Braced Steel Frames Research Articles

Seismic performance evaluation of high‐post‐yield stiffness concentrically braced steel frames under mainshock‐consistent‐aftershock sequences

AbstractStructures may be subjected to earthquake sequences after major mainshock (MS) events in seismically active sites within a short time. As a result, they may be susceptible to damage accumulation, which may hinder their performance under consecutive seismic loading. This study evaluates the effects of earthquake sequences on the seismic performance of seismic‐resistant concentrically braced steel frames designed to Eurocode‐8. The frames under investigation have concentric chevron‐type braces with replaceable hourglass‐shaped pins made of duplex stainless steel. The seismic energy is dissipated through inelastic deformations concentrated in the pins while keeping the other members elastic. The stainless‐steel pins provide the frame with high‐post‐yield stiffness to reduce the residual drifts after a seismic event. The seismic behaviour of the frame is assessed using site hazard‐specific mainshock‐consistent‐aftershock (MS‐AS) sequences selected for a site in Terni, Central Italy. Nonlinear back‐to‐back dynamic analyses are performed at multiple intensity levels while adopting detailed numerical nonlinear models created in OpenSees. We show that the implemented behaviour factor satisfies the life safety assurance objective while keeping the maximum residual inter‐storey drifts below 1/300 to permit an easy substitution of the damaged pins after the design‐level earthquake without being curtailed by the potential following events. We then develop prediction models for the damage accumulation in the pins, considering different energy‐based intensity measures and we show that the cumulative absolute velocity‐based model is the most efficient predictor in this particular case. Finally, the damage accumulation in the pins is evaluated, confirming their superior low‐cycle fatigue capacity under earthquake sequences.

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.

Seismic fragility functions for RC As-built and eccentric steel brace frames

Quasi-static cyclic loading tests were performed on single-story reduced-scale reinforced concrete as-built (un-retrofitted) and eccentrically steel braced (retrofitted) frames. The frames damage mechanism, force–displacement hysteretic response and capacity backbone envelopes were studied and a damage scale was developed. A simplified numerical modeling technique was proposed in the finite element-based package SeismoStruct and the model was evaluated, calibrated and validated with the test data. The proposed modeling technique was employed further to perform incremental dynamic time history analysis on two-story as-built and eccentric steel braced frames. Fragility functions for the frames in terms of immediate occupancy, life safety and collapse prevention were developed using a probabilistic-based methodology. The as-built frame was more fragile in terms of immediate occupancy, life safety and collapse prevention damage states as opposed to the retrofitted frame.

Development of Steel Yielding Seismic Dampers Used to Improve Seismic Performance of Structures: A Comprehensive Review

Seismic excitation can cause significant energy to be released within structures. By using special devices, this energy can be consumed and dissipated without deforming structural members significantly. Due to this, structural damage is minimized, casualties are prevented during earthquakes, and structures are extended in their useful life. Over the past five decades, it has been widely acknowledged that steel yielding dampers are among the best energy dissipation devices. It has been stated that the hysteretic behavior of steel yielding dampers could vary slightly depending on their geometry. From a practical point of view, they are suitable for the improvement of seismic safety in new and existing structures. The purpose of this paper is to present a review related to steel yielding dampers, their development, various types, and applications, in order to help understand the role of these dampers in improving the seismic performance of structures. In terms of their shape, steel yielding dampers can be categorized as steel plate dampers, pipe dampers, curved dampers, and slit dampers. The most common use of steel plate, such as ADAS and TADAS, and pipe dampers is within braced frames, whereas U-shaped, J-shaped, and S-shaped dampers are mostly seen in frames with chevron bracing. Steel curved dampers with a 60° angle in a steel-braced frame, on the other hand, provide the best energy dissipation and frame strength. In this direction, until today, steel slit dampers have been found to be the most commonly used steel yielding dampers.

Seismic performance evaluation of eccentrically braced steel frame buildings

Seismic performance evaluation of eccentrically braced steel frame buildings

Experimental and numerical assessment of a steel frame equipped with Dissipative Replaceable Bracing Connections

In the last decades, high priority has been given by the research community to the development of low-damage structures, and reparability has become fundamental for minimizing the environmental and economic impact of reconstruction. In this context, the European Research Fund for Coal and Steel (RFCS) project DISSIPABLE was carried out, with the aim to test large-scale structures where the dissipation is concentrated on replaceable components introduced in the structure. In this paper, the performance of a six-storey braced steel frame with dissipative systems is analysed. The capacity of withstanding seismic actions relies on Dissipative Replaceable Bracing Connections (DRBrC), used for the brace-column joints. The energy dissipation is ensured by wide hysteresis loops experienced by DRBrC, whose configuration enables an easy replacement after a medium-high intensity earthquake. Results of a wide experimental test campaign on full-scale structures and numerical analyses on refined models are presented and compared. In particular, experimental data were used to validate and to calibrate the simplified numerical laws used to represent the cyclic performance of the dissipative components and it was proved the effectiveness of using a simplified formulation from both a theoretical and a practical point of view.

Conceptual design of anti-seismic devices with metal foam core for CBFs

Metal foam is a relatively new and potentially revolutionary material due to its properties such as the low weight-to-stiffness ratio, high damping and energy dissipation capacity. Even if this material is widely used in mechanical, aerospace and automotive industries, it is still not frequently used in civil engineering applications. The large energy dissipation capacity that metal foams possess could be exploited to control the seismic performance of structures, but only few research investigations have been focused on this matter. Within this context, the present paper investigates a possible solution to realize dissipative aluminium foam dampers to be applied in X-braced steel structure resembling a concentrically braced steel frame (CBF). The bracing system consists of a steel rod or tube linked to the aluminium foam damper. The damper device is constituted by aluminium foam that provides the structure with dissipation capacity when activated under compression. The permanent deformations of the aluminium foam are absorbed by a wedge device, avoiding a pinching behaviour in the global response. Analytical equations governing the global behaviour of the bracing system are herein developed. An X-braced steel frame complying with Eurocode 8 provisions is designed and considered as case-study. The structure is subsequently upgraded by introducing the dissipative device into the bracings. The results of preliminary experimental and numerical tests on the components of the device are herein presented, showing the potentiality of the solution.

Seismic evaluation of concentrically braced steel frames equipped with yielding elements and BRBs

As a way of increasing lateral load resistance, Concentrically Braced Frames (CBFs) are employed in steel structures. Structures with CBF display high lateral stiffness and experience lesser story drifts than structures with Moment Resisting Frame (MRF). Despite the advantages, there are some major problems in the performance of the braced frames, such as low capacity in energy dissipation, buckling of conventional brace members, and degradation of strength and stiffness. To increase the energy dissipation of braced frames, central yielding elements in the intersection of braced members have been proposed in the past. In this study, BRB elements were placed in the bracing system to improve energy dissipation and lateral stiffness and to reduce buckling in the bracing. Designing a structure that includes the stated lateral-load-resisting system necessitates seismic performance parameters to be evaluated. As the study's main purpose, by pursuing the FEMA P695 methodology, seismic parameters, including response modification factor, over-strength factor, and deflection amplification factor (R-, Ω-, and Cd-factor), are specified. Several archetypes with various properties were considered to reflect the possible application of this bracing system. Based on FEMA P695, static pushover analyses are implemented to establish over-strength factors for archetypes. Subsequently, Incremental Dynamic Analyses (IDA) are implemented to specify Adjusted Collapse Margin Ratios (ACMRs). Finally, the acceptance of the abovementioned assumed factors for the system is scrutinized by comparing ACMRs values with the values which are acceptable in FEMA P695.

Behavior of Gusset Plates Connecting Angle Sections in Compression

This paper contains experimental and numerical study on the behavior of the gusset plate in compression. Gusset plate connections are usually used for the transfer of load from one member to another, they are generally used in bridge trusses and steel braced frames. However, due to lack of technical information regarding the behavior of the gusset plate under compression, these have been under research focus. It has been seen that the gusset plate shows a buckling failure by swaying on one side as there is no out of plane restraint. In this paper, a study is initiated aimed at the gusset plate connected with the angle section on a single side. Key items that were investigated in the study were strain, strength, and displacement. A total of 5 specimens were used for the experimental investigation. The parameters which were considered for the study, the number of bolt lines and the thickness of the gusset plate. Based on the experimental results of load and strain, a numerical model was developed and compared with experimental load-carrying capacity of plate and displacements. After going through the study unpredicted behavior was observed for the two-bolt line and suggestions were given accordingly.

Performance Assessment of Different Retrofitting Techniques for Steel-Framed Buildings

The requirement for cost-effective critical infrastructure has made employing nonlinear analysis techniques an appealing choice. Estimating seismic loads on structures necessitates explicit consideration of inelastic structural behavior. Hence, nonlinear approaches such as pushover analyses have become more popular. In this study, the performance of a ten-story steel moment-resisting frame retrofitted with various bracings like diagonal-bracing, chevron bracing, x-bracing and k-bracing is evaluated using linear and nonlinear analysis. The responses, such as base shear, lateral load distribution, displacement, drift, and lateral stiffness, are determined using the ETABS-18 building analysis and design program. The results of the study show that the steel-braced frame systems have substantially greater strength and ductility capabilities than their unbraced or alternate steel-braced moment-resistant steel frame. Further, the unbraced steel frame has a low R-factor as compared to the bracing system due to the increased stiffness at the story level.

Seismic Performance of an Inverted V-type Eccentrically Braced Steel Frames with Slit Dampers Using Shape Memory Alloy

Seismic Performance of an Inverted V-type Eccentrically Braced Steel Frames with Slit Dampers Using Shape Memory Alloy

Performance-based seismic design of laterally braced steel frames

Improving the performance of structural frames is one of the most important focuses of today's researchers. The total base shear capacity of the structural frames will increase by reaching the maximum number of plastic hinges. In this work, the effects of bracing system type on different heights of steel frames were investigated. Static pushover analysis was used to evaluate the performance of 4, 8, and 12-story steel frames with seven structural configuration systems. The results showed that the performance of braced frames increased significantly in terms of number of plastic hinges, total base shear and performance point compared to unbraced frames. The capacity curves were maximum in the one-story X-bracing, multi-story X-bracing, and single diagonal bracing systems.

Seismic performance of steel braced frames with innovative assembled self-centering buckling restrained braces with variable post-yield stiffness

An innovative assembled self-centering buckling restrained brace (ASC-BRB) with variable post-yield stiffness is proposed in this study, aiming to better control structural responses under earthquakes of different intensity levels. The proposed ASC-BRB consists of the buckling restrained brace (BRB) system and self-centering (SC) system, where the SC system provides the variable post-yield stiffness with trilinear elastic behavior. The overall configuration and restoring force model of the brace are first described, and six independent hysteretic parameters are determined to comprehensively describe its hysteretic behavior. A simplified truss model of the ASC-BRB is established using OpenSees, and the experimental and numerical hysteretic curves coincide well with each other, indicating that the simplified model has satisfactory accuracy. The simplified model is then utilized to investigate the effect of the design parameters on the hysteretic behavior of the ASC-BRB. Nonlinear dynamic analyses are also conducted for three 6-story steel frames with different types of braces. The comparison results indicate that the proposed ASC-BRB with high post-yield stiffness can effectively limit the displacement development at the lower floors, but the structural high-mode effect will be further amplified. Finally, system level parametric analyses are performed to study the influence of brace design parameters on the structural responses, and reasonable values for these design parameters are recommended according to the parametric analysis results.

STUDY ON SEISMIC RESPONSE OF STEEL FRAMES INTEGRATED WITH BRACING SYSTEMS

Increasing lateral stiffness and energy dissipation capacity is considered an effective solution to improve the seismic performance of building structures. The bracing system is one of the most effective and reliable solutions in seismic-resistant design for frame structures. In this study, the step-by-step of design for a bracing system according to the displacement-based design method is detailed. A typical study is performed on a steel-braced frame structure, subjected to earthquake ground motion to evaluate the effectiveness of the solution. The obtained results show that the bracing has good effects in reducing the horizontal displacement and shear force for building structures.

Experimental Study of the Seismic Behavior of a Prefabricated Low-Rise Steel Frame Structure with Hinged Joints

This paper investigated the seismic behavior of a prefabricated steel braced frame structure with hinged joints. Six steel frame specimens with different enclosure walls were tested under pseudo-static loading. The results indicated that the vertical load of the hinged braced frame system was mainly resisted by the beam and column members, and the lateral stiffness was completely provided by the bracing members. The final failure mode of all specimens was the failure of the bracings, while the beam-column members and the joints remained largely intact. The rigidly braced specimen was mainly damaged by buckling, yielding, and tearing, and the flexibly braced specimen was mostly damaged by buckling, yielding, and node failure. The energy dissipation of the specimens primarily depended on lateral force-resistant components such as braces and enclosure walls. Different building envelopes exert significant effects on the lateral stiffness and energy dissipation capacity of the structure. The ductility coefficient of all specimens ranged between 1.4 and 1.9, which indicates that the structural system mainly relies on lateral stiffness and elastic deformation to resist earthquakes, rather than structural ductility. The proposed prefabricated steel frame system with hinged connections has wide prospects of application in economically underdeveloped areas because of its convenience in transportation and installation.

Inspection and repair considerations for downtime assessment of seismically isolated buildings

This paper investigates the seismic downtime of seismically isolated buildings with steel moment or braced frames designed by the procedures of ASCE/SEI 7–16. The seismic isolation systems considered in this study are comprised of triple or double friction pendulum isolation bearings with and without moat walls. The seismic downtime is calculated from the damage to structural components and non-structural components, demolition and collapse of buildings. The downtime components (repair and inspection) are defined, and mathematical expressions are provided for the computation of downtime fragility curves, expected annual downtime, and economic losses due to the expected annual downtime. The procedure is then implemented using the results of nonlinear response history analysis from previous studies by the first author. The study demonstrates that the expected annual downtime of seismically isolated buildings is less than that of the comparable non-isolated buildings regardless of the seismic isolation systems used. Among the cases of seismically isolated buildings studied in this paper, it is found that the most effective structural system to mitigate long downtime is the seismically isolated building with seismic isolators with enhanced sizes and with braced frames that are designed to be minimally compliant with the seismic design requirements of ASCE/SEI 7–16.

Dynamic Response of Concentrically Braced Steel Frames to Pulse Period in Near-Fault Ground Motions

Steel braced frame systems (SBFs) having high stiffness and high strength are commonly utilized due to their resistance to lateral seismic forces in regions with high seismicity. In this study, concentrically braced frames (CBFs) having different bracing configurations are used to obtain the significance of the pulse period associated with near-fault (NF) ground motion by time-history dynamic analysis. Besides, far-fault (FF) ground motions are also used to compare with NF ground motion results according to chancing bracing configurations. To achieve dynamic responses of steel frames with different concentric bracings under NF ground motions, which especially have small, medium, and long pulse periods, 3-story and 4-span CBFs having different bracing configurations were selected as an example. 4 FF and 12 NF ground motions having different pulse durations were chosen to evaluate the dynamic response of concentrically braced frames. The results showed that peak ground acceleration (PGA) could be identified as a key parameter that controls the response of braced frames under FF ground motions. In addition, the ratio of the pulse duration to the first mode period is the dominant parameter when this ratio is only greater than 1.0 under the NF ground motions.

Finite Element Parametric Analysis of High-Strength Eccentrically Braced Steel Frame with Variable-Cross-Section Replaceable Link

This paper proposes a new type of variable-cross-section replaceable link that can isolate the plastic deformation. The link and the frame beam connect with the expanding section by a bolt along the flange and web separately. It is easy to design an elastic bolt, reduce bolt slippage, and facilitate link replacement after an earthquake. Considering the large elastic deformation range of high-strength steel and the superior plastic deformation of the new type of replaceable link, a high-strength eccentrically braced steel frame with a variable-section replaceable link is raised by setting a new type of variable-section replaceable link in the eccentrically braced steel frame. Then, the existing end plate connection replaceable link test specimen is simulated and verified by using ABAQUS software. The finite element model of the high-strength eccentrically braced steel frame with a variable-cross-section replaceable link is established, and the bearing capacity, stiffness, plastic rotation, plastic distribution, and other bearing mechanisms of the structure are studied by cyclic loading. The length of the energy-consuming region (e), the steel strength of the link and other components, and the length of the replaceable link (e’) are compared and analyzed with regard to the seismic performance of the structure. The results are of great significance for understanding and exploring the force mechanism, energy dissipation characteristics of the new variable-section replaceable link, and the seismic performance of the high-strength eccentrically braced steel frame, and it also provides a reference for subsequent research.

A Framework to Assess Seismic Resilience of Self-Centering Steel Frame Buildings

Resilience can be treated as an integrated measure to assess the seismic performance of buildings. Available methods to assess the seismic performance of self-centering structures mainly consider inter-story drift ratios, peak floor accelerations, and energy dissipative capacities, however, their seismic resilience is poorly understood. This paper proposes a framework to estimate the seismic resilience of self-centering energy dissipative braced steel frame office buildings. In this context, a new damage ratio assessment model is presented to assess functionality. Unlike the traditional method that only considers the economic loss or repair time in the functionality assessment, the proposed model accounts for the contribution of four performance measures such as downtime. To estimate the repair cost, repair time, casualties, and unsafe placarding, the FEMA P58 methodology is used. Based on the proposed model, the new damage ratios and recovery times of the self-centering structure are estimated. Obtained damage ratios are combined with fragility analysis to calculate the functionality loss. After considering an appropriate recovery model, seismic resilience is assessed. For comparison, the traditional method (i.e. HAZUS) is also used to assess seismic resilience. The whole framework is applied to a three-story self-centering energy dissipative braced steel frame, and the results show that the traditional method often underestimates the seismic resilience, and the proposed method may be more suitable for predicting the seismic resilience of self-centering energy dissipative braced steel frame office building. Sensitivity analysis is performed to identify uncertain parameters to which seismic resilience is most sensitive, and the result indicates that the control time and damage ratio at the extensive damage state have a major influence on seismic resilience. The framework can be modified and extended to assess the seismic resilience of ordinary building structures and other complex structural systems such as isolated structures.

Experimental investigation of braced steel frames equipped with dissipative replaceable connections

Experimental investigation of braced steel frames equipped with dissipative replaceable connections