Eccentrically Braced Frames Research Articles

An analytical model for shear links in eccentrically braced frames

When an eccentrically braced frame (EBF) is subjected to severe earthquakes, the links experience inelastic deformations while beams outside of the link, braces and columns are designed to remain elastic. To perform reliable inelastic analyses of EBFs sufficient analytical model which can accurately predict the inelastic performance of the links is needed. It is said in the literature that available analytical models for shear links generally predict very well the maximum shear forces and deformations from experiments on shear links, but may underestimate the intermediary values. In this study it is shown that available analytical models do not predict very well the maximum shear forces and deformations too. In this study an analytical model which can accurately predict both maximum and intermediary values of shear force and deformation is proposed. The model parameters are established based on test results from several experiments on shear links. Comparison of available test results with the hysteresis curves obtained using the proposed analytical model established the accuracy of the model. The proposed model is recommended to be used to perform inelastic analyses of EBFs.

Residual stress effect on link element of eccentrically braced frame

The effects of residual stress on the performance of link elements as lateral force shield, particularly for earthquake load as in the Eccentrically Braced Frame (EBF), is the main focus of this research. Experimental tests were conducted in two stages: First, the residual stress measurement of the link element with Neutron Diffraction technique using the equipment of DN1 at Center of Science and Technology of Advanced Material - National Nuclear Energy Agency (PSTBM-BATAN); second, two links were tested, a standard link that corresponds with the AISC 341–10 and a modified link. Behaviors of both links were studied based on the residual stress distribution in the first stage. The magnitude of tensile stresses on and around the “k area” caused the initiation of crack on the standard link so that it decreases the performance of the link element. Modifications of the link by replacing the stiffener of the vertical web with the horizontal stiffener can avoid the initiation of crack that may occur on the web plate around the “k area” so that the performance of the link elements can be improved.

Shear slotted bolted connection

Summary Slotted bolted connections (SBCs) have been developed and used as an axial friction damper in braced frames since 1980s. To employ the benefits of SBCs in moment resisting frames (MRFs), rotational slotted bolted connections have been developed more recently with limited application in members that flexural behavior is dominated to shear. In this paper, shear slotted bolted connection (SSBC) is introduced as a new type of friction dampers to employ the benefits of SBCs in lateral load resisting systems with predominant shear behavior members that dissipate energy by traditional yielding mechanisms. The SSBC is a modified bolted connection that dissipates energy through friction in which friction is activated by shear force. The applications of the proposed system as a shear link in link beams of eccentrically braced frames (EBFs), in the beams of MRFs, and coupling beams of coupled concrete shear walls are introduced. To show the efficiency of SSBC, an existing EBF with tubular link beam is equipped with SSBC, and its behavior is studied via models created in general purpose finite element program ABAQUS (SIMULIA, The Dassault Systemes, Realistic Simulation, RI, USA) verified thoroughly against relevant test results. Also, three MRFs with different beam lengths are modified using SSBC, and their monotonic and cyclic behavior are investigated using validated finite element models. The results show that, as expected, SSBC is capable of working as a mechanical shear fuse dissipating energy effectively in both MRFs and EBFs without any material yielding.

Seismic Behavior of Direct Displacement-based Designed Eccentrically Braced Frames

Direct Displacement-Based Design (DDBD) is a performence-based seismic design method that has been proposed and well developed over the past two decades to design RC frame structures, shear walls and bridges. In this method, the behavior of a multi-degree-of-freedom (MDOF) sysytem is approximaeted by an equivalent single-degree-of-freedom (SDOF) substitute structure. Although this method has been utilized to design the abovementioned structures, however, there is just one comprehensive DDBD method to design steel eccentrically braced frames (EBFs) in the literature. The purpose of this study is to investigate nonlinear seismic behavior of the DDB designed EBFs with short, intermediate and long link beams and estimate their seismic demands. To this end, twelve 3, 5, 9 and 12-story EBFs were designed using the DDBD method. To simulate the nonlinear cyclic behavior of link beams, a macro-model proposed in the literature was adopted and validated with the available tests results. In order to describe material nonlinearity of the framing members in the macro-model, distributed plasticity fiber based model was used. After validating the FEM macro-modeling technique of link beams, seismic behavior of the 2D EBFs was investigated with both pushover analysis and nonlinear time-history analysis under a set of selected earthquake records using the structural analysis software OpenSees.The results showed that the DDB designed EBFs generally can reach their anticipated performance level. Also, it was found that the overstrength of the frames depends on their links length ratio.

An analytical model for inelastic cyclic response of eccentrically braced frame with vertical shear link (V-EBF)

Eccentrically Braced Frames (EBFs) have a capability of energy dissipation relying on horizontal link beams in main frame. Use of vertical link (V-EBF) enhances system performance; main frame damaged, easy replaces after earthquake, and it can be rehabilitated minor changes of existing structure. This paper describes experimental and analytical study of the V-EBF system. Experimental results showed that ultimate shear strength of vertical link is more than two times of yielding strength. In analytical model, Kinematic-Isotropic strain hardening for shear, and only Kinematic for moment, showed accurate results with an upper bound for yielding surface to the vertical link.

Effect of stiffener arrangement on hysteretic behavior of link-to-column connections

Link-to-column connections in Eccentrically Braced Frames (EBFs) have critical role in their safety and seismic performance. Accordingly, in this study, contribution of supplemental stiffeners on hysteretic behavior of the link-to-column connection is investigated. Considered stiffeners are placed on both sides and parallel to the link web between the column face and the first stiffener of the link. Hysteretic behaviors of the link beams with supplemental stiffeners are numerically investigated using a pre-validated numerical model in ANSYS. It turned out that supplemental stiffeners can change energy dissipation mechanism of intermediate links from shear-flexure to shear. Both rectangular and trapezoidal supplemental stiffeners are studied. Moreover, optimal placement of the supplemental stiffeners is also investigated. Obtained results indicate a discrepancy of less than 9% in maximum link shear of the numerical and experimental specimens. This indicates that the numerical results are in good agreement with those obtained from the test. Trapezoidal supplemental stiffeners improve rotational capacity of the link. Moreover, use of two supplemental stiffeners at both ends of the link can more effectively improve hysteretic behavior of intermediate links. Supplemental stiffeners would also alleviate the imposed demands on the connections. This latter feature is more pronounced in the case of two supplemental stiffeners at both ends of the link.

Collapse Prevention seismic performance assessment of new eccentrically braced frames using ASCE 41

This paper presents the results of a seismic performance assessment using ASCE 41-06 for six eccentrically braced frames (EBFs) designed in accordance with the 2012 International Building Code. The correlation between ASCE 7-10 and ASCE 41-06 is investigated to compare the seismic performance anticipated by the two standards. Three archetype buildings (4-, 8-, and 16-story) with EBFs along one principal direction are designed for seismic effects: (1) once using the equivalent lateral force (ELF) procedure and (2) a second time using the response spectrum analysis (RSA) procedure. Performance assessments of the frames are conducted using four analysis procedures, static and dynamic analyses performed under both linear and nonlinear analysis regimes. Linear analysis results indicate performance issues in the columns and no issues in the links. In contrast, the nonlinear analysis results show that the links in the 8- and 16-story frames consistently fail to meet the assessment criteria due to significant concentrations of ductility demands. Furthermore, the RSA-designed frames generally perform poorer than their ELF-designed counterpart frames. The contributing factors to the noted performance issues with regards to ASCE 41 are investigated and recommendations are made on how to alter the performance outcome.

Fragility functions for eccentrically braced steel frame structures

Eccentrically braced frames (EBFs) represent an attractive lateral load resisting steel system to be used in areas of high seismicity. In order to assess the likely damage for a given intensity of ground shaking, fragility functions can be used to identify the probability of exceeding a certain damage limit-state, given a certain response of a structure. This paper focuses on developing a set of fragility functions for EBF structures, considering that damage can be directly linked to the interstorey drift demand at each storey. This is done by performing a Monte Carlo Simulation of an analytical expression for the drift capacity of an EBF, where each term of the expression relies on either experimental testing results or mechanics-based reasoning. The analysis provides a set of fragility functions that can be used for three damage limit-states: concrete slab repair, damage requiring heat straightening of the link and damage requiring link replacement. Depending on the level of detail known about the EBF structure, in terms of its link section size, link length and storey number within a structure, the resulting fragility function can be refined and its associated dispersion reduced. This is done by using an analytical expression to estimate the median value of interstorey drift, which can be used in conjunction with an informed assumption of dispersion, or alternatively by using a MATLAB based tool that calculates the median and dispersion for each damage limit-state for a given set of user specified inputs about the EBF. However, a set of general fragility functions is also provided to enable quick assessment of the seismic performance of EBF structures at a regional scale.

Experimental validation of re-centring capability of eccentrically braced frames with removable links

Modern seismic codes allow for inelastic deformation in dissipative zones during design earthquakes, accepting damage to the relevant structural parts to a certain extent. Experience has shown that repair work is needed after moderate to strong earthquakes. The use of structural systems that are easily repairable is therefore beneficial in seismic regions. For a structure to be repairable, in addition to constraining any inelastic deformation to removable dissipative members, any permanent (residual) drift should be eliminated. To realise a re-centring capability in a structure with eccentrically braced frames (EBFs) and removable dissipative members, a dual structural configuration is used, which combines EBFs and moment-resisting frames (MRFs). If the more flexible MRFs are kept elastic, they would provide the restoring force necessary to re-centre the structure upon the removal of the damaged removable links (the dissipative members). This paper presents an experimental validation of the re-centring capability of a dual eccentrically braced frame with replaceable links, based on a full-scale experimental testing programme, performed at the European Laboratory for Structural Assessment (ELSA) at the Joint Research Centre (JRC) in Ispra within the framework of the Transnational Access of the SERIES Project. The experimental set-up, programme, and instrumentation are described. The results attained through the re-centring of the structure are presented, as well as its overall seismic performance and information on the interaction between the steel frame and the reinforced concrete slab in the link region.

INCREASED DUCTILITY OF STEEL STRUCTURE FRAMES WITH ARRANGEMENT OF ECCENTRIC BRACES AND RESISTANT TO BUCKLING UNDER SEISMIC LOAD

One of the goals of seismic design of structure is to control damage to structure in severe earthquakes. Changing the outlook of seismic regulations from increase of the resistance of structure to increase of structure ductility reveals the necessity to attention to seismic assessment of structure regarding the outlook of ductility capacity (Ductility capacity). Due to the fact that the bracing change in height of the structure is less of interest to researchers and structural engineers, most of studies have centered on the brace changes in the structure plan and the braces are used without change in the height, thus study on increased ductility of steel structure frames with arrangement of eccentric braces and resistant to buckling under seismic load has been taken into consideration in this study. If it can improve seismic behavior of the brace by changing the type of brace in the height of structure, it can insure about the seismic function of the brace. Further, it can take action to optimize consumption of steel materials in metal buildings. For this, a number of eccentric braced steel moment frames with different number of storeys(6, 9 and 12 storeys)were undergone the nonlinear dynamic time history analysis after loading and designing based on Iranian standards, under 7 earthquake records including the near-fault earthquake record, and seismic damage to these frames were examined by changing type of bracing in Various height balances of these frames.

Finite element analysis of extended stiffened end plate link‐to‐column connections

AbstractThe applicability of extended stiffened end plate (ESEP) connections used as link‐to‐column connections in eccentrically braced frames (EBFs) with long (flexural yielding) links is examined in this paper. A finite element method (FEM) is used for this purpose, based on a validated parametric FE benchmark. Analysing the numerical model of an ESEP connection designed to the recent seismic design rules for special moment frames reveals that the link‐to‐column connections of EBFs sustain more severe conditions than the moment connections of moment‐resisting systems. The design approach implemented is examined and the results are discussed. The results demonstrate that ESEP connections can be used as a successful alternative for the link‐to‐column connections of EBFs and the system with this type of connection can achieve the required rotations for long or flexural links.

Evaluation of Seismic Response Factors for Eccentrically Braced Frames Using FEMA P695 Methodology

This paper reports details of a numerical study undertaken to evaluate seismic response factors for steel eccentrically braced frames (EBFs) using the FEMA P695 methodology. Six archetypes were designed by making use of the current U.S. specifications, and their behavior was assessed by making use of nonsimulated collapse models. Results indicate that the current values of response factors result in designs with higher collapse probabilities than expected. Two modifications were developed to bring the collapse probability of these archetypes to acceptable levels. The first modification is on the deflection amplification factor while the second one is on the response modification coefficient. Six archetypes were redesigned using the proposed modifications and reevaluated using the FEMA P695 methodology. The results indicate that the proposed modifications are adequate to satisfy the target collapse probability. Maximum and cumulative link rotation angles were observed to be less than the predefined limits.

Fundamental period of irregular eccentrically braced tall steel frame structures

This paper presents the results of investigation on the fundamental periods of eccentrically braced frame (EBF) structures with varying geometric irregularities. A total of 12 EBFs are designed and analyzed. Based on the results obtained from vibration theory, equations for the approximate fundamental periods are put forth for EBFs which take into account vertical and horizontal irregularities. Through statistical comparison, it was found that a 3-variable power model which is able to account for irregularities resulted in a better fit to the Rayleigh data than equations which were dependent on height only. The proposed equations were validated through a comparison of available measured period data for EBFs. These proposed equations will allow design engineers to quickly and accurately estimate the fundamental period of EBF structures by taking into account their irregularities.

Comparison of Seismic Behavior of Eccentrically Braced Frames with Vertical and Knee Links in Retrofitting the Reinforced-Concrete Buildings with Intermediate Moment-Resisting Frame Systems

Given the fact that some reinforced-concrete (RC) buildings have not met resistance against seismic loads, it is mandatory to adopt a suitable and economically cost-efficient method to retrofit them. One appropriate method is application of steel Eccentrically Braced Frames (EBFs). This article is aimed at evaluation of seismic performance of retrofitted reinforce-concrete buildings by EBFs with single and knee links. For evaluation of modeled buildings, different editions of the Iranian Seismic Code (Standard No. 2800) were reviewed. To this end, three Moment-Resisting Frames (MRF) of four-, eight-, and twelve-story buildings with medium ductility were modeled where they were designed for seismic loads based on Standard 2800, 2nd edition. In order to evaluate buildings under modified seismic loads, models were seismically reloaded based on the Standard 2800, 3rd edition. Reanalysis showed that the stress ratios exceeded 1 in most columns. Therefore, buildings were retrofitted using EBFs with knee and single vertical links and their seismic performance were evaluated using nonlinear static analysis. Results indicate that knee bracing systems are more efficient than bracing systems with vertical link in increasing rigidity and controlling displacements, while they significantly reduce ductility. For example, application of knee bracing system in the twelve-story building can cause an 11% reduction in displacement in comparison to bracing systems with vertical links.

Evaluation of Seismic Performance Factors in High Rise Steel Buildings with Dual Lateral Systems Consisting of Buckling Restrained Braced Frames and Intermediate Moment Frames

Dual lateral systems are commonly used in high rise buildings for various architectural reasons. Buckling Restrained Braced Frame (BRBF) is an emerging seismic force-resisting system that is currently being used either as a single seismic force-resisting system or in combination with other seismic force resisting systems. American Society of Civil Engineers (ASCE) provides distinct directions regarding the estimation of Seismic Performance Factors such as, Seismic Response Modification Factor (R), Over Strength Factor (Ωo) and Deflection Amplification Factor (Cd) when Eccentrically Braced Frames (EBF) and Special Moment Resisting Frames (SMRF) are used in combination. But ASCE gives no clear suggestion about what should be the values for R, Ωo and Cd when stiffer BRBF systems are used in conjunction with softer Intermediate Moment Frame (IMF) Systems. Since, these seismic parameters (R, Ωo and Cd) vary significantly for BRBF systems in comparison with IMF systems, it is essential to estimate and evaluate the correct values for them. In conventional practice, ASCE suggests that when a stiffer lateral system is used in conjunction with a softer system in a dual configuration, the lowest Response Modification Factor (R) pertaining to the softer system shall be used.

IRREGULAR STEEL BUILDING STRUCTURES SUBJECTED TO BLAST LOADING

In seismic design, structural irregularity has been found to have a significant influence on structural response. The impact of structural irregularity on the global response of steel frame structures subjected to blast loading has not been examined. In the paper, six seismically designed steel framed structures are considered: moment resisting frames (MRF), concentrically braced frames (CBF) and eccentrically braced frames (EBF) each with geometric irregularity in the plan and with a geometric irregularity in the elevation. The blast loads are assumed to be unconfined, free air burst detonated 15 ft from one of the center columns. The structures are modeled and analyzed using the Applied Element Method, which allows the structure to be examined during and through structural failure. A plastic hinge analysis is performed as well as a comparative analysis observing roof deflection and acceleration to determine the effect of geometric irregularity under extreme blast loading conditions. Two different blast locations are examined. Conclusions of this research are a concentrically braced frame provides somewhat of a higher level of resistance to blast loading for irregular structures and geometric irregularity has an impact on the response of a structure subjected to blast loading.

Effects of near-fault records characteristics on seismic performance of eccentrically braced frames

In this paper the effects of fling-step and forward-directivity on the seismic performance of steel eccentrically braced frames (EBFs) are addressed. Four EBFs with various numbers of stories (4-, 8-,12- and 15-story) were designed for an area with high seismic hazard. Fourteen near-fault ground motions including seven with forward-directivity and seven with fling-step effects are selected to carry out nonlinear time history (NTH) analyses of the frames. Furthermore, seven more far-field records were selected for comparison. Findings from the study reveal that the median maximum links rotation of the frames subjected to three set of ground motions are in acceptable range and the links completely satisfy the requirement stated in FEMA 356 for LS performance level. The arrival of the velocity pulse in a near-fault record causes few significant plastic deformations, while many reversed inelastic cycles result in low-cycle fatigue damage in far-fault records. Near-fault records in some cases are more destructive and the results of these records are so dispersed, especially the records having fling-step effects.

Direct Displacement-Based Seismic Design of Eccentrically Braced Steel Frames

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.

Cyclic Behavior of Very Short Steel Shear Links

AbstractA replaceable coupling beam is proposed comprising of steel hybrid shear links that are shorter than typical shear links in eccentrically braced frames (EBFs). Cyclic loading tests were conducted to examine the behavior of these very short shear links. The test variables included the steel type, length ratio, web stiffener configuration, and loading protocol. The link specimens showed two types of failure modes: link web fracture and fracture at the weld connecting link flange to end plate. The link specimens had an inelastic rotation capacity of approximately 0.14 rad, which is significantly larger than the capacity assumed for EBF links. Links using LY225 steel instead of Q235 steel achieved a 25% increase in inelastic rotation and 44% increase in cumulative plastic rotation. The overstrength factors of the very short shear links reached 1.9, significantly exceeding 1.5, which is the value assumed for EBF links by design provisions. Analysis suggests that large overstrength can develop in very s...

Effect of Link-Beam Stiffener and Brace Flange Alignment on Inelastic Cyclic Behavior of Eccentrically Braced Frames

Finite element analysis was used to investigate the effects of the misalignment of the brace flange-to-beam connection point with the link-end stiffener (referred to as the offset in the paper) on the ductility of eccentrically braced frames (EBF). The offset was speculated to be a possible reason for the unexpected EBF failures observed in the aftermath of the Christchurch earthquake series of 2010 and 2011. EBF models with different detailing at the offset area were analyzed under monotonic and cyclic displacements. Results showed severe stress and strain concentration in the offset area, preventing the EBF from developing its expected ductility, and suggested possible initiation of a failure from the part of link flange located in the offset area. Simulation using the ductile fracture model implemented in ABAQUS resulted in a fracture pattern in agreement with the actual failed EBF. Results from analyses on different detail configurations showed that removal of the offset by modifying the brace section to build an ideal case, or by a simple change in the location of the link stiffener, can mitigate the problem of possible premature failure, with the latter solution being slightly less effective but much easier to be used in practice.