by the diagrid system; only in the last story was the shear absorbed by the internal core. Analytical results show that as the slope of braces increased, the lateral strength decreased. Diagrid structures with an inner braced core indicated greater ductility and strength than diagrid structures with inner moment frame cores. Moreover, the results indicate that the diagrid tube with an inner core produced no significant increase in ductility. To avoid premature buckling of braces, perimeter diagonal braces should be replaced with buckling-restrained elements.

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.

Advances and Challenges in Design of Connections in Steel-Braced Frame Systems with In-Plane Buckling Braces

A new type of connection has been developed for steel-braced frame systems that allows the brace members to undergo compression buckling in the in-plane direction. In addition to the inherent benefits of in-plane buckling (IPB) braces that help in reducing the extent of damage to the non-structural components, the IPB brace system is also considered to be an efficient way of retrofitting existing seismically deficient structures. The use of the compact and thicker gusset plate prevents the distortion of the free edges and the additional torsional force demand on beams and columns. However, IPB braced frame systems are not frequently used in practice, primarily due to the absence of limit state design criteria. As a result, some prominent failure modes observed in IPB frame systems are out-of-plane brace buckling, yielding of gusset plates, interface weld failure, and fracturing of knife plates. Recent studies on the IPB braced system have resolved some of these problems, such as design criteria being developed to prevent OOPB (out-of-plane buckling) of the IPB braced system. Other challenges need to be studied to achieve reliable performance of the braced frame system. This study focuses on recent advances and potential areas of improvement to achieve an efficient IPB braced system in highly seismic areas.

Axial compressive behavior of middle partially encased composite brace

To prevent the buckling and unbalanced tensile-compressive behavior of steel braces, a middle partially encased composite brace (MPEC) is proposed in this paper. In this composite brace, concrete is encased in the middle section of the H-shaped steel, and stiffeners are welded in the steel section for reinforcement. Axial compressive tests were implemented on four MPEC braces and one comparison specimen with considering the flange thickness, longitudinal reinforcement ratio, stirrup spacing and slenderness. The test results revealed that the failure of all composite braces was due to local buckling of the flange plates and separation from the concrete. The presence of concrete within the steel flange effectively limited the buckling of the H-shaped steel and improved the stiffness and compressive capacity. The compressive capacity of the composite braces was primarily influenced by the sectional dimension of the H-shaped steel, while the contribution of the reinforced concrete was limited. Additionally, a higher longitudinal reinforcement ratio improved the ductility of the composite braces, while larger stirrup spacing reduced both the compressive capacity and ductility. The specimen with large slenderness demonstrated poor mechanical properties and experienced a more abrupt failure. The axial compressive capacity of MPEC braces was calculated based on the critical buckling stress of steel plate and the confinement of concrete in partially encased composite columns (PEC). The calculated results showed good accuracy compared to the experimental data, and thus, the formula proposed in this study can be applied in design applications to predict the axial compressive capacity of MPEC braces.

Seismic assessment of chevron braced frames with differently designed beams

Braces in chevron braced frames inevitably suffer large cyclic deformation in the case of severe earthquakes. In a chevron braced frame, one brace buckling and another brace yielding would induce the unbalanced resultant force to braced beams. Capacity design requirements of braced beams in current codes suggest that braced beams totally bear the unbalanced resultant force. However, this requirement causes an excessively large cross section of braced beams and inhabits the application of Special Concentrically Braced Frames. Recent researches show that SCBF with yielding braced beams might also have satisfied seismic performance. To study the effect on seismic performance of concentrically braced frames with different beam strength and stiffness, numerical analysis including pushover analysis, hysteresis analysis and incremental dynamic analysis are conducted in this paper. Different slenderness of braces and different stiffness of braced beams are taken into consideration in the analysis. The results show that the seismic performance of braced frames with 1/3 brace resultant force-bearing beams is similar to braced frames designed by current codes.

Stability assessment and development of design criteria for SCBF systems with in‐plane buckling braces

AbstractSpecial concentrically braced frames (SCBFs) with in‐plane buckling (IPB) braces are often designed using the provisions applicable for out‐of‐plane buckling (OOPB) brace systems. As a result, the IPB brace systems may fail in the undesirable modes as observed in past experimental studies. In this study, a novel simplified analytical formulation based on the rigid beam spring model (RBSM) has been proposed to prevent the out‐of‐plane buckling of IPB braces. A high‐fidelity finite element model (FEM) is generated and validated with the experimental studies incorporating the influence of the residual stress, imperfection, and strain hardening. The variability, sensitivity, and uncertainties involved in predicting the direction of buckling of braces are examined. A parametric study has been carried out to investigate the influence of thickness and clearance of the gusset and knife plates, geometric imperfections, slenderness ratios, compactness ratios, residual stresses, and frame actions considering the most destabilizing effect in the OOPB mode. The third mode of buckling, termed as the mixed‐mode of buckling, has been observed in this study. A design criterion is proposed to get the desired IPB mode of buckling of braces in SCBFs. Multivariate regression analysis is carried out to incorporate the impact of various parameters as well as uncertainties in the formulation of the design criteria. The validity of the proposed equation is verified by comparing the findings of past experimental studies for various loading scenarios. Finally, a resistance factor of 0.8 in the LRFD approach has been proposed for the IPB brace systems.

Energy-based collapse assessment of concentrically braced frames under mainshock-aftershock excitations

Concentrically braced frames (CBFs) collapse mainly because of soft story formation at one or a few stories in which excessive brace buckling occurs. This study provides an intuitive understanding of the collapse mechanism of CBFs with 3–18 stories subjected to mainshock-aftershock sequences. Such understanding will support development of design methods that preclude low-capacity collapse modes specially under multi-shock excitations. This paper assesses the collapse mechanism as a stage in which the imposed seismic energy fails to dissipate and eventually leads to uncontrolled kinetic energy. The investigation focuses on the role and distribution of the various energy measures and different dissipating mechanisms throughout the structures. The employed structural models simulate softening-induced dynamic instability by modeling various inelastic damage modes including buckling of the brace members. Collapse mechanism is identified for various combinations of the utilized mainshock-aftershock pairs that are gradually scaled following the IDA process. The distribution of input and dissipated energies along various stories reveals the role of upper stories in damping the imposed energy. Furthermore, the similarity between the height profile of the residual drifts and the story imposed energies highlights the characteristic of the structures in adapting their drift response to a mode with the highest energy absorption.

Experimental investigation of header end-plate beam-to-column composite connections with single-corner gusset plates

Experiments on 6 full-scale header end-plate beam-to-column connections with concrete slabs and single-corner gusset plates of bracing members were conducted to study the contribution of concrete slabs and gusset plates to the hysteretic behavior of this type of connection. The main failure mode of the connections was the out-of-plane distortion instability of the steel beam caused by the bottom-up beam web crack near the connecting weld between the header end-plate and beam web. The experimental results showed that the connection had a certain energy dissipation capacity. Considering the concrete slab and the gusset plate, the connection behaved as a partially restrained connection and had flexural strength equivalent to 80% and 64% of the plastic moment of the beam under positive and negative moments, respectively. At a connection rotation of 0.02 rad, the connection still had a flexural capacity equivalent to 65% and 30% of the plastic moment of the beam under positive and negative moments, respectively, which provided a reserve capacity to resist seismic collapse of concentrically braced frame structures after brace failure. To make the gusset plate rotate out of the plane freely during brace buckling, some specimens were opened in the slab around the gusset plate. The existence of openings reduced the energy dissipation capacity and the flexural strength of the connection by 6% and 5%, respectively. The beam bottom flange comes into contact and extrudes with the column flange to regain stiffness and strength under negative loads in the later stage.

A variable stiffness energy dissipation device for drift control of steel frames

A novel variable stiffness energy dissipation device is conceptualized and numerically studied in this paper. The proposed device is a passive device that consists of an array of helical springs arranged in the form of a series of von Mises trusses anchored between two tubes. Under an external excitation, the tubes move relative to each other and energy is dissipated when the von Mises trusses undergo snap-through and snap-back actions. After presenting the theoretical underpinning of this device, its behavior is programmed using C++ and incorporated into the framework of an open-source software OpenSees. Non-linear time history analyses are then performed on several shear, moment resisting and braced frames to demonstrate the effectiveness of the device in dissipating energy and reducing structural deformations. The results show that the proposed device is capable of reducing both the peak and residual displacements of shear and moment frames, and preventing brace buckling of braced frames under three different levels of earthquake severity.

Modelling and Seismic Response Analysis of Non-residential Existing Steel Buildings in Italy

ABSTRACT Non-residential older buildings usually have a significant role in the economy of a territory, making their seismic response worth of investigation. To this end, archetype steel buildings were selected, with portal frames and stabilizing braces. Non-linear models were developed including brace buckling and fracture, connections, cladding panels. Both static and dynamic analysis were carried out considering three building sites and two performance levels: usability-preventing damage and global collapse. Results show significant effects of the structural choices on the seismic performance, depending largely on the connection characteristics. Especially, the cladding connections largely affected the exceedance of the usability preventing damage level.

SEISMIC COLUMN SPLICE DEMANDS IN BRACED FRAMES WITH BUCKLING CONTROLLED BRACES

Numerous experimental tests showed that ductile square hollow structural sections (HSS) are susceptible to premature fracture under design-level earthquake ground motions. In addition, recent research has highlighted the vulnerability of column splices to reach their full strength once ductile square HSS braces experience a fracture in braced frames. In this study, the performance of column splices in 5-and 13-story special concentrically braced frames (SCBFs), with X-bracing configuration, located at a distance permitted by the seismic design provisions (i.e., four feet over the concrete floor level), is evaluated under a large number of ground motion excitations. The performance of identical SCBFs with conventional HSS braces and innovative buckling controlled HSS braces is compared. Overall, results from this research indicate that the prevention of brace buckling in SCBFs yields a substantial reduction in the seismically induced bending demands on the column splices, leading to a large margin of safety to remain elastic.

Modification of Buckling Restrained Braces and Evaluating the Deflection Amplification Factor

Buckling is a main problem in every structure. It is a sudden change in shape or deformation of a structural component under load. Under moderate to severe earthquakes, buckling of compressive braces may cause damage to the joints and connections. So Buckling-Restrained Braces (BRBs) have been widely implemented in framed structures to reduce damage during severe earthquakes. Unlike conventional braces that buckle under compression, the core of BRBs yields both in tension and compression under the restraining effect of the casing. A typical buckling-restrained brace (BRB) is composed of a ductile steel core, which is designed to yield in both tension and compression. To avoid global buckling in compression, the steel core is usually wrapped with a steel casing, which is subsequently filled with mortar or concrete. So in this work the deflection amplification factor of these braces are found out. As DAF predicts the maximum capacity of the structure, so a deep study in this field is necessary. DAF is the ratio of in-elastic deformations to elastic deformation. So after finding the DAF of these BRBs and by knowing the elastic deformation of the structure we can easily find the in-elastic deformation. For this works the analysis are carried out using etabs and abaqus software.

A Finite Element Study on Compressive Resistance Degradation of Square and Circular Steel Braces under Axial Cyclic Loading

In this study, detailed finite element analysis was conducted to examine the seismic performance of square and circular hollow steel braces under axial cyclic loading. Finite element models of braces were constructed using ABAQUS finite element analysis (FEA) software and validated with experimental results from previous papers to expand the specimen’s matrix. The influences of cross-section shape, slenderness ratio, and width/diameter-to-thickness ratio on hysteretic behavior and compressive-tensile strength degradation were studied. Simulation results of parametric studies show that both square and circular hollow braces have a better cyclic performance with smaller slenderness and width/diameter-to-thickness ratios, and their compressive-tensile resistances ratio significantly decreases from cycle to cycle after the occurrence of the global buckling of braces.

Conventional I-shape brace member bolted connections under seismic loading: Laboratory study

In many regions of the world, a conventional low-ductility concentrically braced frame (CBF) is commonly chosen as the seismic force resisting system. The energy dissipation is not restricted to yielding and buckling of the braces, as would be required for ductile braced frames; rather, energy dissipation is assumed to occur through localised yielding and friction in connections, as well as limited member yielding. The objective of this study was to characterize by means of laboratory testing the inelastic response of full-scale conventional I-shape braces and their bolted connections under reversed-cyclic seismic loading. Six brace specimens comprising two commonly used bolted connection types, flange plate and flange angle, were designed following the Type CC provisions prescribed by CSA S16. The brace specimens achieved storey drift ratios of 1%–2%, due to inelastic deformations in both the braces and the connections, even though capacity based design provisions were not incorporated in their design. Energy dissipation resulted from overall and local brace buckling, gusset plate buckling, tension yielding of flange plates, angles and gusset plates, block shear failure of brace webs and angles, bolt fracture and fracture of the gusset plates, along with bolt slip. However, the observed performance of these braces and connections in this laboratory based study is not necessarily guaranteed to be replicated for all conventional CBF designs due to the conservative nature and variability in accuracy of the existing CSA S16 design equations that were relied upon to design and detail the test specimens.

Parametric study on the I-shape brace connection of conventional concentrically braced frames

Concentrically braced frames (CBFs), designed using the conventional linear elastic method without seismic proportioning and detailing requirements, are referred to as Conventional CBFs (CCBFs) in this study. They are widely used in moderate and low seismic areas in North America due to the ease of design and economy. Without a code specified dedicated fuse member to dissipate earthquake induced energy, or a prescribed yield/failure hierarchy, the brace connection of a CCBF is usually the weakest link in the lateral load-carrying path and prone to fracture. The brace connection is therefore determinant for the structural seismic performance. In this paper, a parametric study based on a validated numerical simulation procedure was carried out on a typical I-shape brace connection, i.e. the flange plate connection. Three key design parameters, namely, the gusset plate thickness, the flange lap plate thickness, and the web lap plate thickness, were varied to study their effects on both the compressive and tensile behaviour of the brace and connection assembly. Various possible failure modes were revealed both in compression and in tension. The results showed that the brace end restraint provided by flange plate connections in CCBFs was significant; the pinned-end assumption would lead to conservative estimation of the brace buckling resistance, which might trigger detrimental gusset plate buckling. The tensile overstrength of the flange lap plate, due to the presence of transverse tensile stress along the net section, was quantified using the von Mises criterion. Design recommendations are proposed with regards to attaining better deformation capacity.

Performance of a New Model of Rectangular Damper in Diagonal Concentric Brace

In this study, the performance of rectangular added damping and stiffness (RADAS) dampers has been introduced in the diagonal element site to avoid brace buckling and energy dissipation by dampers. The innovation of this research is in the type of site of steel plates and the way of connection in the vicinity of gusset plates which can be easily replaced. To investigate the cyclic performance of the ADAS damper, 15 numerical examples have been simulated by ABAQUS software. The study of cyclic behavior on the single-bay and single-floor steel frame has been done, and the sensitivity of cyclic behavior based on the thickness, length and ratio of dimension to the thickness has been studied. The studied thicknesses of the damper were 12, 21 and 30 mm, the studied lengths of the damper were 400, 500 and 600 mm, the studied geometry of the damper was rectangular, and the studied thicknesses of the brace were 12, 21 and 30 mm. The results of this study showed that this type of damper has good behavior in the energy dissipation of the frame and the total stiffness of the steel plates forming ADAS dampers must be less than the stiffness of the brace to show acceptable frame performance. By changing the damper length parameter from 600 to 500 and 400 mm, each of the indicators of stiffness, ultimate strength and energy has increased by a maximum of 144, 46 and 149%, by changing the damper thickness parameter from 12 to 21 and 30 mm, each of the mentioned indices has increased by a maximum of 147, 52 and 160%, and by changing the brace thickness parameter from 12 to 21 and 30 mm, each of the mentioned indices has increased by a maximum of 5, 7 and 9 percent. The values ​​of the damper thickness and length, when they are less stiff than the stiffness of the brace, cause the plastic hinge formation to be made on the damper, and the damper performance is optimized.

Hybrid braced frame with buckling-restrained and strong braces to mitigate soft story

The steel concentric braced frames (CBFs) are the common structural system for buildings. Unlike high strength and stiffness, brace buckling in CBFs results in high nonlinear behavior, which causes damage concentration in one or a few stories during a severe earthquake. This paper proposes a hybrid CBF bracing system to reduce soft-story mechanisms and improve the structural behavior of the CBF system. The proposed system is composed of a buckling restrained brace (BRB) or an energy dissipation device and a strong brace (SB) in one span. The strong braces are designed to have an elastic performance during an earthquake. The direction of BRB and SB are changed, inversely, story by story in a zigzag form. This configuration leads to paradoxical results: stiffness and ductility. This improves both the soft story mechanisms and the buckling capacity of CBFs. In this study, a set of 6-story structural models with different bracing patterns including inverted-V, multi-X, zipper, hexa (hexagonal bracing configuration), and strongback braced frames were evaluated through nonlinear static (pushover) and dynamic analyses (IDA). The collapse performance of structural models was assessed by comparing the seismic behavior of conventional CBFs with similar bracing configuration based on FEMA P695 criteria. From the evaluation of collapse fragility curves, the hybrid bracing system had more safety margin against collapse during severe earthquakes than conventional CBFs.

Development, testing and performance evaluation of steel beam-through framed connections with curved knee braces for improving seismic performance

An innovative beam-through framed connection equipped with curved knee braces is presented in this article. Curved knee braces with pinned connections are employed in semi-rigid beam-to-column joints. The proposed connection is expected to exhibit great deformability and energy dissipating capacity. The working mechanism and the theoretical behavior of the connection are firstly introduced, followed by an experimental study on three full-scale specimens under quasi-static cyclic tests. The test specimens exhibited stable behavior and great ductility. Damages were mainly concentrated on the knee braces within 0.02 rad drift ratio. The failure appeared when loading amplitude was above 0.07 rad drift ratio. The proposed connection is then introduced into beam-through steel frames (BTFs) which use strip braces as lateral force resisting system. To evaluate the contribution from knee braces, nonlinear response history analyses were performed on four demonstration BTFs. Based on the results of analyses, it is found that traditional BTF would exhibit high mode effect due to compressive buckling of strip braces, the inter-story drift is nonuniform along building height. Employing knee braces on traditional BTF stabilizes hysteretic behaviors and improves post-yielding strength for the system, global response of the system is reduced obviously and soft-story mechanism is mitigated. Moreover, results also indicate that satisfying peak transient inter-story drift and deformation uniformity can be achieved when strength demand from knee braces is adopted as 25% of total design inter-story strength demand.