Eccentrically Braced Frames Research Articles

Seismic performance of eccentrically braced frames with high strength steel combination

Eccentrically braced frames (EBFs) often use conventional steel with medium yield strength. This system requires structural members with large cross-sections for well seismic behavior, which leads to increased material costs. In eccentrically braced frames with high strength steel combination (HSS-EBFs), links use Q345 steel (specified nominal yield strength 345 MPa), braces use Q345 steel or high strength steel while other structural members use high strength steel (e.g., steel Q460 with the nominal yield strength of 460 MPa or steel Q690 with the nominal yield strength of 690 MPa). For this approach can result in reduced steel consumption and increased economic efficiency. Several finite element models of both HSS-EBFs and EBFs are established in this paper. Nonlinear hysteretic analyses and nonlinear time history analyses are conducted to compare seismic performance and economy of HSS-EBFs versus EBFs. Results indicate that the seismic performance of HSS-EBFs is slightly poorer than that of EBFs under the same design conditions, and HSS-EBFs satisfy seismic design codes and reduce material costs.

Seismic response of EB-frames with inverted Y-scheme: TPMC versus eurocode provisions

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.

Relationship between hardness and plastically deformed structural steel elements

A field based non-destructive hardness method is being developed to determine plastic strain in steel elements subjected to seismic loading. The focus of this study is on the active links of eccentrically braced frames (EBFs). The 2010/2011 Christchurch earthquake series, especially the very intense February 22 shaking, which was the first earthquake worldwide to push complete EBF systems into their inelastic state, generating a moderate to high level of plastic strain in EBF active links for a range of buildings from 3 to 23 storeys in height. Plastic deformation was confined to the active links. This raised two important questions: what was the extent of plastic deformation and what effect does that have on post-earthquake steel properties? A non-destructive hardness test method is being used to determine a relationship between hardness and plastic strain in active link beams. Active links from the earthquake affected, 23-storey Pacific Tower building in Christchurch are being analysed in the field and laboratory. Test results to date show clear evidence that this method is able to give a good relationship between plastic strain and demand. This paper presents significant findings from this project to investigate the relationship between hardness and plastic strain that warrant publication prior to the completion of the project. Principal of these is the discovery that hot rolled steel beams carry manufacturing induced plastic strains, in regions of the webs, of up to 5%.

AN INVESTIGATION OF THE EFFECTIVENESS OF THE FRAMING SYSTEMS IN STEEL STRUCTURES SUBJECTED TO BLAST LOADING

The effectiveness of different framing systems for three seismically designed steel frame structures subjected to blast loading is investigated. The three faming systems considered are: a moment resisting frame (MRF), a concentrically braced frame (CBF) and an eccentrically braced frame (EBF). The blast loads are assumed to be unconfined, free air burst detonated 15 ft (4.572 m) from one of the center columns. The structures are modeled and analyzed using the Applied Element Method, which allows the structure to be evaluated during and through failure. Failure modes are investigated through a plastic hinge analysis and member failure comparison. Also, a global response analysis is observed through comparison of roof deflections and accelerations. A conclusion of this research is that braced frames provide a higher level of resistance to the blast loading scenario investigated in this research. Both the CBF and EBF had a smaller number of failed members and plastic hinges compared to the MRF. They also had smaller roof deflection and acceleration. The CBF yielded the fewest number of plastic hinges but the EBF had a slightly fewer number of failed members.

Reliability assessment of compression columns in seismic EBFs

In recent years, eccentrically braced frames (EBFs) have widely been used in steel structures in different countries. These systems are known to provide relatively high ductility, strength, and stiffness against seismic loads and to offer architectural flexibility by giving large openings. Seismic performance of EBFs is subjected to statistical uncertainties of both input excitations and performance of the system. This study aims at examining the current seismic design provisions and the reliability level of compression columns in EBFs with short shear links. An analytical model of minimally designed EBFs has been developed which is based on the LRFD design approach. For reliability analysis, analytical equations have been derived. The studies show that the current seismic provisions result in a less safe design relative to the target values for a wide range of the EBF columns. To cover this, an empirical equation is proposed for resistance factor of such columns. The study also includes a sensitivity analysis for five stochastic parameters that are presented and discussed.

Non-destructive method to investigate the hardness-plastic strain relationship in cyclically deformed structural steel elements

A non-destructive hardness testing method is being developed to determine plastic strain in steel elements that have been subjected to inelastic seismic loading. The focus of this study is on the active links of eccentrically braced frames (EBFs). The 2010/2011 Christchurch earthquake series, especially the very intense February 22 shaking, was the first earthquake worldwide to push complete EBF systems into their inelastic state, generating a moderate to high level of plastic strain in EBF active links for a range of buildings from 3 to 23 storeys in height. Plastic deformation was confined to the active links. This raised two important questions: 1) what was the extent of plastic deformation; and 2) what effect does that have to post-earthquake steel properties? To answer these questions a range of actions is being taken. A non-destructive hardness test method is being developed to determine a relationship between hardness and plastic strain inactive link beams. Active links from the earthquake affected, 23-storey Pacific Tower building in Christchurch has been hardness and material property tested to determine the changes in the steel, and cyclic testing of active links to defined levels of inelastic demand is underway. Test results to date show clear evidence that the hardness based method is able to give a good relationship between hardness measurements and plastic strain. This paper presents recent significant findings from this project. The principal of these is the discovery that hot rolled steel tested beams, all carry manufacturing induced plastic strains, in regions of the webs, of up to 5%.

Forensic Analysis of Link Fractures in Eccentrically Braced Frames during the February 2011 Christchurch Earthquake: Testing and Simulation

The earthquake on February 22, 2011, in Christchurch, New Zealand, resulted in the first documented field fractures of links in eccentrically braced frames (EBFs). A comprehensive forensic analysis of these fractures, which occurred in the parking garage of the Christchurch Hospital, is presented. The analysis is based on mechanical and spectrochemical testing and three-dimensional (3D) scanning of procured physical samples of the fractured links. The analysis features nonlinear time history simulations to characterize deformation demands and continuum finite-element simulations to determine the capacities based on a sophisticated fracture mechanics model. The exercise represents a multiscale end-to-end simulation of the system and provides insight regarding the observed fractures. The analysis reveals the inherent challenges in determining the proximate cause of the fractures because the fractures occurred because of a confluence of several interacting factors, primarily the intensity of shaking (several times the intensity that was expected during a design-level event) and the frame geometry, which severely amplified the imposed demands. In addition, the fractured links also suffered from an erection (fit-up) error, in which the link stiffener was not located (as specified) directly above the brace flange, producing a severe strain concentration. This flaw significantly reduced the deformation capacity; however, the simulation of hypothetical scenarios indicates that, even with this flaw, the links would (with high likelihood) have survived a design-level event. Strategies for mitigation include stricter tolerances for stiffener location, consideration of frame geometry to reduce amplification of rotation, and enhancement of the seismic hazard used for design. The limitations of the study are outlined.

Displacement amplification factors for steel eccentrically braced frames

SummaryInelastic deformation capacity of links is a factor that significantly influences design of steel eccentrically braced frames (EBFs). The link rotation angle is used to describe inelastic link deformation. The link rotation angle is generally calculated by making use of design story drifts that in turn are calculated by modifying the elastic displacements by a displacement amplification factor. This paper presents a numerical study undertaken to evaluate the displacement amplification factor given in ASCE7‐10 for EBFs and the rigid‐plastic mechanism used for calculating link rotation angles. A total of 72 EBFs were designed by considering the number of stories, the bay width, the link length to bay width ratio, and the seismic hazard level as the prime variables. All structures were analyzed using elastic and inelastic time history analyses. The results indicated that the displacement amplification factor given in ASCE7‐10 provides unconservative estimates of the story drifts. On the other hand, the rigid‐plastic mechanism provides conservative estimates of link rotations. Based on the results of the numerical study, a new set of displacement amplification factors that vary along the height of the structure and modifications to the rigid‐plastic mechanism were developed. In light of the proposed modifications, the EBFs were redesigned and analyzed using inelastic time history analysis. The results indicated that the proposed modifications provide improvements for the displacement amplification factor and link rotation angle calculation procedures. Copyright © 2014 John Wiley & Sons, Ltd.

On the optimum performance-based design of eccentrically braced frames

Abstract. The design basis is being shifted from strength to deformation in modern performance-based design codes. This paper presents a practical method for optimization of eccentrically braced steel frames, based on the concept of uniform deformation theory ( UDT ). This is done by gradually shifting inefficient material from strong parts of the structure to the weak areas until a state of uniform deformation is achieved. In the first part of this paper, UDT is implemented on 3, 5 and 10 story eccentrically braced frames (EBF) subjected to 12 earthquake records representing the design spectrum of ASCE/SEI 7-10. Subsequently, the optimum strength-distribution patterns corresponding to these excitations are determined, and compared with four other loading patterns. Since the optimized frames have uniform distribution of deformation, they undergo less damage in comparison with code-based designed structures while having minimum structural weight. For further investigation, the 10 story EBF is redesigned using four different loading patterns and subjected to 12 earthquake excitations. Then a comparison is made between link rotations of each model and those belonging to the optimized one which revealed that the optimized EBF behaves generally better than those designed by other loading patterns. Finally, efficiency of each loading pattern is evaluated and the best one is determined.

Fundamental periods of steel eccentrically braced frames

SummaryThis paper describes formulation of a hand method that can be used to estimate the computed fundamental periods of vibration of building structures in general and steel eccentrically braced frames (EBFs) in particular. The developed method uses the Rayleigh's method as a basis and utilizes the roof drift ratio (RDR) under seismic forces as a parameter. To obtain RDR, more than 4000 EBFs were designed by considering the seismic hazard, number of stories, braced bay width and link length to bay width ratio as prime variables. A model was developed to estimate RDR, which depends on the rigid plastic deformation mechanism for a typical EBF. The method was verified using design data produced as a part of this work as well as data published in literature. The verifications indicate that the proposed formulation is capable of providing acceptable estimates of the computed period. When compared with existing empirical period–height relationships, the proposed formulation offers closer estimates with reduced scatter. The method was further refined to derive new period–height relationships for two different seismicity regions. The accuracy of the relationship for high seismic regions was verified using measured periods of EBF buildings. Copyright © 2014 John Wiley & Sons, Ltd.

Innovative energy dissipation systems (FUSEIS 1-1) — Experimental analysis

Structural elements in multi-story steel building frames may be damaged and in need of repair after strong earthquakes. Conventional structural systems, like moment resisting frames, concentric or eccentric braced frames, are not advantageous in respect to reparability. For that reason two innovative systems with dissipative fuses were introduced and studied in the frame of a European Research Program with the name “FUSEIS” [1,2]. The first, FUSEIS 1, which is described here, is composed of two closely spaced strong columns rigidly connected to a number of beams between floors. Energy dissipation and potential damage during strong ground motions concentrate in these short beams only that constitute the fuse elements. Repair work, if needed, is therefore limited to the fuses only. Actually, the fuses may be completely replaced, an easy task given their small dimensions and the fact that they are connected by bolted connections to the columns.The FUSEIS system combines strength, stiffness, ductility and architectural transparency. It is cost-effective in respect to initial installation and replacement/repair. The fuse elements and the overall system were studied experimentally and analytically. Experimental investigations on the fuse behavior to cyclic loading were performed at the Technical University of Aachen, while on the overall system at the Technical University of Athens. This paper presents the experimental research and the accompanying numerical investigations carried out for the overall system.

Stability of beams in steel eccentrically braced frames

When an eccentrically braced frame (EBF) is subjected to a severe seismic event, large axial force and bending moments are produced in the beam outside of the link. Designers face significant difficulties in meeting the capacity design requirement to keep these beams elastic. On the other hand, previous research suggests that controlled yielding in the beams is not detrimental to EBF performance as long as stability of the beam is maintained. A computational study was undertaken to investigate the stability of cyclically loaded EBFs. A total of 51 EBF sub-assemblage models, none of which satisfies the capacity design requirement, were selected and investigated through three-dimensional, nonlinear finite element analysis. The results indicate that the link overstrength factor should be a function of the link length for performing capacity design of the beam outside of the link. This is because flexure yielding links, which are more problematic to beam stability, tend to develop smaller overstrength compared to shear yielding links. Furthermore, designs with demand-to-capacity ratios greater than unity were found to be viable provided that the stability of the beam is maintained by making use of a slenderness limit developed herein.

An Overview on the Seismic Design of Braced Frames

Seismic design of steel braced frames in the modern building codes follow the capacity design approach where some of the members are obliged to dissipate energy whereas others are taken care to be protected. In this paper the seismic design methodologies used by European and American approaches for Concentric Cross Braced Frames (CCBF) and Eccentric Braced Frames (EBF) are highlighted. Synoptic tables for the design of such frames of the most advance seismic codes i.e., Eurocode 8 and the seismic provisions of American Institute of Steel Construction (AISC) are provided. Emphasizes are made on the provisions of Eurocode 8 both for CCBF and EBF. Finally, a general conclusion is drawn when dealing with CCBF and EBF

Overstrength and rotation capacity for EBF links made of European IPE sections

Overstrength factor and inelastic rotation capacity are the key parameters in eccentrically braced frames (EBFs) design which may affect significantly the design economy or safety. The overstrength factor given in the 2005 AISC Seismic Provisions is based on the previous tests on wide-flange link beams commonly used in the united states. Despite the extensive research conducted on EBFs link beams, it is not known whether the overstrength factor given by the Provisions can be used for the design of EBFs having IPE link beams. In this paper a three dimensional finite-element model using ABAQUS is developed for the inelastic nonlinear analysis of IPE link beams. The model is used to investigate the applicability of the overstrength factor and inelastic rotation capacity given by the Provisions to design of EBFs with IPE link beams. It was found that the strain hardening overstrength factor of short link beams made of European IPE sections with closely spaced stiffeners is greater than the Provisions' factor. Therefore, using the Provisions' overstrength factor may lead to unconservatie design of EBFs having such links. However, the overstrength factor given by the Provisions can be used conservatively to design EBFs with intermediate and long IPE links. Also, it was shown that links made of IPE sections can sustain much larger rotations than the rotations required by the Provisions.

Numerical seismic vulnerability analysis of mid-height steel buildings in Bogotá, Colombia

A number of mid-height steel buildings have been erected recently in Bogotá. Their seismic risk might be high, given the present microzonation of Bogotá and the lack of comprehensive previous studies; noticeably, the response reduction factors were commonly obtained only from general recommendations. The objective of this work is to investigate the seismic performance of these buildings. This study is carried out on eighteen representative prototype buildings. All these edifices have plan symmetry and are uniform along their height. The eighteen considered prototype buildings are generated by combining the values of three parameters: span-length (6 and 8m), number of floors (5, 10 and 15) and earthquake-resistant systems (moment-resistant frames, concentrically-braced frames and eccentrically-braced frames — using chevron braces). The structures of each of these eighteen prototype buildings have been designed according to the former and to the current Colombian seismic design codes; in the former code two seismic zones are considered and in the current code such zones are coarsely subdivided in three and in five zones, respectively. The structural properties of the buildings designed with the former and the current Colombian codes are compared to investigate the practical repercussions of the new regulation. The vulnerability of these buildings has been evaluated by 2-D “push-over” analyses. The main objective of these analyses is to determine the response reduction factor; the obtained values are compared with the recommendations of the current Colombian seismic design code. As well, the seismic performances of the three considered earthquake-resistant systems are compared.

Design overstrength of steel eccentrically braced frames

The paper reports an analytical study on the design overstrength of steel eccentrically braced frames (EBFs). The study aimed at examining the influence of geometrical factors and seismic hazard on the design overstrength of EBFs. Pursuant to this goal a computer program which facilitates EBF designs was developed. The algorithm of the program adopts the lightest uniform frame design and library of link-beam-brace sub-assemblages concepts. The design output from the program was compared with published solutions and the results indicate that the algorithm developed as a part of this study is capable of providing lighter framing solutions. A parametric study was conducted using the developed computer program. The results indicate that the frames considered in this study have on average higher overstrength values when compared with the codified value even without considering potential increases due to material overstrength and strain hardening. The design overstrength was found to be influenced primarily by the link length to bay width ratio and the bay width, and secondarily by the building height and seismic hazard level.

Direct displacement-based seismic design of steel eccentrically braced frame structures

This paper details a direct displacement-based design procedure for steel eccentrically braced frame (EBF) structures and gauges its performance by examining the non-linear dynamic response of a series of case study EBF structures designed using the procedure. Analytical expressions are developed for the storey drift at yield and for the storey drift capacity of EBFs, recognising that in addition to link beam deformations, the brace and column axial deformations can provide important contributions to storey drift components. Case study design results indicate that the ductility capacity of EBF systems will tend to be relatively low, despite the large local ductility capacity offered by well detailed links. In addition, it is found that while the ductility capacity of EBF systems will tend to reduce with height, this is not necessarily negative for seismic performance since the displacement capacity for taller EBF systems will tend to be large. To gauge the performance of the proposed DBD methodology, analytical models of the case study design solutions are subject to non-linear time-history analyses with a set of spectrum-compatible accelerograms. The average displacements and drifts obtained from the NLTH analyses are shown to align well with design values, confirming that the new methodology could provide an effective tool for the seismic design of EBF systems.

역V형 가새골조의 반응수정계수

철골구조에서 지진력 저항시스템으로 많이 사용되고 있는 가새골조시스템은 중심가새골조(Concentrically Braced Frames, CBF)와 편심가새골조(Eccentrically Braced Frames, EBF)로 구분하여 KBC2009 건축구조기준에서는 서로 다른 반응수정계수를 적용하여 설계하도록 되어있다. 현행 내진설계기준에서 편심가새골조에 대하여 링크 타단 접합부의 종류에 따라 모멘트 저항접합일 경우 반응수정계수는 R=8, 비모멘트 저항접합일 경우 반응수정계수 R=7로 적용하여 설계하도록 규정하고 있을 뿐 편심길이의 영향을 고려하지 않고 있다. 본 연구에서는 편심가새골조 중에서 역V형가새골조를 대상으로 비탄성 거동을 파악한 다음 편심길이에 따른 반응수정계수를 산정하여 내진설계 기준에서 제시한 값과 비교한 결과 편심가새골조에 대하여 동일한 반응수정계수를 적용하는 것은 적절하지 않음을 확인하였다. In this study of Eccentric Braced Frames have identified the following target eccentricity on the length of the inelastic behavior of the reaction by calculating the correction factor by comparing it to the value suggested by the earthquake provided material for the rational design aims to There are. As a variable-length V-braced frame analysis model stations were set up. Eccentricity faults in the model according to the length stiffness ratio, the maximum amount of energy dissipation were analyzed base shear and multi-layered model of the reaction from the eccentricity correction factor calculated on the length of the building standards proposed by KBC 2009 in response eccentricity correction factor calculated from The length varies. does not have the same response modification factor was confirmed.

Evaluation of hysteretic behavior of eccentrically braced frames with zipper-strut upgrade

This paper describes the seismic behavior of Eccentrically Braced Frames (EBF) upgraded using zipper-struts which connect the mid-point of shear links in all stories.The numerical modeling of the zipper-strut-equipped model is conducted using finite-element method software under cyclic load, with the goal of evaluating the behavior of shear links. The interaction of shear links and zipper-struts is also studied. Furthermore, the prototypes are subjected to nonlinear static (Pushover) analysis in two configurations; one with moment-resisting connections and one with pinned connections. Finally, the increase of ductility coefficient for the prototypes with zipper-strut is studied. The prototypes are also subjected to time-history analysis using scaled earthquake records, with the purpose of studying the energy parameters of the models.The unbalanced shear force is distributed among all stories due to the added continuity among shear links. This results in the extension of structural displacement capacity according to acceptance criteria. Moreover, a noticeable delay occurs until shear links experience enough rotation to meet Collapse Prevention acceptance criteria. By imposing acceleration on the structure and conducting time history analysis, it is observed that the zipper-strut-equipped system shows a stronger tendency to form shear links, leading to greater dissipation capacity in plastic zone.

Seismic design of steel frames using multi-objective optimization

In this study a multi-objective optimization problem is solved. The objectives used here include simultaneous minimum construction cost in term of sections weight, minimum structural damage using a damage index, and minimum non-structural damage in term of inter-story drift under the applied ground motions. A high-speed and low-error neural network is trained and employed in the process of optimization to estimate the results of non-linear time history analysis. This approach can be utilized for all steel or concrete frame structures. In this study, the optimal design of a planar eccentric braced steel frame is performed with great detail, using the presented multi-objective algorithm with a discrete population and then a moment resisting frame is solved as a supplementary example.