Concentrically Braced Frames Research Articles

Concentrically Braced Frames: European vs. North American Seismic Design Provisions

A critical review of seismic design provisions for concentrically braced frames (CBFs) in both European and North American (i.e. US and Canadian) codes is presented in this paper. Indeed, even though those codes are based on capacity design philosophy, different requirements and different approaches are used to guarantee the hierarchy of resistances between dissipative and non-dissipative elements, thus leading different overall seismic performance. In detail, the main issues critically discussed are (i) the ductility classes and the correlated force-reduction factors; (ii) the structural analysis methods permitted by different codes; (iii) the modelling aspects of braces; (iv) the detailing rules for both dissipative (bracing members) and non-dissipative elements. Synoptic tables summarizing the corresponding assumptions and requirements in different codes are provided.

Dual-concentrically Braced Frames Using High Strength Steel – Seismic Response

The recent technological advances on steel production process allowed introducing in construction market steel grades with significantly high yield strength. These new materials are known as High Strength Steel (HSS). The use of these steel grades offers economical and mechanical benefits compared with mild carbon steel (MCS). Consequently, their use is constantly increasing especially for seismic applications that are the rational field to exploit the high performance of HSS, by means of the “dual-steel” concept, which combines the HSS with MCS in order to provide overstrength to non-dissipative element and ductility to dissipative ones, thus controlling the global frame behaviour into a ductile overall failure mode. In this paper, a comprehensive parametric study devoted to investigate the seismic performance of Eurocode 8 compliant dual-steel chevron Dual-Concentrically Braced Frames (D-CBF) is presented and discussed. This structural typology is composed of two dissipative sub-systems acting in parallel, namely Moment Resisting Frames (MRFs) and Concentrically Braced Frames (CBFs). Static nonlinear pushover analyses were carried out in order to assess the seismic performance of the D-CBFs. The examined parameters cover both geometric and mechanical variables, as the type columns, span length, number of storeys and spectral shape. The analyses showed that the use of HSS in Eurocode 8 compliant D-CBFs is effective to avoid the damage in non-dissipative members. On the other hand, the use of HSS leads to design flexible members, especially for the braced-intercepted beams, resulting in poor performance of bracing members due to significant damage concentration. The economic evaluation shows that the use of HSS allows reducing the material consumptions and dropping the total constructional costs.

Hybrid Testing of the Stiff Rocking Core Seismic Rehabilitation Technique

AbstractThe use of a stiff rocking core (SRC) has been proposed as a seismic rehabilitation technique to mitigate soft-story response in low-rise to midrise steel concentrically braced frame (CBF) ...

Structural Behaviour of Steel Building with Diagonal and Chevron Braced CBF (Concentrically Braced Frames) by Pushover Analysis

This paper presents diagonal and chevron braced steel structure design with pushover analysis conducted following ATC-40 guidelines. In this study, the lateral load used is increased gradually until plastic hinge and collapse occur. The results are capacity curve, performance point, and the process of plastic hinge until the building collapse. There are five steel structure models designed based on the various types of bracing and bracing configuration. Model 1 is steel structure without bracing, model 2 and 3 are chevron braced steel structure, model 4 and 5 are diagonal braced steel structure. The results of this study indicate that the performance level of five models structure for earthquake in x direction and y direction according to ATC-40 is IO (Immediate Occupancy). Model 4 has the best effectiveness in terms of plastic hinge mechanism compared to other models. In the last step, model 4 is able to reduce plastic hinge 84% from model 1 for earthquake in x direction and 79% from model 1 for earthquake in y direction. Model 3 has the greatest ability to carry the forces of an earthquake compared to other models. It is 185% bigger than model 1 for x direction and 181% bigger than model 1for y direction. Model 3 has the greatest effectiveness in reducing lateral displacements compared to other models in the amount of 65.47% of model 1 for x direction and 72.14% of model 1 for y direction. Model 3 has the greatest effectiveness in reducing storey drift compared to other models in the amount of 73.89% of model 1 for x direction and 80.55% of model 1 for y direction.

Seismic Rehabilitation of Concentrically Braced Frames Using Stiff Rocking Cores

AbstractExisting steel concentrically braced frame (CBF) buildings subjected to seismic hazards can exhibit an undesirable soft-story response, in which drift and damage are concentrated in a singl...

Seismic behavior of concentrically braced frames designed to AISC341 and EC8 provisions

Steel concentrically braced frames (CBFs) are frequently used as efficient lateral load resisting systems to resist earthquake and wind loads. This paper focuses on high seismic applications where the brace members in CBFs dissipate energy through repeated cycles of buckling and yielding. Widely-used seismic provisions have somewhat different approaches for the seismic design of CBFs. The present study evaluates in detail the similarities and differences between the design philosophies and provisions used in the United States and Europe for these systems. The requirements of both provisions applied during a full design procedure are summarized and compared. Furthermore, X-braced, split X-Braced, and V-braced archetypes are designed accordingly and the differences in the design outcomes are investigated regarding section sizes and the weight of steel used in each design. Finally, inelastic structural models of the designed archetypes are developed and subjected to a large set of ground motions to study their seismic behaviors. The results of a total of 880 nonlinear time history analyses are then synthesized to investigate the way in which the requirements of these provisions affect the seismic behavior of the designed CBF. Notable differences are observed between the performances of the CBFs designed using American and European provisions. The similarities and differences as well as drawbacks of the provisions are thoroughly discussed. Recommendations and future research needs are suggested to enhance the seismic performance of steel CBFs designed according to these provisions.

Seismic performance of steel frames equipped with buckling-restrained braces (BRBs) using nonlinear static and dynamic analyses

The current paper compares the seismic performance of buckling-restrained braced frames (BRBFs) and ordinary concentrically braced frames (CBFs) through nonlinear static and time history analysis. Two groups of BRBFs and CBFs including 5, 10 and 15 story frames are modeled and nonlinear pushover analysis is carried out. To further investigate the performance of frames, a selection of near-fault and far-field ground motion records are used for time history analysis. The results indicate the acceptable seismic performance and conformity of low-rise CBFs with BRB frames under far-field records. However, BRB frames present more satisfying results with regards to inter-story drifts, residual drift responses and energy dissipation in case of near-fault seismic excitations. Moreover, the implementation of BRBs provides better performance against the Basic Safety Earthquake 1 (BSE-1).

Progressive collapse analysis of buildings with concentric and eccentric braced frames

In this study, the susceptibility of different symmetric steel buildings with dual frame system to Progressive Collapse (PC) was assessed. Some ten-story dual frame systems with different type of braced frames (concentrically and eccentrically braced frames) were considered. In addition, numbers and locations of braced bays were investigated (two and three braced bays in exterior frames) to quantitatively find out its effect on PC resistance. An Alternate Path Method (APM) with a linear static analysis was carried out based on General Services Administration (GSA 2003) guidelines. Maximum Demand Capacity Ratio (DCR) for the elements (beams and columns) with highest DCRs (DCRmoment and DCRshear) is given in tables. The results showed that the three braced bays with concentric braced frames especially X-braced and inverted V-braced frame systems had a lower susceptibility and greater resistance to PC. Also, the results represented that the beams were more critical than columns against PC after the removal of column.

New expressions to estimate damping in direct displacement baseddesign for special concentrically-braced frames

New expressions to estimate damping in direct displacement baseddesign for special concentrically-braced frames

Full-Scale Cyclic Testing of Low-Ductility Concentrically Braced Frames

AbstractTwo full-scale, two-story, low-ductility steel concentrically braced frame (CBF) systems were tested to evaluate failure mechanisms, postelastic frame behavior, reserve capacity, and overal...

Role of compression diagonals in concentrically braced frames in moderate seismicity: A full scale experimental study

According to European building code provisions, compression diagonals should be neglected during the analysis stage of concentrically braced frames (CBFs) with X and N type bracings, and the inelastic capacity of the tension bracings only should be considered in the design. This provides simplifications at analysis and design stages for practising engineers. Such an assumption can be rational in the high-seismicity context, where the compression bracings undergo buckling at the early stages of the seismic event, and the shear demand is very high. On the other hand, in moderate seismicity areas (that is estimated as the 90% of the seismic regions of the world), where the shear deformation demand for braced frames and the number of high-amplitude cycles are very limited, it may be reasonable to consider both tension and compression diagonals in the analysis. Accounting for compression diagonals at the analysis stage, and exploiting their post-buckling resistance and dissipative contribution in design, may allow using a higher behaviour factor, and increase the economic efficiency of CBF structures in moderate seismic regions. To understand the real seismic performance of braced frames in moderate seismicity areas, a sound characterization is needed, focusing on behaviour of compression diagonals. In the literature, many tests have been performed to analyse the behaviour of bracing elements, but they were mostly designed to meet high seismicity criteria with significant connection over-strengths.The European research project RFSR-CT-2013-00022 MEAKADO investigated the influence of compression diagonals on the global performance of CBF structures, by means of experimental and numerical studies. This paper presents the results of full scale tests performed within this research project, focusing on the stiffness and post-buckling performances of double-angle bracings with bolted connections, which are the most common bracing configurations in the European construction market characterized by low-to-moderate seismicity. Experiments have shown that extra stiffness and strength provided by the contribution of the compression diagonals to the structural response are significant for moderate seismicity drift and shear demands, and may be worth considering in the analysis and design phases.

New expression to estimate out-of-plane displacement of special concentrically-braced frames

Induced non-structural damages due to strong ground motion can affect the performance levels of a building. Special concentrically-braced frames are used in high and moderate seismicity regions because of high ductility and energy absorption capacity. In special concentrically-braced frames, storey drift ratio and the out-of-plane displacement are the most important parameters defining structural and non-structural damage. Various expressions for out-of-plane displacement without inertial effects are available in the literature. This study describes the development of new expressions to determine out-of-plane displacement for special concentrically-braced frames based on the seismic response of 16 different single-storey models: eight inverted-V and eight X-braced frames. Initially, out-of-plane displacement is calculated based on cyclic loading and then modified for earthquake excitations. A simplified iterative procedure is proposed to calibrate out-of-plane displacement expression using revised effective mass. Two new expressions for determining out-of-plane displacement are developed as a function of storey drift ratio, slenderness ratio, and the ratio of the tension to compression strength.

Cyclic behavior and finite element modeling of wide flange steel bracing members

Wide-flange (WF) bracing members are being used increasingly in concentrically braced frames (CBFs). The limited number of tests WF members performed to date highlights the need for a detailed finite element model with a reliable fracture rule to understand the hysteresis behavior and to predict the fracture of WF braces subjected to cyclic loading. Building a reliable finite element model is cost effective compared to the high expense of experiments, especially when brace sizes are large and thus require a high capacity test frame with powerful actuators. Nine WF quasi-static cyclic brace tests (Fell et al., 2006; Tremblay et al., 2008; Richard, 2009) are used to validate the finite element model presented here. Material variations on the cross-sectional level are considered. Results indicate that the current model is able to simulate the overall hysteresis behavior of the WF braces accurately. Fracture in the plastic hinge that developed at the mid-length of specimens tested by Tremblay and Richard is predicted through a calibrated cyclic void growth model (CVGM) for ultra-low cycle fatigue. In addition, the current model is able to simulate the shift in the location of mid-length plastic hinge. The effects of mesh density, number of integration points, boundary conditions, initial imperfection, initial yield strength, and loading protocol on the hysteresis behavior of WF bracing members are also investigated.

Analytical investigation on cyclic response of buckling-restrained braces with short yielding core segments

Buckling-restrained braced frames (BRBFs) are considered to be one of the efficient lateral force-resisting systems used in buildings located in the seismically active regions. Nearly symmetric hysteretic response of buckling-restrained braces (BRBs) in tension and compression due to the yielding of steel core plates helps the BRBFs to withstand the seismic excitations without causing any extensive damage to the primary frame members. However, BRBFs may exhibit the excessive post-earthquake residual drift response due to the low axial stiffness of BRBs under the strong ground motions. A reduction in the yielding core segments of BRBs results in the improved elastic and post-elastic axial stiffness of BRBs which may help in controlling the excessive residual drift response. This study is focussed on the analytical evaluation of hysteretic response of BRBs of varying lengths using finite element (FE) software. The results of FE study are further used to evaluate the inter-story and residual drift response of a 3-story braced frame fitted with BRBs of short core lengths (referred as SBRBF). Nonlinear dynamic analysis results of SBRBF are compared with the conventional BRBF and concentrically braced frame (CBF). It is concluded that the optimum reduction in yielding core lengths of BRBs can improve the overall seismic response of BRBFs with a reduction in the residual drift response.

Evaluation of loading sequences on testing capacity of concentrically braced frame structures

Static cyclic loading tests are commonly used to evaluate the seismic capacity of a structure. However, different loading sequences would result in different structural behavior. To quantify the effects of loading sequences on the structural capacity, we investigate the experimental behavior of braces in concentrically braced frames (CBF) under different loading sequences. The variables for testing program includes two different pipe sections (different slenderness as well) and three loading protocols, namely, loading sequence for beam-to-column moment connections (LS1), loading sequence for link-to-column connections (LS2) and proposed loading sequence for CBF structures (LS3). After tests, we compare the critical performance parameters of different specimens including the failure modes, maximum deformation, cumulative energy, etc. Test results show that the specimens fracture at different deformation stages under different loading sequences. The specimens accumulate energy the fastest under LS1. The maximum cumulative energy is less correlated to the loading sequences. The test results also show that the rate of cumulative deformation under LS1 is too fast for CBF structures; the rate under LS2, on the contrary, is a little slow for CBF structures. LS3 is more suitable for braces in regard to failure modes, cumulative deformation rate, cumulative energy rate, and deformation amplitude.

Secondary frame action in concentrically braced frames designed for moderate seismicity: a full scale experimental study

European seismic design codes do not take into account the strength and stiffness of the secondary frame action provided by bracing gusset plates of concentrically braced frames (CBFs). This is an attractive assumption for practicing engineers, as it provides simplifications during the analysis and design phases. However, when efficiency and economy are concerned, especially in low-to-moderate seismic regions, this normally neglected frame resource may be interesting to consider in design. Gusset plates can provide a certain degree of stiffness and strength following the bracing failure, and may even prevent global collapse. In particular, when the shear deformation demand of the braced cell remains limited, as in the case of low-to-moderate seismic actions, it may become reasonable to take this extra stiffness and strength into account. Ongoing research project RFSR-CT-2013-00022 MEAKADO investigated this phenomenon by means of experimental and numerical studies with the perspective of setting new inputs for the design rules of the future generation of Eurocodes. This paper presents the results of full scale tests performed inside this research project, which characterized resistance, stiffness, and ductility resources of CBF systems designed for moderate seismicity. The paper also quantifies the effective contribution of the frame action, provided by gusset plate connections, to the global performance of CBF frames.

Redundancy Factors for the Seismic Design of Ductile Reinforced Concrete Chevron Braced Frames

In this paper the authors summarize the results of a study devoted to assess, using nonlinear static analyses, the impact of increasing the structural redundancy in ductile moment-resisting reinforced concrete concentric braced frames structures (RC-MRCBFs). Among the studied variables were the number of stories and the number of bays. Results obtained were compared with the currently proposed values in the Manual of Civil Structures (MOC-08), a model code of Mexico. The studied frames have 4, 8, 12 and 16-story with a story height h=3.5 m. and a fixed length L=12 m., where 1, 2, 3 or 4 bays have to be located. RC-MRCBFs were assumed to be located in soft soil conditions in Mexico City and were designed using a capacity design methodology adapted to general requirements of the seismic, reinforced concrete and steel guidelines of Mexican Codes. From the results obtained in this study it is possible to conclude that a different effect is observed in overstrength redundancy factors respect to ductility redundancy factors due to an increase of the bay number considered. Also, the structural redundancy factors obtained for this particular structural system varies respect to the currently proposed in MOC-08.

Seismic Behavior of X-Shaped Drawer Bracing System (DBS) and X-Braced Frames with Heavy Central Core

In this work, seismic performance of conventional X-braced frames is enhanced by using Drawer Bracing Systems (DBS). DBS is an innovative structure, which increases ductility and energy absorption capacity of the X-braces through elimination of the harmful effects of local and global buckling and by converting the induced axial forces inside diagonal arms to flexural moments. Two half-scale specimens are tested under cyclic loading and the seismic performance of an X-shaped DBS is compared to that of an X-braced frame. Both braced frames are designed for equal nominal base shears and have similar frame sizes and dimensions. Test results confirm that converting the axial force to flexural moments in rational dissipative elements inside braces helps prevent the global and local buckling of braces in X-shaped DBS. Consequently, ductility and energy absorption capacity of the Concentrically Braced Frames (CBFs) is increased remarkably.

Seismic Performance Assessment of Multitiered Steel Concentrically Braced Frames Designed in Accordance with the 2010 AISC Seismic Provisions

AbstractMultitiered steel concentrically braced frames (CBFs) are commonly used to provide lateral resistance for tall single-story commercial, performing arts, sports, and industrial buildings. The seismic response of these frames is studied in this paper. A set of seven special concentrically braced frames (SCBFs), ranging from 9 to 30 m tall with two to six tiers, located in a high seismic area was designed according to the 2010 AISC Seismic Provisions. Fundamental behavior of the two- and four-tiered frames was investigated using three-dimensional (3D) finite element models with shell elements, with particular focus on the buckling response of the columns. The seismic frame response and column stability were then studied more broadly for all frames using more computationally efficient 3D finite element models with fiber-based beam-column elements, which were validated against the shell element models. Multitiered CBFs designed according to current multistory CBF procedures are shown to develop drift c...

Experimental Investigation of Chevron Concentrically Braced Frames with Yielding Beams

AbstractSteel concentrically braced frames designed prior to the implementation of capacity design principles in seismic design provisions may exhibit poor inelastic response under seismic excitation. These older, nonductile concentrically braced frames (NCBFs) used several configurations, with the chevron configuration being one of the most common. The response of chevron-configured NCBFs is unknown, as relatively large axial and flexural demands are imposed on the beam after brace buckling. Current code requirements for special concentrically braced frames (SCBFs) promote full yielding of the braces while the beam remains elastic, but NCBFs develop a mechanism in which the beam yields, deforms plastically, and limits tensile elongation of the brace. However, if ductile, this plastic mechanism may meet current performance limits and not require retrofitting. To examine this issue, four tests of two-story NCBFs were conducted at the National Center for Research on Earthquake Engineering in Taipei, Taiwan....