Eccentric Bracing Research Articles

The Seismic Investigation of Off-Diagonal Steel Braced RC Frames

Abstract Steel bracing may be used to enhance the seismic strength of RC frames. Concentric steel bracing generally reduces ductility, which is a key component of seismic design. To overcome the problem, ductile steel brace-RC frame systems are therefore usually introduced in the form of eccentric braces. In the present study, the Off-Diagonal Bracing System (ODBS), which works as a concentric type of bracing, is investigated. In this paper the response of ODBS elements to cyclic loading is first explored and compared with those of other types of bracing such as X-bracing and inverted-V bracing systems. The time history analysis responses and cyclic hysteresis responses of a number of lowrise to mid-rise RC frames retrofitted with different types of bracing systems are then evaluated and compared. It is shown that under seismic excitation, a much reduced base shear is experienced by frames retrofitted with ODBS compared to other bracing systems. The results of time history and cyclic hysteresis response analyses also indicate a far greater energy dissipation capacity and ductility for the ODBS compared to other bracing systems. It is also concluded that ODBS performs best in lowrise frames. The out-of-plane buckling response of the ODBS is also investigated, and it is shown that a double-plated central connection can control such an adverse response.

Development of Eccentrically Interconnected Braced Frame (EIC-BF) for seismic regions

This study aims to develop an Eccentrically Braced Frame (EBF) system, which is so called Eccentrically Interconnected Braced Frame (EIC-BF), by proposing an innovative Structural Fuse (SF) using a very thin mid-height gusset plates with specified details for seismic regions. In the proposed structural system, as a low damage or repairable system, the mid-height gusset plate, as a replaceable centric shear link that connects four eccentric brace segments, behaves as a SF so concentrates most of large deformation and stress demand and dissipates the large amount of energy imposed by lateral loading. In order to investigate the effect of the proposed SF with the specified details, i.e. mid-height gusset plates with proposed details, on the seismic behavior of EIC-BF, nine three-fourth scale cyclic experiments were conducted. This research describes the results of the experiments elaborately and evaluates the cyclic responses of the studied EIC-BFs with respect to their utilized details. The results show that establishing the specified details and design parameters including eccentricity value of braces at mid-height gusset plate, mid-height gusset plate thickness and the stiffener configuration at mid-height gusset plate improves the seismic performance of EIC-BF in term of story drift angle by up to 32% as well as reduces the considerable pinching in cyclic response; while prevents main structural elements including beam, columns and braces from undergoing inelastic behavior.

On the Evaluation of the Use of EKBs to Improve Seismic Performance of Steel Frames

In this paper a new eccentric and knee bracing system named Eccentrically Knee Brace (EKB) is introduced and its merits to increase seismic performance of the steel frames is investigated. To assess the seismic behavior of this system compared to other steel braced systems, numerical modeling using finite element software is implemented. Subjected to moderate earthquakes, the knee element exclusively dissipates the seismic energy through plastic shear distortion, while the other structural elements remain in elastic range. Under severe earthquakes, plastic deformation of the knee element will be halted by the stopper subsequently causing yielding of the link element. Finally, a simplified analytical model is derived to predict the bilinear behavior of the EKBs.

Effect of Eccentric Lateral Bracing Stiffness on Lateral Torsional Buckling Resistance of Wooden Beams

An energy-based solution is developed for the lateral torsional buckling (LTB) analysis of wooden beams with flexible mid-span lateral bracing offset from section mid-height and subjected to uniformly distributed or mid-span point load. The study shows that such beams are prone to two potential buckling modes; symmetric or anti-symmetric. The symmetric mode is shown to govern the capacity of the beam for low bracing stiffness while the anti-symmetric mode governs the capacity when the bracing stiffness exceeds a threshold value. Using the present formulation, the threshold bracing stiffness required to suppress the symmetric mode and maximize the critical moments is directly obtained by solving a special eigenvalue problem in the unknown bracing stiffness. The technique thus eliminates the need for trial and error in standard solutions. A parametric study is conducted to investigate the effect of bracing height, load height, and bracing stiffness on the critical moments. A large database of runs is generated and used to develop simple expressions for determining the threshold bracing stiffness required to maximize the elastic LTB resistance.

Optimum Design of Braced Steel Space Frames including Soil-Structure Interaction via Teaching-Learning-Based Optimization and Harmony Search Algorithms

Optimum design of braced steel space frames including soil-structure interaction is studied by using harmony search (HS) and teaching-learning-based optimization (TLBO) algorithms. A three-parameter elastic foundation model is used to incorporate the soil-structure interaction effect. A 10-storey braced steel space frame example taken from literature is investigated according to four different bracing types for the cases with/without soil-structure interaction. X, V, Z, and eccentric V-shaped bracing types are considered in the study. Optimum solutions of examples are carried out by a computer program coded in MATLAB interacting with SAP2000-OAPI for two-way data exchange. The stress constraints according to AISC-ASD (American Institute of Steel Construction-Allowable Stress Design), maximum lateral displacement constraints, interstorey drift constraints, and beam-to-column connection constraints are taken into consideration in the optimum design process. The parameters of the foundation model are calculated depending on soil surface displacements by using an iterative approach. The results obtained in the study show that bracing types and soil-structure interaction play very important roles in the optimum design of steel space frames. Finally, the techniques used in the optimum design seem to be quite suitable for practical applications.

Experimental Assessment of Retrofitted RC Frames With Different Steel Braces

Due to lack of sufficient concrete strength or change in design guidelines, some RC structures are in need of retrofitting. In the past few decades, using steel braces as a means with which to retrofit RC structures has become the subject of more attention and the reason can be attributed to fast implementation of the system as well as a significant increase in the stiffness and the strength of the structure. By adding different types of braces to moment resisting RC frames, the seismic properties of the structure including its ductility, strength reduction factor, stiffness, and strength undergo change. Retrofitting a building and designing it is only possible if the behavioral properties of the new hybrid seismic resisting system are known. This study experimentally investigates the effect of adding different types of steel braces on the behavioral properties of RC moment resisting frames. Eight RC moment resisting frames with identical steel bar configuration and concrete strength were built and seven of which were retrofitted with different braces such as the X, the knee, the chevron, the eccentric brace and the chevron brace with a vertical link. All the frames were subject to cyclic loading and their hysteresis load-displacement diagrams were plotted. Strength, stiffness, crack expansion, ductility, energy dissipation, and the strength reduction factor of all the frames were assessed. From the ductility and strength reduction factor viewpoints, the results indicate that the eccentric brace has a better performance compared to the other specimens. However, from the stiffness, strength, and cracking control standpoints, the behavior of the X brace is more desirable.

Assessment of eccentrically braced frames strength against progressive collapse

One of the most important and effective factors of structural strength against the risk of progressive collapse is the type of lateral load bearing system of a building. In this research, strength of dual steel moment frames equipped with a variety of eccentric bracings against progressive collapse was evaluated by using nonlinear static alternate path method. 6-floored building samples were designed with steel frame using a dual steel moment system together with 3 different types of bracing, including inverted eccentrically V-shaped bracing (chevron bracing), eccentrically V-shaped bracing and eccentrically X-shaped bracing, each with two different kinds of arrangement of bracings in the structural plan, in form of alternate and neighbor. The effects of sudden removal of columns on different floors of these buildings were examined. These studies showed that dual steel moment frames equipped with eccentric bracings generally exhibited desirable strength against progressive collapse. A change in the type of bracing resulted in significant changes in the system capacity in the progressive collapse. Among the different types of braces assessed, chevron type eccentrically brace showed higher strength against progressive collapse. Also, that alternate arrangement of bracings in structure plan demonstrated better performance than neighboring arrangement.

Experimental study on seismic performance of reinforced concrete frames retrofitted with eccentric buckling-restrained braces (BRBs)

As a new type of energy dissipation component with excellent mechanical performance, the Buckling-Retrained Braces (BRBs) were gradually applied in retrofitting and improving seismic performance of reinforced concrete structures in China. In order to investigate the seismic performance of reinforced concrete structures retrofitted with BRBs, quasi-static test of two single-bay and 3-story reinforced concrete frames specimens was conducted and introduced in this paper. Two 1/2 scaled specimens were designed to reflect real prototype structure. For comparison, one control specimen was designed without BRBs, and the other specimen was retrofitted with BRBs. And particularly, for the specimen retrofitted with BRBs, the BRBs were eccentric layout instead of usually concentric or x-shaped layout, aiming to be more suitable for large-span frames. In the test, the failure mode, carrying capacity, deformability, ductility and energy dissipation ability of both two specimens were investigated. Based on the test results of the measured hysterical curves, skeleton curves, the seismic performances such as bearing capacity, plastic deformability, energy dissipation ability and ductility of two specimens were fully studied. And from the test results, it was indicated that the specimen retrofitted with BRBs showed much better seismic performance than the control specimen without BRBs, and the BRBs could effectively improve the seismic performance of the reinforced concrete frame. For the specimen retrofitted with BRBs, the BRBs firstly yielded before the beam-ends and the column-ends, and an expected yielding process or yielding mechanism as well as good seismic performance was obtained. For the specimens without BRBs, though the beam-ends yielded prior to the column-ends, the seismic performance was much poor than that of the specimen with BRBs.

Numerical modeling of links behavior in eccentric bracings with dual vertical links

Configuration and geometry of bracing systems affect the seismic performance of structures significantly. Recently, the current authors have introduced a new configuration for eccentric bracing of structural frames that may be assumed as the combination of inverted Y-type and rotated K-type EBFs. The resulted braced frame is called EBF-DVL, consisting of two vertical links attached together by a horizontal link. For evaluating the vertical links of EBF-DVL in seismic conditions, a total number of 4 cyclic quasi-static tests were performed on the specimens with different link lengths, sections and material grades. In this research, a summary of tests performed on experimental specimens are presented at first. Afterwards, the results of numerical modeling using ABAQUS software are described and compared with the experimental results. Cyclic loading was applied on each model, similar to the deformation history which was used during the tests. The effects of both material and geometry nonlinearities are considered. The diagrams of Load-displacement resulted from numerical analyses have a good agreement with the experimental ones; although there is more inconsistency in cases with steeper softening behavior. Also, the behavior of the links' components and yielding sequences are the same as experimental specimens.

Cost-competitive hysteretic devices for seismic energy dissipation in steel bracings: experimental tests and low-cycle fatigue characterisation

This paper presents the results of experimental tests intended at investigating the behaviour of two cost-competitive solutions for realising dissipative devices to be adopted as link elements in Y-shaped eccentric braces (EB), widely employed both for seismic protection of newly designed structures and retrofitting of existing ones. These structural components, generally referred to as Steel Slit Shape (SSS) and Short Link (SL) devices, were realised through standard welding and cutting procedures generally employed in steel workshops. Experimental tests with both constant and variable displacement amplitude protocols were carried out for observing the mechanical performance for these devices under cyclic actions. Since a fast degradation in shear strength was generally observed for all the tested specimens, a low-cycle fatigue was identified: it resulted that the same curve can be employed describing the number of cycles leading to low-fatigue failure for a given amplitude of the imposed cyclic displacement history. This curve is an essential tool for describing the behaviour of this devices that, although characterised by poorer mechanical performance under cyclic actions, have the potential to be utilised as dissipative components, for instance in Y-shaped steel bracings adopted for retrofitting reinforced concrete structures in medium-to-low seismic intensity area.

SEISMIC PERFORMANCE OF R C BUILDINGS ON SLOPING GROUNDS WITH DIFFERENT TYPES OF BRACING SYSTEMS

Structure are highly susceptible to serve damages in earthquake scenario, so choosing an appropriate lateral force resisting bracing systems will have a significant effect on performance of the structure. So this present study is aimed at evaluating and comparing various types of eccentric steel bracings for 12 storey RC frame building resisting on sloping ground configurations. For this 5 types of bracing systems like X-Bracing, Diagonal bracing, K- bracing, V-bracing and inverted V bracing are considered on the outer periphery of the buildings with step back and set back – step back type configurations are modeled and analyzed. The models are compared for different aspects within the structure, such as the maximum storey displacement, base shear, storey drift and storey shear, the structure is analyzed for seismic zone V and medium soil condition as per IS 1893:2002 using ETABS software. Results conclude that on sloping ground due to irregularity on ground surface, the structures are more vulnerable to earthquakes. Hence use of eccentric steel bracing is an effective and economical way to resist earthquake forces, Inverted V type bracing performs well compared to other bracing types. By using inverted V type bracing in step back buildings types maximum storey displacement of 70% and storey drift of 66% are obtained. Similarly for setback – step back configuration maximum storey displacement of 74% and storey drift of 70% are obtained respectively.

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.

Nonlinear Seismic Analysis of Masonry Infill RC Buildings with Eccentric Bracings at Soft Storey Level

Eccentric bracings are used in steel structures for long time where it serves as lateral load resisting system and improve strength and stiffness of frame along with effective energy dissipation. In the present study, eccentric bracings are used as a means to reduce soft storey effect in masonry infill reinforced concrete (RC) building. Masonry infill buildings with open first storey are usual choice for almost every general multi-storey construction in India, despite the building's palpable vulnerability to strong ground motion earthquakes. Among other strengthening methods, eccentric bracings could be an advantageous scheme as it provides lateral stiffness and ductility to structure with greater economy and also provides free space for commuting of vehicle at soft storey level. It has been seen that introduction of soft storey in building leads to concentration of damage in that storey while the building suffers only slight damage. Hence, eccentric bracings in soft storey need to be designed in such a way that they act as fuse during major earthquake events. The seismic performance of eccentric bracings for a seven storey building located in Indian seismic zone – V as per Indian standard code 1893-2002 are investigated using nonlinear static pushover analysis. A parametric study involving parameters such as shape of eccentric bracing, area of section, amount of eccentricity etc. are performed for selecting the type of eccentric bracing. The results of pushover analysis, reported in terms of storey drift demand and collapse fragility curve, showed that buildings with eccentric bracings have lower drift demand and probability of collapse.

Determining the optimal topology for intermediate steel moment resisting frames with eccentric braces in a hybrid system

Steel structures with intermediate ductility level are one of the systems widely employed all over the world. Given the significance of construction costs and its association with the selected structural system, number of the floors, length of spans, location of columns, and the soil type, this paper investigates the optimum values of these parameters. The present article aims at discovering the optimal topology with respect to architectural and applicability considerations. To this end, models of steel moment resisting frames with eccentric bracing with 5, 10, and 14 floors were selected. The selected models were assumed with equal area and two different soil types are considered. In order to provide parking space based on the urban development regulations, models were considered with span length of 5.6, 7.5, and 11.2 metres for two, three, and four car parks, respectively. Having analysed the models and designed the structural members and foundations, amount and cost of the building material were estimated based on the available rates and the results were compared using figures.

The effect of number and position of braced frames on column behavior of the dual steel structural system (MRF and EBF) (With a view on amplified seismic load)

In seismic design of structures, determination of number and position of braced frames, considering the architectural scheme of projects, is usually confronted by obstacles. Due to this fact, in some cases, selecting the best location and number of braced bays has led to mistakes in determination of their adjacent members (columns) design loads. One of the seismic design requirements of lateral resisting system is to control the columns adjacent to braced bays for load combinations of amplified seismic load, which is a function of over-strength factor of the structure. This research aims to present and introduce the best structural model of number and position of braced frames in a structural system, such as steel moment resisting frame and eccentric braces dual system; because in 3rd revision of Iranian 2800 standard of seismic provision, there are statements and criteria provided only for capacity of moment frame, not for braces. Though the amplified seismic load function is controlled in models which columns are connected to braces in 2 directions, and seismic loads are applied in those 2 directions, number of damage hinges (Exceeding CP) is significantly increased in comparison to the models with straggly braces. As the increase in axial force of these columns leads to decrease in their moment capacity (despite controlling the amplified seismic load provision), columns in dual systems that resist flexure, would be damaged and exceed the collapse threshold much sooner than other columns. This important fact is not presented in Iranian or even American codes and provisions.

Numerical comparison of the seismic performance of steel rings in off-centre bracing system and diagonal bracing system

During a seismic event, a considerable amount of energy is input into a structure. The law of energy conservation imposes the restriction that energy must either be absorbed or dissipated by the structure. Recent earthquakes have shown that the use of concentric bracing system with their low ductility and low energy dissipation capacity, causes permanent damage to structures during intense earthquakes. Hence, engineers are looking at bracing system with higher ductility, such as chevron and eccentric braces. However, braced frame would not be easily repaired if serious damage has occured during a strong earthquake. In order to solve this problem, a new bracing system an off-centre bracing system with higher ductility and higher energy dissipation capacity, is considered. In this paper, some numerical studies have been performed using ANSYS software on a frame with off-centre bracing system with optimum eccentricity and circular element created, called OBS_C_O model. In addition, other steel frame with diagonal bracing system and the same circular element is created, called DBS_C model. Furthermore, linear and nonlinear behavior of these steel frames are compared in order to introduce a new way of optimum performance for these dissipating elements. The obtained results revealed that using a ductile element or circular dissipater for increasing the ductility of off-centre bracing system and centric bracing system is useful. Finally, higher ductility and more energy dissipation led to more appropriate behavior in the OBS_C_O model compared to DBS_C model.

A new design of automatic vertical drilling tool

Abstract In order to effectively improve penetration rates and enhance wellbore quality for vertical wells, a new Automatic Vertical Drilling Tool (AVDT) based on Eccentric Braced Structure (EBS) is designed. Applying operating principle of rotary steering drilling, AVDT adds offset gravity block automatic induction inclination mechanism. When hole straightening happens, tools take essentric moment to be produced by gravity of offset gravity lock to control the bearing of guide force, so that well straightening is achieved. The normal tool's size of the AVDT is designed as 215.9 mm，other major components' sizes are worked out by the result of theoretical analysis, including the offset angle of EBS. This paper aims to introduce the structure, operating principle, theoretical analysis and describe the key components' parameters setting of the AVDT.

Critical review of the EC8 design provisions for buildings with eccentric braces

The objections raised by researchers to the design provisions reported in Eurocode 8 make the efficient seismic performance of the eccentrically braced structures designed according to this code unlikely. Given the rationality and the number of the objections, this paper aims to summarize the criticism of researchers and report the opinion of the Authors. The objections raised to the design procedure of eccentrically braced structures regard aspects common to the design of steel structures and aspects specifically related to the design of eccentrically braced structures. The significance of these objections is also shown by means of exemplary cases.

Comparative Analysis of the Energy Dissipation of Steel Buildings with Concentric and Eccentric Braces

A comparative study has been done to analyze the behavior of regular steel building structures of 4, 6, 8 and 10 stories, located in seismic zone 5 and soil type S1. The structures were upgraded with different brace configurations according to current Venezuelan codes. A total number of 24 numerical models were analyzed considering non-linear static and incremental dynamic analysis (IDA). The buildings were initially designed as moment resisting frames, and upgraded with six different bracing configurations: concentric braces in “X” and inverted “V”; eccentric braces inverted "V" with horizontal links, inverted “Y” and “X” with vertical links. Short length links were used to ensure a shear failure. The used methodology is based on obtaining the capacity, IDA curves, and bilinear approximations of these curves that allow the determination of yield and ultimate capacity points, in order to estimate important parameters of seismic response: overstrength and ductility; and considering these areas under the curves to estimate elastic deformation energy, energy dissipated by hysteretic damping and equivalent damping. According to the results, the cases with no brace enhancement showed the lowest lateral strength and lateral stiffness and high deformation capacity. On the other hand, the concentric bracing cases, resulted with the highest stiffness and strength and the lowest deformation capacity, therefore they have low ductility and energy dissipation capacity under seismic loading. Structures with links showed intermediate stiffness and strengths, resulting in the best performance in terms of ductility and energy dissipation capacity. The present study provides a better understanding of the benefits of eccentrically braced systems.

Cost-competitive Steel Devices for Seismic Retrofitting of RC Frames: Model Identification and Nonlinear Analysis

Seismic retrofitting of existing reinforced concrete (RC) buildings, designed in the last decades in seismic areas, is one of the most complex tasks for structural engineers: in fact, it includes several issues, such as quantifying the capacity of existing members, designing the supplemental ones and analysing the whole structure. This paper is intended as a contribution to clarifying some of those issues. First of all, a model based on using 1D finite elements with fiber section discretization is proposed for simulating the behaviour of a cost-competitive steel device that can be employed as a link in Y-shaped eccentric bracings (EB): particularly, the cyclic response and the low-cycle fatigue degradation is modelled, based on the results of obtained in a previous experimental research carried out at the University of Salerno. Secondly, the global response of an existing RC frame equipped with the aforementioned devices is investigated via Non Linear Time History (NLTH) analyses. Taking into account the lowcycle fatigue often leads to significantly more severe seismic displacement demand value on the retrofitted structure: a close correlation is unveiled between some specific features of the seismic signals adopted in the NLTH and the actual influence of low-cycle fatigue.