Bracing Configurations Research Articles

Modified seismic design of concentrically braced frames considering flexural demand on columns

SummarySpecial concentrically braced frames (SCBFs) are considered as one of the most economical and effective lateral force‐resisting systems in structures located in the regions of high seismicity. Steel braces in a braced frame undergo large axial deformations in tension and compression to dissipate the seismic energy. However, past studies have shown that SCBFs exhibit the soft‐story hinge mechanisms and unpredictable failure patterns under earthquake loading conditions. These inelastic responses along with the use of continuous structural sections as columns over consecutive floors induce flexural demand that is not considered in the current design practice. In this study, the evaluation of seismic performance of nine SCBFs designed as per the current practice has been carried out for three different story heights (i.e., three‐story, six‐story, and nine‐story) and three types of brace configurations (namely, chevron, splitX, and singleX). Three additional design techniques are also explored based on (i) the inclusion of column moments in the design; (ii) the theory of formation of plastic hinges; and (iii) the design of braces considering the forces computed at their post‐buckled stages. Nonlinear dynamic analyses of these study frames have been evaluated numerically using a computer software Perform‐3D for a suite of 40 ground motions representing the design basis earthquake and maximum considered earthquake hazard levels. Analyses results showed that the SCBFs designed as per the modified procedures achieved the desired performance objectives without the formation of soft‐story mechanism. Copyright © 2017 John Wiley & Sons, Ltd.

Prediction of lateral stiffness and fundamental period of concentrically braced frame buildings

In this paper, a simple hand calculation method to predict lateral stiffness and fundamental period of concentrically braced frame buildings with either fixed or pin base, which can also be used for unbraced frame buildings, is theoretically derived. To verify the reliability of the developed equations, the estimated lateral stiffness and fundamental periods are compared with the corresponding values obtained from pushover and Eigenvalue analyses for a wide range of low to medium rise buildings (i.e. up to ten storeys) with varying geometrical configurations. Systematic investigation of parameters affecting the fundamental period of frame buildings is also carried out. Although the basic formulation considers X and chevron brace configurations explicitly, its applicability to other types of brace configurations is also investigated. The suitability of the proposed equation for the braced frames with linear variation of section properties along the building height and pin beam-column connections is also scrutinized. Reliability of the developed hand calculation method is verified using the experimental test and numerical analysis results available in the literature. Finally, a flowchart and two worked examples are provided to explain the sequence of steps to calculate the fundamental period of concentrically braced frame buildings.

Seismic progressive collapse of MRF–EBF dual steel systems

Designing structures to resist progressive collapse during an earthquake is a key challenge for structural engineers working in seismic zones. This paper introduces a more appropriate brace configuration based on an upper-bound estimation of the probability of seismic progressive collapse in moment-resisting, eccentrically braced frames designed to the third edition of the Iranian seismic code. The code currently only has provisions for the capacity of moment frames, with no detail given for bracing elements. A numerical modelling study of low- to mid-rise residential and office buildings was undertaken. The results show that for five-storey buildings the optimum number of braced bays is 10–20% of the building's perimeter bays, whereas for eight-storey buildings the optimum proportion is 20–30%. Results also show that regular bracing in internal bays is more appropriate for resistance against progressive collapse.

Analysis of Transmission Towers for Optimal Bracing Configuration

Analysis of Transmission Towers for Optimal Bracing Configuration

Response modification factors for steel buckling restrained braced frames designed as per the 2010 National Building Code of Canada

This paper evaluates the overstrength, ductility, and response modification factors for low to mid-rise buckling restrained braced frames (BRBFs) designed as per the 2010 National Building Code of Canada. In addition to nonlinear static pushover analyses, dynamic time history analyses are performed to assess the seismic performance of four-, six-, and eight-story BRBFs. Different bracing configurations, including chevron (inverted-V) and split-X braces, are considered for the building frames with varied frame span lengths of 6 m and 8 m. The results confirm that the prescribed design values for overstrength and ductility factors provide reasonable estimations of the lower bound for these factors. The response modification factor obtained in this study ranged from 4.8 to 6 for different frames. The results also indicate that the response modification factor decreases with the increase of story height and span length. Moreover, bracing configurations may slightly affect the response modification factor for BRBFs.

End-Warping Bracings during the Construction of Steel Bridges

AbstractSlight bracing of steel bridges to control end warping of the compression flanges near the end supports can be very effective in enhancing the load-carrying capacity of the main girders that are involved. Relatively little information is available, however, regarding the stiffness and strength requirements of end-warping bracings. The results of seven large-scale laboratory tests of different configurations of bracings, including plan bracing and corrugated metal sheets, used in a twin I-girder bridge to resist end warping at support points are presented here. Bracing forces generated in the plan bracings of the test bridge were also compared with those obtained through an approximate analysis for the preliminary analysis of such bracings. Moreover, the bracing forces present in the cross beam of the test bridge were obtained for the different end-warping brace configurations that were investigated. The bracings, the plan bracing type and corrugated metal sheets, were found to effectively enhance ...

Retrofitting of a seismically deficient building

Most available seismic retrofitting methods have certain degree of difficulty to be implemented on site and are thus more expensive. A relatively new retrofitting technique coupling the conventional braces with the recently developed viscoelastic dampers is proposed to improve the seismic performance of deficient structures in more effective and efficient way. The relatively new damper is tubular in shape as opposed to the flat shape seen in the conventional dampers. The effectiveness of the proposed retrofitting technique is validated through the three-dimensional finite element analyses performed on real structures, in which three different brace configurations (diagonal, X and K) and small to strong earthquakes are considered. The vibration periods, interstorey drifts and formation sequences of plastic hinges are compared to each other between the original and the retrofitted structures. The study results show that the energy dissipation resulting from the proposed retrofitting scheme is significant, especially for X-type bracing systems. The proposed retrofitting method also alters the overall framing behaviour from strong beam–weak column to the more desirable strong column–weak beam. The retrofitted structure is shown to satisfy the modern seismic design requirements. The proposed retrofitting technique is also shown to be easier to be installed and cost-effective.

Seismic analysis of steel structure with brace configuration using topology optimization

Seismic analysis for steel frame structure with brace configuration using topology optimization based on truss-like material model is studied. The initial design domain for topology optimization is determined according to original steel frame structure and filled with truss-like members. Hence the initial truss-like continuum is established. The densities and orientation of truss-like members at any point are taken as design variables in finite element analysis. The topology optimization problem of least-weight truss-like continuum with stress constraints is solved. The orientations and densities of members in truss-like continuum are optimized and updated by fully-stressed criterion in every iteration. The optimized truss-like continuum is founded after finite element analysis is finished. The optimal bracing system is established based on optimized truss-like continuum without numerical instability. Seismic performance for steel frame structures is derived using dynamic time-history analysis. A numerical example shows the advantage for frame structures with brace configuration using topology optimization in seismic performance.

Theory of Plastic Mechanism Control for MRF–EBF dual systems: Closed form solution

In this paper, the closed form solution of the Theory of Plastic Mechanism Control (TPMC) is extended to the case of Moment Resisting Frames–Eccentrically Braced Frames (MRF–EBF) dual systems. As it is known, Eurocode 8 does not provide any specific design criterion regarding such structural typology, so that practitioners commonly carry out the design process by combining the design rules suggested for simple MRFs and EBFs. Therefore, the aim of this work is to provide a complete and exhaustive design procedure for MRF–EBF dual systems, considering all the brace configurations commonly adopted and with the goal of assuring the development of a collapse mechanism of global type. The design procedure to assure this ambitious design goal is based on TPMC whose aim is to derive the column sections required to assure the desired collapse mechanism starting from the knowledge of the dissipative zones. In order to point out the accuracy of the proposed design approach and the differences between itself and Eurocode 8, a number of MRF–EBF dual systems have been designed and their performances have been evaluated by means of push-over analyses.

Validation of Fiber-Based Distributed Plasticity Approach for Steel Bracing Models

Nonlinear analysis approach is not anymore limited only to research purposes, but becoming more popular as a tool that can be used during design, thanks to the increased efficiency of computer software and hardware. An accurately calibrated numerical model may simulate the behaviour of buildings in a quite realistic way, which helps designers understand better the performance of their structures. However, the feasibility of the nonlinear analysis approach is limited by the complexity of the numerical model, and the aim of any researcher or engineer is to obtain the most useful information in a reasonable amount of time. This study focuses on the validation of a simplified numerical modelling approach to simulate the nonlinear behaviour of steel bracings. The paper presents a comparison between two different modelling approaches; a refined finite element model using volumetric elements, and fiber-based model using beam elements with distributed plasticity. The numerical models calibrated with the experimental result from existing literature, reproduce the behaviour of cold formed square, and hot rolled open section steel elements under inelastic cyclic loading. The hysteresis loops obtained from two models show that the accuracy obtained by simpler fiber-element formulation is quite close to the more refined volumetric model. Finally, in order to assess the accuracy of the fiber-based modelling approach to estimate the nonlinear cyclic response of full-scale braced frame configurations, two real scale frames are analysed, and the results are compared with the results of the experiments performed on the test frames. In terms of computation time and accuracy, distributed plasticity model is much more efficient, and can be a good option to perform nonlinear analysis of multi-level buildings, which would be quite cumbersome with volumetric modelling approach. This study has been realized thanks to the research fund received from European commission with the contract MEAKADO RFSR-CT-2013-00022.

Fragility assessment of buckling-restrained braced frames under near-field earthquakes

This study presents an analytical investigation on the seismic response of a medium-rise buckling-restrained braced frame (BRBF) under the near-fault ground motions. A seven-story BRBF is designed as per the current code provisions for five different combinations of brace configurations and beam-column connections. Two types of brace configurations (i.e., Chevron and Double-X) are considered along with a combination of the moment-resisting and the non-moment-resisting beam-to-column connections for the study frame. Nonlinear dynamic analyses are carried out for all study frames for an ensemble of forty SAC near-fault ground motions. The main parameters evaluated are the interstory and residual drift response, brace displacement ductility, and plastic hinge mechanisms. Fragility curves are developed using log-normal probability density functions for all study frames considering the interstory drift ratio and residual drift ratio as the damage parameters. The average interstory drift response of BRBFs with Double-X brace configurations significantly exceeded the allowable drift limit of 2%. The maximum displacement ductility characteristics of BRBs is efficiently utilized under the seismic loading if these braces are arranged in the Double-X configurations instead of Chevron configurations in BRBFs located in the near-fault regions. However, BRBFs with the Double-X brace configurations exhibit the higher interstory drift and residual drift response under near-fault ground motions due to the formation of plastic hinges in the columns and beams at the intermediate story levels.

Correlating Stiffness and Shear Lag Behavior with Brace Configuration of Tall Truss Tube Buildings

As the height of buildings increases, effect of shear lag also becomes considerable in the design of high rise buildings. In this paper, shear lag effect in tall buildings of heights, i.e., 120, 96, 72, 48 and 36 stories of which aspect ratio ranges from 3 to 10 is studied. Tube in tube structural system with façade bracing is used for designing the building of height 120 story. It is found that bracing system considerably reduces the shear lag effect and hence increases the building stiffness to withstand lateral loads. Different geometric patterns of the bracing system are considered. The best effective geometric configuration of a bracing system is concluded in this study. Lateral force, as wind load, is applied on the buildings as it is the most dominant lateral force for such heights. Wind load is set as per Indian standard code of Practice IS 875 Part 3. For analysis purposes, the SAP2000 software program is used.

Bracing Configurations Effect on BucklingRestrained Braced Frames

Buckling restrained bracing systems are effectively being used to reduce seismic response during recent years. But often configurations of these systems are ignored while evaluating their behaviour. Aim of this study is to consider the effect of different bracing configurations on the behaviour of Buckling Restrained Braced Frames(BRBFs). Five-storey three-bay braced frames with three different patterns are modelled using Finite Element Analysis software ANSYS. The main variables of this study are the bracing pattern and location of bracing. The effects of bracing configurations on seismic behaviour of buckling restrained braces are also considered. The best combinations of bracing pattern and location along with specific bay width and storey height in terms of seismic response are suggested finally.

Nuanced Robustness Analysis with Limited Information

This paper presents a nuanced robustness analysis for structures when only limited information is available. A new methodology based on fuzzy set theory is proposed to cope with scarce information as a major problem in the performance assessment of existing structures. The developed robustness measure provides information on the relationship between structural robustness and the magnitude of uncertainty in the damage of the structure. This feature is enabled through a nuanced consideration of imprecision in the damage assessment through alpha-level discretization. An entropy-based robustness measure is formulated as a function of imprecision in the damage state. On this basis, different design solutions can be compared in a one-swoop analysis with respect to their robustness for different magnitudes of damage. Furthermore, this approach can be used to assess the effort for inspection versus gain in precision of the predicted structural performance. The development is of a general nature. In this paper, it is elucidated in the context of a typical offshore engineering problem to demonstrate its application in practical cases. Fixed offshore platforms with different brace configurations are compared in light of robustness with respect to damage from corrosion.

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.

English

The structural system of the building has to support the lateral loads due to earthquake and wind in addition to gravity loads. A lateral load develops high stresses and produces sway causing vibration and drift. If the buildings are not designed to resist the lateral loads, then they may be collapse resulting into the loss of life or its content. Therefore it‟s important for the structure to have not only sufficient strength against gravity loads but also the adequate stiffness to resist lateral forces. Literature review reveals that LLRSS (Lateral load Resisting structural system) is provided in the form of devices like base isolation and dampers which controls the seismic vibration and lateral drift. But these devices are very costly and effective only for high rise buildings. Hence there is a need to study the LLRSS or technology suitable for a particular height of building. The objective of this research is to propose simple but innovative and effective LLRSS or structural technology and methodology for the seismic control which can be used in new as well as old building structures. It is reviewed that since North earthquake (1994), concentrically braced steel frame has gained the popularity as a LRSS in the seismic areas. In spite of increasing popularity, analytical study of braced frame structure and its detailed requirement to control the seismic response is limited in India. Also RC building involves heavy dead load due to large member size which intern is more prompt for seismic loss. Hence, it is proposed to study the response of steel buildings/frames with different types of steel bracings configurations as a LLRSS to control the vibration and storey drift. The structural response parameters selected for the study are time period, natural frequency, and roof displacement. The research work deals with the parametric study of response of Non-linear time history analysis (NLTHA) of 3D industrial steel buildings braced with different bracing configurations using software (Sap-2000) under Bhuj earthquake. The bracing configuration used are SDB, CDB, VVB and INVB in concentric bracing to suggest suitability of particular bracing configurations for the stability of the building structure under seismic loading.

Seismic design and experiment of single and coupled corner gusset connections in a full‐scale two‐story buckling‐restrained braced frame

SummaryA gusset plate is subjected to forces induced from a buckling‐restrained brace (BRB) and frame action. In this study, a performance‐based design method of the gusset connections incorporating a BRB and frame actions is investigated. The force demands resulting from the BRB axial force are computed from the generalized uniform force method. The force demands induced from the frame action effects primarily result from beam shear. A conservative method, which considers the beam axial force effect and the thereafter reduced beam flexural capacity possibly developed at the gusset tips, is adopted in estimating the maximum beam shear. An improved equivalent strut model is used to represent the gusset plate subjected to the frame action effect. The total force demands of the gusset connection are combined from the BRB force and the frame actions. For design purposes, the stress distributions on the gusset interfaces are linearized. The maximum von Mises stress combining the normal and shear stresses is considered as the demand for the gusset plate design. In order to verify the effectiveness of the proposed design method, experiments on a two‐story full‐scale buckling‐restrained braced frame (BRBF) were performed. The chevron and single diagonal brace configurations were arranged in the second and the first stories, respectively. Two different corner gusset connection configurations including one single corner gusset and one coupled corner gusset connection, where two braces in adjacent stories joined at the same beam‐to‐column joint, were tested. The BRBF specimen was subjected to cyclically increasing lateral displacements with a maximum frame drift of 0.04 rad. The maximum story drifts reached 0.035 and 0.061 rad. in the first and the second stories, respectively. At the end of the tests, no fractures were observed on any of the gusset interfaces. Along the gusset interfaces, the normal and shear stress distributions computed from the proposed design procedures and the FEM analysis correlated well with the experimental results. This paper concludes with the procedure and recommendations for the performance‐based design of gusset connections. Copyright © 2015 John Wiley & Sons, Ltd.

Traditional and viscous dissipative steel braced top addition strategies for a R/C building

A study concerning a double-storey extension on top of an existing reinforced concrete (R/C) apartment building is carried out for two different steel structural solutions. A traditional bracing configuration, and a dissipative bracing technology incorporating pressurised fluid viscous spring-dampers, are selected for the two hypotheses. The gravity load-bearing system is assumed to be coincident, in order to directly compare the seismic response capacities of the bracing systems. Furthermore, the following mutual design objectives are fixed: attaining operational, and immediate occupancy non-structural performance levels at frequent design earthquake and serviceability design earthquake levels, respectively; and limiting strengthening interventions on the R/C structure to jacketing maximum 20% of columns, and adding a thin R%C slab over the foundations. These objectives are reached at virtually coinciding costs. On the other hand, the viscous-dissipative bracing design shows substantially higher seismic performance at basic design and maximum considered earthquake levels, as well as lower architectural intrusion, as compared to the traditional bracing solution.

Experimental and numerical developing of reduced length buckling-restrained braces

Buckling-restrained braced frames (BRBFs) have been widely used as an efficient seismic load resisting system in recent years mostly due to their symmetric and stable hysteretic behavior and significant energy dissipation capacity. However, buckling-restrained braces (BRBs) are heavier and more expensive in comparison to other concentric bracing systems. In order to facilitate the use of BRBs, the idea of reducing the length of the core and the encasings which can result in lighter and more replaceable BRBs is proposed and experimentally investigated in this paper. Two relatively similar all-steel reduced length BRBs (RLBRBs) are designed, detailed and constructed using a special debonding and stopper mechanism. The design and construction procedure is accomplished by paying special attention to low-cycle fatigue (LCF). The specimens were tested under the quasi static loading protocol, and withstood high axial strains of 4–5% without any global or local failure. The hysteretic responses of the specimens were stable and symmetric. Moreover, numerical models were developed and nonlinear cyclic analyses were performed to provide better insight into the core and encasing performance as well as application of the RLBRB in the brace configuration.

Comparative study on the behaviour of Buckling Restrained Braced frames at fire

In this paper, the use of Buckling Restrained Brace systems (BRBs) in preventing the progressive collapse of the structural frame against fire is investigated using the stiffness reduction technique. Four-storey steel structures fitted with different bracing configurations were modelled using the Vulcan programme. For comparative purpose, the efficiency of BRBs in preventing the progressive collapse of the structure is compared with the Ordinary Concentrically Brace systems (OCBs) in the presence of edge and central bay fire exposures. In order to consider the effect of bracing member stiffness on the collapse prevention of the frame and to provide a comprehensive scheme of progressive collapse mechanism under fire condition, several cross sections of BRBs under different fire scenarios are considered. The results indicate that BRBs provide a higher global collapse temperature for the frame, owing to a greater stiffness and more symmetrical performance offered as compared to OCBs, and thus providing better progressive collapse resistance. Moreover, it is observed that BRBs are stiff enough to redistribute the sustained load by heated columns to adjacent members without any buckling occurrence in the bracing member, maintaining the stability of the whole frame through both heating and cooling phases of fire.