Eccentric Bracing Research Articles

Design and analysis of eccentrically braced steel frames with vertical links using shape memory alloys

In recent years, the philosophy behind traditional methods by which structures were designed to withstand natural disasters has gone through major changes. Shape memory alloys (SMAs) are characterized by their superelastic behavior, which enables them to recover their original shape after experiencing large deformations. This characteristic provides an ideal reversibility capacity that can be used in the passive control of buildings exposed to earthquakes. This article has attempted to identify the effects of changing the hysteretic characteristics of SMA materials when they are used as passive control systems in eccentric bracing frames. By evaluating the numerical analysis results obtained from the modeling of an experimental sample and the modeling of the proposed EBF system, the accuracy of the above-stated notion was confirmed. Moreover, the results that pertain to the nonlinear pushover analysis, time-history dynamic analysis and seismic damage analysis of eccentric steel bracing frames of 4-, 9- and 14-story structures indicate that the use of SMA superelastic materials, in addition to effectively improving their ductility, stiffness and lateral strength, provides an excellent reversibility capacity, which considerably reduces both the maximum relative displacement and the residual deformation in the structure.

Optimal design of seismic retrofitting of RC frames with eccentric steel bracing

This paper focuses on optimal design for dissipative steel bracing in seismic retrofitting of gravity load designed RC frame buildings. An optimized iterative force/strength-based design procedure is presented, for simultaneous yielding of bracing of all storeys, in order to induce an global damage mechanism in the building which maximizes the hysteretic damping effect. The procedure complies with the capacity design rule, whereby the bracing system enters the plastic range before the existing RC frame. The iterative design procedure of the system is applied here to a regular RC shear frame, with various design behavior factors.

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

Proposal of modifications to the design provisions of Eurocode 8 for buildings with split K eccentric braces

This paper evaluates the effectiveness of the procedure suggested in Eurocode 8 for the design of eccentrically braced structures and compares the seismic response of buildings designed according to this procedure to the response of buildings designed according to the modifications proposed by the Authors. The proposed procedure modifies that of Eurocode 8 in order to fill gaps and make the new procedure consistent with the intent of the code. The seismic response of the buildings is obtained by incremental dynamic analysis. To ensure the general validity of the results, the buildings are considered to be founded on hard or soft soil and are characterised by different values of the link length and number of storeys.

Global Behavior Factor of Frames with Eccentric Bracings and Relationships with the Ductility Requirements

During alternate loading cycles, as during earthquakes, the structure behavior becomes inelastic. This is particularly the case for steel structures. They show a better ability to absorb earthquake energy. This energy dissipating capacity is described by the q global behavior factor as used in the Eurocode 8. However, in other earthquake codes, it is expressed by R letter and in others only the X and V shapes bracing systems are taken into account in the seismic design. In this work, a method for determining the global q behavior factor is presented for eccentrically braced steel frames that are designed to resist seismic loads.

Evaluation of Equivalent Bracing Rigidity on Elasto-Plastic Buckling Load for H-Shaped Compression Members with Eccentric Braces

Evaluation of Equivalent Bracing Rigidity on Elasto-Plastic Buckling Load for H-Shaped Compression Members with Eccentric Braces

Dynamic Response Control of a Building Model using Bracings

Abstract Construction of tall and flexible structures has become easier now-a-days because of advances in analytical techniques and availability of more efficient materials. However, these structures are more sensitive towards dynamic loadings. Dynamic response of such structures under various excitations can be quantified by displacement and acceleration response time history. Excessive dynamic response of structure leads to substantial damage to the structural components and hence it need to be controlled. Passively, this is achieved by adding either mass or stiffness or both of them. This paper covers experimental study on controlling dynamic response of a building model using two types of bracings, namely, Concentric- and Eccentric bracing. A building model is fabricated using aluminum flats and plates and bracings are fabricated using linear springs of moderate stiffness. A model represented as Single Degree of Freedom (SDOF) system is subjected to sinusoidal excitations of various frequencies through small scale shake table. Acceleration response of a uncontrolled building model (model without bracings) is obtained using accelerometers and Data Acquisition System (DAQ) – LabVIEW 8.0. Displacement response is derived through numerical integration from acceleration response using LabVIEW. Acceleration and displacement response of controlled building model (model with bracings) are extracted and compared with uncontrolled system. Dynamic properties like Natural Time Period and Damping Ratio for both types of bracing systems are obtained and comparisons are made among them. It is found that natural time period and damping ratio of controlled building model is increased as compared to uncontrolled system due to stiffness addition. Displacement and Acceleration response are reduced appropriately for controlled system as compared to uncontrolled system.

Shear Failure Behavior of Y-Eccentrically Brace in RC Frame Structures under Earthquake Action

In order to study the mechanical characteristics and the shear failure behavior under earthquake action, a single-storey single-span RC frame structure strengthened with Y-eccentrically brace were designed and manufactured to be 1/3 scale. The pseudo-dynamic testing method was carried out under El-Centro earthquake action with different peak acceleration adjusted by Code for Seismic Design of Buildings in mainland China. The testing result indicates that RC frame structure strengthened with Y-eccentrically steel brace presents perfect seismic performance under strong earthquake action, at one time, the seismic performance maybe affected by the length of outsourcing steel at the joint between energy dissipation element of eccentric steel brace and RC frame beam. The joint should be considerably designed to make sure that shear failure can firstly occur in energy dissipation element.

Analytical and numerical study of plastic overstrength of shear links

Shear links are widely used in eccentric bracing of steel buildings and, recently, for seismic protection of existing bridges and buildings. Experimental tests carried out for classic eccentric bracing of steel buildings have consistently shown that peak inelastic shear forces up to 1.4–1.5 times the plastic shear strength can develop at plastic link rotations of about 0.08–0.1rad (plastic overstrength). However, more recent tests have shown that larger forces could be developed. Three basic parameters are devised as influencing shear overstrength: (i) axial forces acting on the link, (ii) the ratio of link flange over web area and (iii) the ratio between link length and cross section depth. In this paper only tensile axial forces induced by the presence of axial restraints and due to nonlinear geometric effects are dealt with. Numerical analysis of detailed finite element models has been carried out in order to ascertain the combined influence of these factors on the plastic overstrength of short links. A simple analytical model is proposed on the basis of finite element model analysis results. The analytical predictions are compared with the results of available experimental test results, showing good agreement.

Seismic Strengthening of Non-Ductile R/C Structures Using Infill Wall or Ductile Steel Bracing

This study deals with the seismic assessment and retrofitting of existing non-ductile R/C structures and the issue of selecting effective strengthening solutions is addressed. A displacement based procedure using nonlinear static pushover analyses is adopted in order to assess the main characteristics of the original structure and to select valuable retrofitting strategies. The procedure is applied to a four-storey R/C frame tested at the JRC ELSA Laboratory some years ago. The R/C frame was designed for gravity loads without specific earthquake-resistant provisions. Accurate numerical models are developed to reproduce the seismic response of the R/C frame in both the original and retrofitted configurations. The effectiveness of two strengthening techniques is examined. First, a retrofitting intervention is carried out by adding a concrete shear wall to the short bay of the frame. This solution is efficient in controlling global lateral drift and thus reducing damage in structural members. Then, a retrofitting solution involving the introduction of eccentric steel bracing is investigated in order to reduce the displacement demand and to increase the energy dissipation capacity of the frame. Nonlinear dynamic analyses are performed to assess and compare the seismic response of the frame in the original and retrofitted configurations.

A design procedure for dual eccentrically braced systems: Analytical formulation

Dual systems constituted by eccentrically braced frames and moment resisting frames are considered in this paper to improve the seismic behaviour of structures with conventional eccentric bracings. The design procedure proposed for such dual structures follows the force–design approach and complies with the capacity design principles. It is formulated so as to ensure seismic performances which are more satisfactory than those usually produced by current code regulations. In particular, the proposed procedure aims at inelastic responses characterised by fairly uniform ratios of required to available plastic link rotations and by substantial insensitivity to realistic variations of the mechanical properties of steel and dynamic characteristics of the earthquake ground motion.

Height Effect on Response Modification Factor of Open Chevron Eccentrically Braced Frames

The necessity of making buildings taller and height of new buildings is now one of the major problems in building design and codes of interest is located. With height, the need for further ductility and the capability of energy dissipation can be a significant role in the proper and safe design of structures to play. It seems that changes the height of buildings can be treated directly on the response modification factor of structures and thus the seismic performance of structures under severe earthquake to be effective. This research is an attempt to evaluate the ductility, over-strength and response modification factors of the Open Chevron Eccentrically Braced Frames (EBFs) with 5, 10, 15 and 20 stories under Tabas, Naghan, Bam and El-Centro strong ground motions. These frames were analyzed by using static pushover analysis, linear dynamic and incremental nonlinear dynamic analysis and the values of these factors for different models have been determined separately and it is resulted that with increasing the height of buildings, the ductility reduction factor and the response modification factor of EBFs, decrease, but this variation hasn't any influence on the over strength reduction factor.

Nonlinear Dynamic Response Analysis of the Braced Steel Frame with Wedge Devices

In order to research the nonlinear dynamic respond analysis of a new braced steel frame with wedge devices under the action of earthquake, its damping mechanism is analyzed, and the computational model is obtained. Based on the mechanism of multiple resistant lateral system, the explicit nonlinear dynamic analysis and dynamic contact algorithm are adopted to separately analyze the steel frame with no brace, with centric and eccentric brace, and with the new braced wedge block. During the analysis, in order to take the material and geometrical bi-nonlinear into account, the material model is chosen as the bilinear equivalent strength, and the explicit centered difference algorithm is adopted. It can be obtained from structural deformation and energy and so on. The results show that the stiffness of structure decays after plastic deformation in the earthquake effect, and the hysteresis energy consumption and system dumping appear. The nonlinear dynamic response of steel frame is affected by resistant lateral stiffness, plastic deformation, and system damping. The braced steel frame with wedge block regulates the displacement and acceleration response with yield energy dissipation of brace, as it provides resistance lateral stiffness to control the deformation. This kind of structure has strong adaptability to earthquake intensity and good seismic performance.

Extensive study of plastic hinges in composite steel-concrete members subjected to shear and/or bending

The accuracy of concentrated plasticity models to be applied in the dissipative zones of composite steel-concrete beams in case of joints from Moment Resisting Frames (MRF), and in case of links from the Eccentric Braced Frames (EBF) is still a matter of discussion. In the current practice it is avoiding to install connectors in the expected plastic zones, and to consider a symmetric plastic hinge for the steel beam or to link only. However, since the reinforcing bars are active although the connectors have been suppressed but also due to friction contact between the concrete slab and beam or link flange, the assumption of the ‘nominal symmetric’ plastic hinge could be false.

Compressive Buckling Behavior of Plate-Section Columns with Eccentric Lateral Bracing

Compressive Buckling Behavior of Plate-Section Columns with Eccentric Lateral Bracing

Effects of eccentric steel bracing systems on seismic fragility curves of mid-rise R/C buildings: A case study

In this study, the seismic reliability of a mid-rise reinforced concrete (R/C) building retrofitted using eccentric steel braces is investigated through fragility analysis. As a case study, a six storey mid-rise R/C building was selected. The design of selected sample building was made with reference to 1975 version of the Turkish Seismic Code. The effectiveness of using different types of eccentric steel braces in retrofitting the building was examined. The effect of distributing the steel bracing over the height of the R/C frame on the seismic performance of the retrofitted building was studied. For the strengthening of the original structure, D, K, and V type eccentric bracing systems were utilized and each of these bracing systems was applied with four different spatial distributions in the structure. For fragility analysis, the study employed a set of 200 generated earthquake acceleration records compatible with the elastic code design spectrum. Nonlinear time history analysis was used to analyze the structures subjected to this set of earthquake accelerations generated in terms of peak ground accelerations (PGA), whilst monitoring four performance limit states. The fragility curves were developed in terms of PGA for these limit states; namely: slight, moderate, major, and collapse with lognormal distribution assumption. The improvement of seismic reliability achieved through the use of D, K, and V type eccentric braces was evaluated by comparing the median values of the fragility curves of the existing building before and after retrofits. As a result of this study, the improvement in seismic performance of this type of mid-rise R/C building resulting from retrofits by different types of eccentric steel braces was obtained by formulation of the fragility reduction.

EFFECT OF LATERAL-ROTATIONAL RESTRAINT AND STRENGTH FOR CONTINUOUSLY ECCENTRIC BRACES ON BUCKLING STRESS FOR H-SHAPED COMPRESSION MEMBERS

��������������� � � � � * �� � � � ** When the non-structural members such as folded-roof plate are directly jointed to the structural members, the non-structural members may be used as the eccentric bracings. This paper clarifies the effect of lateral and rotational restraint for continuously eccentric braces on buckling stress for H-shaped compression members. The elasto-plastic buckling stress is evaluated with the buckling curve for design codes using the modified equivalent slenderness ratio. Then the elastic buckling load with energy method is applied to the slenderness ratio. Finally, the relationship between the rigidity and strength of bracings is expressed as the approximation.

EXPERIMENTAL ANALYSIS OF STEEL DISSIPATIVE BRACING SYSTEMS FOR SEISMIC UPGRADING

Energy dissipating devices, such as metallic ductile dampers, could represent one reliable system for seismic performance upgrading of reinforced concrete (RC) structures. This paper illustrates the significant improvement to the seismic response of RC structures equipped with dissipative bracing systems, such as eccentric braces (EBs) and buckling restrained braces (BRBs). In fact, the results of experimental tests carried out on two similar two‐storey one‐bay RC structures, respectively equipped with EBs and BRBs, are described. Referring to EBs, 3 lateral loading tests have been performed. Each test is characterized by shear links with bolted end‐plate connections. Different design criteria have been applied in the design of the connections. In the first test, capacity design criteria have not been considered. In the second test, a capacity design criterion has been applied, with a link shear over‐strength factor equal to 1.5. In the third test, a design criterion similar to the one adopted for the second test has been implemented, but with a larger over‐strength factor. In case of BRBs, two types of ‘only‐steel’ braces have been tested: one type was made using two buckling‐restraining rectangular tubes that are fully welded together with steel plates; the other type is detachable, being made again with two buckling‐restraining rectangular tubes but joined together by means of bolted steel connections. In both cases, the internal yielding core was a rectangular steel plate. The experimental results of both bracing systems are encouraging about the possibility to use these devices for improving the seismic resistance of existing RC structures. Santrauka Didinant gelžbetoninių pastatų atsparumą seisminiams poveikiams, gali būti naudojami energiją sugeriantys metaliniai slopintuvai. Straipsnyje parodoma, kaip tokios slopinančios sistemos, kaip necentriniai ryšiai ir klupimą suvaržantys ryšiai, gali iš esmės gerinti pastato seisminę elgseną. Pateikiami dviejų panašių vienos angos gelžbetoninių pastatų su minėtaisiais ryšiais eksperimentiniai tyrimų rezultatai. Tyrimai parodė, kad abi suvaržymo sistemos gerina eksploatuojamų gelžbetoninių pastatų atsparumą seisminiams poveikiams.

EFFECT OF LATERAL AND ROTATIONAL RESTRAINT FOR ECCENTRIC BRACES ON FLEXURAL-TORSIONAL BUCKLING LOAD WITH WEB DEFORMATION FOR H-SHAPED COMPRESSION MEMBERS

When non-structural members are used as eccentric braces in real structures, flexural and torsional deformation of compression members may occur due to the eccentric joints to the braces. This paper clarifies the buckling behavior with web deformation for H-shaped compression members with laterally and rotationally eccentric braces. The elasto-plastic buckling stress is evaluated with the buckling curve for design codes using the modified equivalent slenderness ratio. Then the elastic buckling load with energy method is applied to the slenderness ratio. In addition, the relationship between the required rigidity and strength of braces is evaluated.

Rehabilitation of RC frame joints using local steel bracing

Reinforced concrete (RC) structural frames that were built prior to the 1970s generally do not meet current design code requirements and may behave in a non-ductile manner. The lateral load carrying capacity of these structures is often insufficient due to non-ductile reinforcement detailing, which includes insufficient or no beam-column joint transverse reinforcement and inadequate anchorage for the beam's bottom reinforcement. Experience shows that such frames are prone to earthquake damage and often suffer shear and bond slip non-ductile modes of failure. Recent earthquakes have demonstrated that the beam-column joint safety is an important factor in keeping the integrity of the entire structure. Concentric steel bracing systems have been used for the rehabilitation of non-ductile RC buildings in a number of countries. However, the application of eccentric (local) steel bracing in the rehabilitation of RC structures still lags behind due to the limited research and information on the design, modelling and behaviour of such a combined concrete-steel system. In this paper a new beam-column joint rehabilitation technique using local steel brace members is proposed. Two full-scale specimens representing a standard joint and a rehabilitated joint were made and tested under reversed cyclic load. Their behaviour was compared to that of a non-rehabilitated specimen. It is shown that the rehabilitation technique was successful in enhancing the overall performance of the deficient joint and upgrading it towards a close to current standard performance.