Eccentric braced frames: seismic response and design challenges

Eccentrically Braced Frame (EBF) is a structural system that is advised to be built in seismically active areas since they are characterized by good stiffness and ductility. A large and stable hysteretic curve, which corresponds to good seismic performance, is produced by the combination of improved stiffness and ductility in EBF. The diagonal component of EBF, known as a brace, contributes to its stiffness. Meanwhile, the short beam, also known as the link element, provides ductility in EBF. One element that is essential as an energy dissipator in EBF is a link element. By displaying a sizable and steady hysteretic curve, a prior study found that EBF with a flexural link could effectively dissipate the seismic energy. But to achieve a higher EBF, the seismic performance still needs to be enhanced. An analysis of various EBF models in Inverted V configurations was conducted in this paper. Each model was prepared with different shear link characteristics. Installing web stiffeners in the link to improve its seismic performance was also taken into consideration in this study. To obtain seismic performance, the cyclic loads were employed to each model under conditions of yield displacement control. Analysis of the data resulted in the load-displacement hysteretic curve. Next, using the hysteretic curve, the three seismic performance parameters, i.e., strength, stiffness, and dissipation energy were further developed. The investigation showed that compared to earlier studies, the EBF with shear links showed a bigger and more stable hysteretic curve which means better dissipated energy. Additionally, adding web stiffeners significantly increases the EBF's seismic capability. Therefore, because of the improved seismic characteristics, it is advised to establish the EBF using a shear link reinforced by web stiffeners in an earthquake-hazard area.

Eccentrically Braced Frames (EBF) have been researched upon and also implemented in practice worldwide, in the last three to four decades. However, the Indian Steel design code is still silent on this topic and does not provide any design provision for EBF. This paper is aimed to propose IS code compatible model design guidelines for EBF buildings. The guidelines are proposed after conducting a comparative study of design provisions from various national codes viz. American, European and New Zealand.The adequacy of proposed model guidelines is evaluated by modeling and designing suitable example buildings and subjecting them to non-linear pushover analysis. Suitable variations were introduced in examples by varying the number of storeys as five and ten; and designing the buildings as EBF with simple beam-column connections and moment resisting frame-EBF dual systems. The observations from analysis of different buildings are presented which indicate that EBF buildings demonstrated the ‘Life Safety’ performance level for the Design Basis Earthquake (DBE) but failed to achieve the desired ‘Collapse Prevention’ performance at the Maximum Considered Earthquake (MCE). Performance of buildings with dual lateral load resisting system, improved but was still not satisfactory along one direction at MCE hazard level. The paper discusses the efficacy of proposed model design guidelines for application of lateral loading and the potential issues in the satisfactory design of EBF buildings.

Moment-resisting frames (MRFs) and eccentrically braced frames (EBFs) can be effectively combined in multi-story steel buildings. In fact, MRFs provide redundancy and ductility, while EBFs provide initial and post-yield stiffness with high energy dissipation capacity. In addition, if detachable links are designed as dissipative fuses, it is possible to activate their almost contemporary yielding and prevent the collapse of structures during severe earthquakes, ensuring easy repair and rehabilitation measures. The seismic responses of dual frames with MRFs and EBFs are investigated by means of pushover analyses. Different buildings were designed, varying the number of floors (two, four, and eight stories), the number of MRF spans (zero, one, and two), and the length of the short links (0.8 Mp/Vp and 1.6 Mp/Vp). The obtained results confirmed the superior behavior of dual frames with respect to simple EBFs. In addition, it was observed that the frames with the shorter links exhibited higher resistance and rigidity but smaller overall displacement capacity because of the anticipated failure of the links.

A series of eccentrically braced frames (EBF) are designed and subjected to nonlinear analyses to highlight ambiguities and differences in current seismic design provisions for EBF structures. This provides motivation to implement better guidance for the checking of local displacement demand considerations and move towards a displacement-based design approach. A recently proposed direct displacement-based design (DDBD) procedure for EBFs is then described and further developed in this article through the calibration of a spectral displacement reduction factors that relate the displacement of an inelastically responding structure to that of the equivalent linear representation used in the DDBD of EBFs. Such an expression is calibrated as part of this study using an experimentally validated numerical model also proposed here for the EBF links such that the actual hysteretic behavior of the links is well represented. The DDBD guidelines are applied to EBF systems from 1–15 stories in height and their performance is verified via nonlinear dynamic analyses using two different sets of design spectrum compatible ground motions. The results of the study indicate the robustness of the proposed DDBD method in limiting the interstory drifts to design limits for a variety of EBF systems with short links, thus demonstrating that the proposed DDBD method is an effective tool for seismic design of EBFs.

During severe earthquakes, the ductility and energy dissipation capacity of shear links in Y-shaped eccentrically braced frames (EBFs) are crucial to achieve desirable seismic performance and structural safety. In this study, an innovative SSL-SSBC combining the advantages of a shear slotted bolted connection (SSBC) and a very short shear link is developed to improve the seismic performance of Y-shaped EBFs. Finite element (FE) analysis methods are first validated through comparison with existing experimental results for slotted bolted connection and very short shear links. Further, FE simulations of the SSBC, very short shear link with shear slotted bolted connection (SSL-SSBC), and Y-shaped EBF are conducted to determine their mechanical behavior systematically. The FE results demonstrate that a reasonable SSBC can effectively increase the ductility and energy dissipation capacity through friction slipping, allowing the SSBC to maintain elastic behavior throughout the entire range of seismic loading, while the very short shear link in the SSL-SSBC could undergo severe damage to dissipate seismic energy. As expected, the SSL-SSBC is capable of working as both a shear dissipation damper and a metallic ductile damper in the Y-shaped EBF. This study also clearly identifies that no yielding or damage of the very short shear links occurs during small and medium earthquakes, and the deformation and damage of the very short shear link can be minimized during severe earthquakes, allowing the seismic resilience capacity of the Y-shaped EBF to be dramatically enhanced.

Resistant structures, such as Eccentrically Braced Frames (EBFs), are required to be established in earthquake-hazard areas. In EBFs, an essential beam component, known as link, plays a crucial role in determining the performance of these structures. As the seismic energy dissipator, link experiences failure to prevent heavy damage to other EBFs members, such as beams and columns. Although several studies have been carried out to enhance the seismic performance of link, there is still limited regarding these elements. Therefore, this study aimed to investigate several IWF flexural links constructed in FE models of Inverted-V EBFs. A total of three flexural links with different stiffener spaces were considered namely non-spacing, 250 mm, and 100 mm stiffener spacing links. To assess their performance, cyclic loading with yield displacement control was used in EBFs models. Observations were mainly conducted on EBFs performances consisting of strength, stiffness, and dissipation energy. The results showed that the best seismic performance was specified on a 100 mm-stiffener spacing link by presenting the largest and most stable hysteresis curve. Based on these results, the configuration of the narrow space stiffener in the link element was recommended to improve the seismic performance of EBFs.

The Theory of Plastic Mechanism Control (TPMC) has been recently extended to the case of Eccentrically Braced Frames (EBFs) with inverted Y-scheme, i.e., EBFs with vertical links. In this paper a further validation of the design procedure, based on TPMC, is provided by means of Incremental Dynamic Analyses (IDA) pointing out the fulfilment of the design goal, i.e., the development of a pattern of yielding consistent with the collapse mechanism of global type where all the links are yielded and all the beams are yielded at their ends while all the columns and the diagonal braces remain in elastic range with the only exception of the base sections of first storey columns. In particular, a study case is designed according to both TPMC and Eurocode 8 provisions and the corresponding seismic performances are investigated by both push-over and IDA analyses. The results show the different performances obtained in terms of pattern of yielding, maximum interstorey drift, link plastic rotation demand and sharing of the seismic base shear between the moment-resisting part and the bracing part of the structural system. The seismic performance improvement obtained by means of TPMC, compared to Eurocode 8 provisions, is pointed out.

In Eccentrically Braced Frames (EBFs), link beams in lower storeys may experience a high risk of vulnerability due to the nondistribution of plastic hinges in the frame height and this problem leads to the formation of the soft storey. To mitigate such vulnerabilities, successive link beams can be tied throughout the structure height employing tie elements to develop a tied EBF (TBF) system. Unlike many studies on K-TBFs with shear link beams, limited investigations have evaluated the performance of single diagonal TBFs and K-TBFs with flexural link beams. In this study, K-EBFs/TBFs refer to EBFs/TBFs having link beams with two braces and single diagonal EBFs/TBFs refer to EBFs/TBFs having link beams with one brace. K-EBFs with flexural link beams have drawn the attention of designers because these frames make more space in the architectural design to create larger openings. Accordingly, improving the performance of K-EBFs with flexural link beams is one of this research objectives. The results comparison for 18 2D EBFs with different storeys including K-EBF with shear behavior, K-EBF with flexural behavior, and single diagonal EBF with shear behavior indicated that tie elements in these models can increase energy dissipation capacity and distribute plastic hinges in the structure height. The results exhibited that not only this system performance was considerably improved using tie elements, but also offered better resistance against progressive collapse.

In the present work, the results of a number of experiments carried out on single-span, single-storey eccentrically braced frames (EBFs) have been reported. Full-scale braced frames have been tested in order to investigate the overall behaviour of EBFs, consisting of the link beam, bracing members and columns. The effects of connections, bracing members and their interaction to the behaviour of the link beam have been taken into consideration. I-Profile Européenes (IPE) sections to DIN-1025 have been used as the link beams of the test specimens in order to investigate their competence for use as link beams of EBFs. At the same time, the effects of the distances between the web stiffeners in such shear links as well as the distances between their lateral supports have been addressed. It has been shown that the IPE sections may be used as link beams of the EBFs to achieve the expected performance with the appropriate provision of lateral supports and web stiffeners. Attention has also been paid to the ductility capacity and the observed modes of failure, in relation to their effects on the energy absorption capability of the system owing to premature buckling or fracture of the members and connections. The observed behaviour of the test specimens has been demonstrated clearly and some subject matters of interest have been discussed and some aspects of the existing seismic design provisions for the EBFs have been assessed.

Seismic performance assessment of novel self-centering friction-based eccentrically braced frames

This study investigates the seismic performance of reinforced concrete Eccentrically Braced Frames (EBF) with vertical and horizontal link beams subjected to cyclic loading. The main objective is to compare strain histories and evaluate energy dissipation in vertical and horizontal link configurations. Experimental results indicate that vertical links, especially those with shorter lengths (15 cm), exhibit higher strain values, leading to significant plastic deformation and enhanced energy dissipation, though at the cost of increased damage and repair needs. Conversely, horizontal links remain largely within the elastic range, maintaining better structural integrity but offering less energy dissipation during seismic events. The CBF (Concentrically Braced Frame) control specimen showed minimal deformation and lower energy absorption. The findings suggest that vertical links are more suitable for energy dissipation in seismic design, while horizontal links offer greater durability and lower post-event maintenance. The balance between energy absorption and repairability is crucial for optimizing EBF systems in earthquake-prone areas. (explain the findings related to the purpose of this paper: to determine the effects of link orientation on strain distribution, energy dissipation, and structural integrity during seismic events).

Eccentrically Braced Frames (EBFs) are the structural systems that are recommended to be constructed in Earthquake Hazard Areas. This system combines good characteristics in withstanding the seismic load, i.e., stiffness, ductility, and dissipation energy. Link is a component in EBFs that play the role of the seismic energy dissipator. As far, several studies have been conducted to enhance the link performance, but the information is still limited. This study observed the structural performance of the IWF flexural link, where this link will experience failure at the flange due to seismic energy dissipation. The investigation was conducted by establishing three Finite Element (FE) models of flexural link. Each link was designed with the same length, i.e. 1000 mm. Three-link models were distinguished by the differences in web stiffener configuration, i.e., non-stiffener link, 250 mmstiffener spacing link, and 100 mm-stiffener spacing link, respectively. All models were examined under cyclic loading with yield displacement controlled. The research discovered that adding the web stiffener on the IWF flexural link significantly escalated the EBFs’ seismic performance, i.e., strength, stiffness, ductility, and dissipation energy. The best seismic performance was presented by a 100 mm-stiffener spacing link. Accordingly, it is indicated that arranging the web stiffeners at the narrow space in the IWF link can be suggested to improve the seismic performance of EBFs.

Comparison of thermal performance of steel moment and eccentrically braced frames

In this work, a tied braced frame (TBF) was developed to achieve uniform inelastic deformation in an eccentrically braced frame (EBF) by connecting links across the entire frame height with tie members. Herein, a TBF design method is proposed, considering a new lateral force distribution pattern. To better evaluate the seismic performance, and verify the design advantages of the TBF, nonlinear time-history analysis and fragility analysis were conducted using 6-, 10-, and 20-story TBF models designed using this method, as well as EBF models for comparison. It was found that the maximum inter-story displacement angles of the TBF model were reduced by 10%, 3.3% and 6.3% at the 84th percentile at 6, 10 and 20 stories, respectively, and the DCF values were also reduced by about 5.5%, indicating that the design of the TBF structure is more reasonable. The results revealed that the TBF models featured more uniform distributions of the normalized link shear forces and inter-story drift ratios, resulting in a better damage distribution and more ductile behavior. Furthermore, under earthquakes, the tie axial forces were similar to those calculated using the design equation, thereby indicating the reliability of the design method. Under the same seismic conditions, the PGA values of the TBF structure are about 10~15% lower at 50% exceedance probability compared to the EBF structure; the CMR values of the 6-story, 10-story, and 20-story models are 1.12, 1.09, and 1.06 times higher than those of the EBF structure, respectively. Notably, based on a comparison of the exceedance probability from the fragility analysis results for the TBF and EBF models, the TBF model exhibited better anti-collapse performance.

Seismic response of steel multi-tiered eccentrically braced frames