Eccentrically Braced Frames Research Articles

Earthquake-induced economic loss estimation of eccentrically braced frames through roof acceleration-based nonmodel approach

This paper develops a nonmodel-based approach for seismic loss assessment of buildings with eccentrically braced frames (EBFs). An extensive database is utilized to predict engineering demand parameters (EDPs), including, peak and residual story drift ratios, peak story link rotations, and also peak floor absolute accelerations, along with the height of the buildings. The estimated EDPs are used to assess the total economic losses associated with the collapse, demolition, and repair of structural and/or non-structural components. The database includes seismic intensity measure, improved wavelet-based refined damage-sensitive feature (rDSF) assembled only by the roof absolute acceleration response, geometric information, and EDPs of the 4-, 8-, and 16-story prototype models, extracted from incremental dynamic analyses subjected to 44 far-field ground motions. The nonlinear model of the aforementioned structures was already developed by the authors to capture the response of the structures up to collapse. Symbolic and Bayesian regressions are carried out in addition to ordinary least square linear regression to develop the empirical equations. Moreover, a thorough study is performed on the optimal selection of the intensity measures proposed in the literature as the input variable of the predictive equations through reliability-based sensitivity analysis. To estimate the first mode period of the considered structures to compute the improved wavelet-based rDSF and promote a nonmodel-based approach, Auto-Regressive model with exogenous input is employed. The results show that the story-based EDPs and also the corresponding expected economic losses of EBF buildings are accurately predicted compared with those obtained from incremental dynamic analyses. Consequently, the proposed nonmodel-based approach paves the path for rapid earthquake-induced economic loss assessment of EBF buildings in emergency operations.

Effects of cyclic strain hardening on performance of eccentrically braced frames

Performance based seismic design (PBSD) procedures have been developed in recent years. Studies conducted in the past revealed that strain hardening ratio (SHR) used in numerical modeling can be influential on the performance of steel structural systems. In this paper, the effects of SHR on the performance assessment of eccentrically braced frame (EBF) buildings were studied by using FEMA P-58 methodology. The numerical model was validated using the pseudo-dynamic test results of the DUAREM structure. Seismic performance of six archetype EBFs with different number of stories and link length to bay width (e/L) ratios were evaluated under design basis earthquake (DBE), maximum considered earthquake (MCE), and collapse level earthquake (CLE). Irreparable residual drift, repair cost and unsafe placards were calculated for SHR ranging between 0 and 0.01. The results showed that the residual interstory drift ratio is more influenced by the SHR when compared with the interstory drift ratio, link rotation angle and story acceleration. The replacement decision was found to depend on the SHR, the number of stories, the e/L ratio, and the seismic hazard. Under MCE and CLE level events, almost all of the archetypes with SHR ≤ 0.003 required replacement while replacement can be avoided for some of the archetypes with SHR > 0.003. Under DBE level events, 3-story archetypes and 6-story archetypes with SHR ≥ 0.002 did not require replacement. Nine-story archetype with e/L = 0.08 required replacement for SHR = 0 only, while archetypes with e/L = 0.12 required replacement for SHR ≤ 0.003. The study demonstrated the strong influence and sensitivity of the repair cost to the SHR.

A study on mid-panel of steel shear-walls with dual openings

The presence of openings in steel shear-walls is oftentimes necessary due to different design requirements. This paper investigates the behaviour of a panel in between two openings (mid-panel or MP) in shear-walls with dual openings under monotonic loading. The panel is analogised to link-beams in eccentrically-braced frames. To examine the structural response of these structures, two established conditions (elastic force and displacement) are addressed. Theoretical formulations are developed for a new condition (flexural stiffness of stiffeners to shear stiffness of a plate) in addition to the two conditions above. Furthermore, an equation was derived to calculate the shape factor of sections with box-shaped flanges, which is used to obtain the shear displacement (yield) of the mid-panel's plate. A comprehensive parametric study (44 models) is conducted using Finite Element Analysis (FEA) to examine the effect of six different geometric factors on the structural response of the mid-panel. The mid-panel's dominant structural responses (deformation and stress distribution) are classified as (i) flexural, (ii) shear, and (iii) combined (shear-flexure). The increase of the MP's width, the box-stiffeners' web height, flange's width and thickness causes a shear response, while the increase of the MP's height and the thickness leads to a flexural behaviour. Each structural response is carefully addressed theoretically and numerically leading to consistent results. The variables of the MP's plate bring about higher effects on the behaviour of the MP comparing to the parameters of the stiffeners. Overall, a shear response is structurally desirable, which enhances the MP's energy dissipation and post-buckling capacity.

Cyclic loading in vertical stainless steel links

AbstractVertical links are dissipative devices in form of beam‐column segments that are included in eccentrically braced frames (EBF). Studies related to different configurations of links have been presented for carbon steel elements. Moreover, stainless steel is a nonlinear metallic material with particular appeal for its structural use due to a considerable amount of capabilities such as corrosion, circular life‐cycle, strength, ductility and aesthetics. Despite the increase on the use of stainless steel for structural elements in construction, the behavior of this material for structural purposes is not described as carbon steel in all applications. The elastic stiffness, maximum load, shear deformation capacity as well as the energy dissipation in long links are generally affected by the defined geometrical parameters. Due to the material nonlinearity, this influence may be affected considerably. In this paper, a numerical model aimed at depicting the cyclic behavior of vertical short and long links subjected to seismic lateral loads is presented. The model accounts for the material nonlinearity, the geometrical nonlinearity and the detailed definition of boundary conditions from the link connection to the EBF.

On the topological optimization of horizontal links in eccentrically braced frames

This paper presents an implementation of structural optimization algorithms for the design of horizontal links of Eccentrically Braced Frames (EBFs) when subjected to monotonic shear loading. The objective is to study optimized geometries that open new design paths for these types of elements. With the advent of new additive manufacturing techniques such as WAAM (wire and arc additive manufacturing), energy dissipators may become one of the strategic applications for 3D printing of steel structures. For this purpose, an optimization procedure based on a displacement-controlled lateral load (push-over) applied to a single-storey EBF has been created using Abaqus FEA and Abaqus Optimization Module (ATOM). The performance of the optimized links has been compared with the performance of original links in terms of initial stiffness, yield strength, maximum strength and energy dissipation. The results show a set of promising geometrical configurations for horizontal links. Further research lines related to cyclic loading, anisotropic behaviour and advanced loading protocols are suggested.

Development of a novel haunched link for eccentrically braced frames

The seismic performance of eccentrically braced frames (EBFs) is strongly influenced by the length of their links. In these frames, short links undergo shear yielding, long links experience flexural yielding, and intermediate links go through a combination of shear and flexural yielding. Hence, shorter links have a greater inelastic rotation capacity than longer links. However, having the option to use longer links is architecturally preferable as it allows the designer to place larger openings in the walls of braced bays. This paper proposes the use of haunched links instead of intermediate and long conventional links in order to accommodate architectural considerations without undermining seismic performance. In the proposed design, the depth of the middle section of the link is reduced so as to ensure that shear yielding will always take place in this area. Finite element modeling, validated with experimental data, is used to study the behavior of the haunched link and compare it with conventional links. The results show that the proposed haunched link has a higher inelastic rotation capacity and energy dissipation capacity than conventional links. They also indicate that an EBF with haunched links will have a higher deformation capacity and ductility than a similar EBF with conventional links.

The influence of the axial restraint on the overstrength of short links

Eccentrically braced frames (EBFs) constitute a good structural solution located between Moment Resisting Frames (MRFs) and Concentrically Braced Frames (CBFs) because they exhibit an adequate lateral stiffness joined with a high ductility capacity, especially for what concerns short and very short links. Link design plays an important role in the effective dissipation capacity of the structure that is well designed only if the link members are involved in plastic range while the other members remain elastic. A proper design of the non-dissipative members needs to account for the link overstrength that results to be very high, especially for very short links.In this paper, the influence of the axial restraints on the evaluation of the link overstrength and ultimate rotation is investigated. A wide parametric analysis is carried out based on a set of shapes selected in the European Standards. FEM simulations are used to develop the parametric analysis after a calibration carried out on experimental tests selected from a literature review. Empirical formulations are also proposed with the scope of providing a more accurate evaluation of the overstrength factor for short and very short links in the framework of EU “I shaped” standard profiles.

Development of Performance Based Plastic Design of EBF Steel Structures Subjected to Forward Directivity Effect

Performance-based plastic design (PBPD) is a method which considers the inelastic behavior of a structure for the seismic design that follows a pattern in which a pre-selected yield mechanism is defined for each type of structural system. Despite the general adequacy of this method compared to the code-based method, the need to further improve this method against Forward-Directivity, the well-known characteristic of near-fault ground motions, is of high importance. On the other hand, Eccentrically Braced Frame (EBF) is considered to be an efficient conventional steel-framed system that combines the advantages of moment-resisting frame and concentrically braced frame. In this study, attempts have been made to improve the conventional PBPD method in order to consider the effect of forward-directivity. To this end, the energy modification factor (γ) was modified by recalculating ductility demand µ and ductility reduction factor Rµ of three 6-, 12-, and 18-story EBF structures. Using incremental dynamic analysis, these structures were subjected to 12 well-known near-fault ground motions, and accordingly, the modified energy factor (γ′) was obtained for each set. Then, an equation was proposed to calculate the γ′ based on the fundamental period of the structure. As expected, the code-based designed structures exceeded the allowable drift limits. Although the structures which were designed based on conventional PBPD followed the pre-selected yield mechanism and performed better, they exceeded the drift limits as well. Consequently, the structures which were re-designed using modified energy factor γ′ could well satisfy the acceptance criteria as well as achieving the desirable yield mechanism.

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

Eccentrically braced frame (EBF) is a robust seismic force resisting system. EBF provides strong stiffness and clear energy dissipation to resist the earthquake shaking. However, after strong earthquake shaking EBF may be difficult or too expensive to repair. This could result in prolong down time and significant repair costs. In this paper, a novel energy dissipation device named self-centering conical friction damper (SCFD) is implemented within EBFs to dissipate the earthquake energy and to reduce the residual drift of EBF. The new system is named self-centering friction-based EBF (SCFB-EBF). The performance of the SCFB-EBF is compared with conventional EBF. Three prototype buildings with 3-, 6-, and 9-stories were designed using conventional EBF according to the AISC341-16/ASCE7-16 specifications and SCFB-EBF by replaced the links of the EBF with SCFD. The prototypes were subjected to a suit of 44 ground motions at both the design based earthquake (DBE) and maximum considered earthquake (MCE) shaking intensities according to the FEMAP695 procedure. Detailed nonlinear time history analyses were carried out using OpenSees. The results of the nonlinear time history show that SCFB-EBF has superior performance compared to the conventional EBF. In addition, the residual drifts of the SCFB-EBF were reduced significantly compared to the conventional EBF. Performance-based earthquake engineering (PBEE) assessment was also conducted to examine the performance of the prototype buildings. The results show that SCFB-EBF has lower repair cost at both DBE and MCE levels. Hence, the study shows that the SCFB-EBF can be used as an efficient alternative compared to seismic force resisting system.

Investigation of Steel Frames Equipped with Steel Eccentric Braces and Steel-Concrete Buckling-Restrained Braces Having Moment Link

Background: Different bracing systems of steel Eccentric Braces (EBs) and steel-concrete Buckling-Restrained Braces (BRBs) can be used in steel frames in order to make the frames stronger in resisting lateral loads. These steel frames with EBs or BRBs are generally called Eccentrically Braced Frames (EBFs) or Buckling-Restrained Braced Frames (BRBFs), respectively. Objective: This study aims to investigate steel frames with bracing systems of steel EBs and steel-concrete BRBs having moment link. Methods: The EBFs and BRBFs are nonlinearly analysed employing the finite element software ABAQUS. Experimental tests of the EBF and BRB are utilised for the validation of their modelling. The modelling is validated by comparing the modelling results with experimental tests results. Then, an EBF and a BRBF are designed having moment link. The extreme earthquake records of Tabas, Chi-Chi, and Northridge are selected for the dynamic analyses of the EBF and BRBF. The validated modelling method is applied to analyse the designed EBF and BRBF under the selected earthquake records. Results: The achieved results from the analyses are lateral displacements, base shears, and energy dissipations of the EBF and BRBF and moment link rotations. These results are compared and discussed. Conclusion: It is concluded that the hierarchy of the lateral displacements of the analysed EBF and BRBF, having moment link, is related to the Tabas, Chi-Chi, and Northridge records because the lateral displacements of the frames are directly proportional to the peak ground accelerations of the records, and there is the same hierarchy for the records in terms of their peak ground accelerations. Lower lateral displacements are witnessed for the BRBF than the EBF subjected to the Tabas and Chi-Chi records. However, larger lateral displacement is observed for the BRBF than the EBF under the Northridge record. The same procedure as the lateral displacements is also revealed for the effectiveness of the BRBF with regard to its link rotations compared with the EBF. Moreover, the BRBF improves the base shear capacities and energy dissipations of the frame compared with the EBF. Consequently, the BRBF is generally demonstrated to be superior to the EBF from the structural performance point of view. Thus, the BRBF can be used more efficiently in structures subjected to large lateral loads compared with the EBF.

Extended end-plate connections for replaceable shear links

Extended end-plate moment connections are used in a number of applications including the beam-to-column connections in seismic moment resisting frames (MRFs) and replaceable link-to-frame connections in eccentrically braced frames (EBFs). While the extended end-plate connections have been extensively studied for MRF applications, little is known about their performance in EBFs. The loading conditions and the acceptance criterion are different for these connections when they are used in MRFs or in EBFs. This paper presents an experimental and numerical study undertaken to investigate the performance of extended unstiffened and stiffened end-plate connections used in replaceable shear links. Pursuant to this goal, 10 nearly full-scale EBF tests were conducted where the thickness, width and stiffening of the end-plate were considered as the variables. The results showed that end-plates designed according to the AISC guidelines or Eurocode provisions show acceptable performance in terms of the target link rotation angle. Due to strain hardening effects, thinner plates than the ones suggested by the codes were also found to show satisfactory performance. Finite element simulations were conducted to investigate the bending strains for different plate thicknesses and to study the levels of axial forces developed in the links. Modifications to the AISC design guidelines have been proposed to determine the plastic resistance of end-plated connections more accurately.

Seismic behavior of double-HSS links for eccentrically braced frames

This study experimentally and analytically investigates an innovative double-HSS link (DHL) used as the active link in eccentrically braced frames (EBFs). Hollow structural sections (HSS) are mechanically efficient and are used in resisting compression, torsion, and bending. Their high torsional rigidity generally eliminates lateral-torsional buckling (LTB). As a result, the plastic rotational capacity and strength degradation rate are mainly governed by flange local buckling (FLB) and web local buckling (WLB). The proposed configuration consists of a double-HSS connected to braces or columns through a center gusset plate secured by flare-bevel groove welds. The advantages of the proposed configurations are: (1) They effectively reduce the width-thickness ratios (b/t) of the flanges by using two HSSs with much shorter flanges, which considerably increases the compactness thereby enabling a large rotational capacity. (2) They provide simple and practical connection details between the link and braces or columns. (3) Typically, double-HSS sections have an e/(Mp/Vp) ratio much larger than that of wide-flange sections due to the fact that double-HSS sections have four webs which significantly increase the shear capacity. Consequently, a DHL can yield in flexure even with a length similar to that of a wide-flange shear yielding link. This in turn eliminates the need for web stiffeners. (4) The welding requirement for DHLs is less than that of the conventional shear links since no stiffeners are required. (5) The overstrength factor due to strain hardening produced by flexural yielding links is generally less than that produced by shear yielding links leading to more economical designs for the elastic portion of the frame, and (6) the iteration design process is minimized by balancing the link length and rotational demand because of a single flexural yielding mode. Furthermore, experimental and analytical results indicated that the novel DHLs can sustain larger plastic rotations and meet AISC 341′s acceptance criterion for shear yielding links. In addition, DHLs can be potentially used in D-braced EBFs with links attached to columns because the link-to-column connection of DHL remains essentially elastic. DHLs can also be used as a replaceable link which is attached to the beam and column through bolted end plates.

Progressive collapse in an eccentric braced frame through extended progressive collapse analysis algorithm

Progressive collapse in an eccentric braced frame through extended progressive collapse analysis algorithm

Nonlinear Analysis of Eccentrically Braced Frames against Progressive Collapse

Eccentrically braced frame (EBF) incorporates the advantages of lateral resisting systems from moment resisting frame (MRF) and concentrically braced frame (CBF) into a single structural system. In order to investigate the effects of brace and link length in resisting progressive collapse, a finite element model of 10-story planar steel frame were built and analyzed with the SAP2000 software through nonlinear static analysis and nonlinear dynamic analysis respectively. The results show that brace can obviously improve the performance of the structure no matter which kind of brace is adopted. The inverted V-shaped (IV) eccentrically braced frame has the highest capacity against progressive collapse and the Y-shaped eccentrically braced frame has the highest energy absorption capacity of the structure. Increasing the link length leads to an increase in deformation and energy absorption of the structure. The results of nonlinear static analysis could reflect the structural performance basically.

Mid-spliced end-plated replaceable links for eccentrically braced frames

Eccentrically braced frames (EBFs) can be re-used after a major seismic event by replacing the link members. Recent years have witnessed the development of numerous replaceable links. Among various details developed for this purpose, extended end-plated replaceable links are found to be the most efficient. The use of these links enables the engineer to minimize the size and the weight of the part to be replaced. In addition, the performance of end-plated links is similar to that of conventional links. Research reported to date has shown that end-plated links have disadvantages in terms of removal and replacement. Large axial forces can develop within the link member. Removal and replacement operations may require the use of hydraulic jacks to push the ends of the collector beams and the flame cutting of the links to gradually release the residual stresses and deformations. More important are the difficulties associated with link replacement under residual frame drifts. A novel detachable, replaceable link is proposed in this study which employs a splice connection at the mid-length of the link. The splice connection consists of channel sections welded to both parts of the replaceable link. The detail employed provides an erection tolerance which facilitates easy removal and enables replacement under residual frame drifts. Proof-of-concept testing of the proposed links was performed on 3 specimens where the type of force transfer in the splice connection was considered as the prime variable. All specimens failed at link rotation angles that were significantly higher than the link rotation angle required by AISC341 and demonstrated the potential of the proposed link concept. Complementary finite element parametric studies were conducted to validate the design procedure developed for the proposed replaceable link concept.

Seismic response of steel multi-tiered eccentrically braced frames

Steel eccentrically braced frames (EBFs) are commonly used as a lateral load-resisting system, in particular in high seismic regions, due to their high ductility, stable and reliable yielding mechanism, and architectural versatility as compared to concentrically braced frames. This article presents the use of EBFs in tall single-storey structures where multiple braced panels with intermediate links are stacked along the frame height, which is referred to as multi-tiered eccentrically braced frames (MT-EBFs). A set of nine frames having different heights, number of tiers, relative tier heights, and brace-to-beam connections are designed in accordance with the Canadian steel design standard. The seismic performance of the frames is then examined using nonlinear response history analysis (NLRHA) with an emphasis on the stability of intermediate links and columns as well as in-plane tier drift and column moment demands. As potential solutions to this problem, alternative intermediate link designs are examined and their seismic performance is presented. The results show that lateral frame deformations tend to non-uniformly distribute between tiers due to uneven link shear yielding, which impose large moments on outer beams and in-plane and out-of-plane bending demands on the columns, which in some cases leads to column buckling. The unbraced intermediate links are found to be prone to lateral out-of-plane buckling. Alternative design strategies except MT-EBFs with laterally-braced links did not appreciably improve link and column stability. Intermediate link stability and column flexural demands shall be considered in seismic design of MT-EBFs.

Evaluation of Damage Index of Eccentrically-Braced Frames according to Link Beam’s Length with Soil-Structure Interaction Effects

Evaluation of Damage Index of Eccentrically-Braced Frames according to Link Beam’s Length with Soil-Structure Interaction Effects

The dynamic load test of two seismic resistance of steel structures EBF and RBS

The non-destructive dynamic loading test has been conducted on two seismic resistance of steel structures, reduced beam section (RBS) and eccentric braced frame (EBF). Eccentric mass shaker was used to excite the 1-storey 1-bay steel structure. Two dynamics parameters were obtained which were the natural frequency and the damping ratio. A finite element analysis was performed using OpenSees software to get a comparison result. Similar natural frequency and displacement response were obtained from the test and the FE Analysis. The Natural frequency of RBS and EBF was around 3.614 Hz which is similar to the FE analysis as 3,623 Hz. Meanwhile the damping ratio is 3.149%. The natural frequency of the EBF structure is around 6.0 to 6.13 Hz, while the FE analysis result is 6.13 Hz. The damping ratio from test results is between 2.04 – 2.06%.

Study of Eccentric and Buckling-Restrained Bracing Systems Used in Frames

The present paper studies eccentric braces (EBs) and buckling-restrained braces (BRBs) used in steel frames. The eccentrically braced frames (EBFs) and buckling-restrained braced frames (BRBFs) that have respectively employed the EBs and BRBs are considered with different types of links as shear, moment-shear and moment links depending on their link length which is an important factor in the design of the EBFs and BRBFs. The BRB consists of a steel core and its surrounding steel tube filled with concrete. The concrete confinement prevents buckling of the steel core. The analysed EBFs and BRBFs by the finite element software ABAQUS under earthquake records are taken into account. Effects of the links of the EBFs and BRBFs on the performance of the frames are discussed. The results uncover that most of the lateral displacements of the EBFs and BRBFs having the shear link are smaller than their counterparts having the moment-shear and moment links, whilst, all the base shear capacities of the former are the greatest. However, majority of the EBFs and BRBFs with the moment link dissipate less energy than their counterparts with the shear and moment-shear links, whereas, most of the link rotations of the former are smaller than the latter. In addition, the BRBFs generally demonstrate better performance than their EBF counterparts.

Study of inelastic behaviour steel structure of special moment frame (SMF) and eccentrically braced frame (EBF) with pushover analysis

High rise buildings should be made as sufficient seismic performance, so it won’t immediately collapse when an earthquake occurs. Therefore, lateral strengthened stories are required in high rise buildings to enhance the lateral rigidity of structure. The use of lateral strengthening has a great effect on the entire seismic performance. A 20-stories high rise steel structure building was evaluated on this research comparing the structures’ inelastic behaviour using two seismic force-resisting system, i.e., special moment frame and eccentrically braced frame. The analysis of inelastic behaviour in this research using static non-linear analysis, i.e., pushover analysis. By using pushover analysis, the inelastic behaviour of structures, including story drift, shear story, displacement, plastic joint formation, and ductility, were obtained. In the elastic condition, the shear story of the eccentrically braced frame system is smaller than the special moment frame system. While in plastic condition, the shear story of the eccentrically braced frame system is larger than the special moment frame system. The story drift and the displacement of the special moment frame system are indicated to be larger than the eccentrically braced frame system. These results show that the eccentrically braced frame system has better rigidity than the special moment frame system.