Concentrically Braced Frames Research Articles

Development of an innovative metallic damper for concentrically braced frame systems based on experimental and analytical studies

SummaryAlthough concentrically braced frames (CBFs) possess high lateral stiffness and strength, the buckling of compressive members indicates that a system does not have appropriate ductility. Therefore, CBF is not a successful system for high seismic regions. Comprehensive studies have been performed on this drawback concerning CBF systems. The use of a metallic damper is considered an appropriate method to enhance the behavior of CBFs. While metallic dampers improve the hysteretic behavior of the CBF, they impose additional costs on the structures. Therefore, in this study, a steel damper, which is economical and straightforward to construct and replace after a severe earthquake, was developed. The proposed damper was investigated experimentally and numerically. In addition, a parametric study was performed to evaluate the effect of the three types of damper mechanisms (shear, shear‐flexural, and flexural) with three types of buckling (elastic, inelastic, and plastic) on the behavior of the proposed damper. The experimental results, as well as the numerical results, indicate that the shear damper exhibits better performance than the other dampers in terms of strength and stiffness. Except in the case where the shear damper is accompanied by plastic buckling, it has a higher energy dissipation capability. In addition, a flexural damper with elastic buckling cannot be considered as an energy‐dissipating device because of the fracturing of the damper due to displacement near the elastic zone. Fundamental relationships were used to predict the behavior of these dampers, and these predictions showed good agreement with the finite element results.

Performance-based rules for the simplified assessment of steel CBFs

In this work, a simplified method for the evaluation of the seismic performance of steel Concentrically Braced Frames (CBFs) in everyday practice is presented. This method, applicable in the immediacy of a seismic event and for quick mapping of the seismic vulnerability of a built area is fully analytical, allowing the representation of the capacity curve of a structure through a trilinear approximation and considering the second order-effects. To set the methodology, extensive parametric analysis is performed on 420 simple CBFs designed according to 3 different approaches namely the Theory of Plastic Mechanism Control ensuring the design of structures characterized by global collapse mechanism (Global Concentrically Braced Frames (GCBFs), the Eurocode 8 provisions (Special Concentrically Braced Frames (SCBFs)) and the design for seismic forces only (Ordinary Concentrically Braced Frames (OCBFs)) without any hierarchy criterion. The capacity-demand checking procedure according to the code-compliant approach and Nassar & Krawinkler is also reported.

Fracture prediction for square hollow section braces under extremely low cycle fatigue

This paper examines the extremely low cycle fatigue (ELCF) fracture of concentrically loaded square hollow section (SHS) braces subjected to cyclic loading. Numerical analyses are presented for both individual bracing members and bracing members integrated into concentrically braced frames (CBFs). The behaviour of the individual members was predicted using solid finite element (FE) simulations that employed a ductile fracture model and a nonlinear damage evolution rule. The solid FE model, which was validated using data from experiments, could adequately predict both the hysteretic response and the ELCF fracture cracking process. The coupled effects of instabilities (i.e. local and global buckling) and fracture on the ELCF performance of the braces were assessed, and the rotation capacity prior to fracture was quantified. This quantified rotation capacity was then incorporated into fibre-based FE models of CBFs as a member-level fracture criterion. The structure-level simulations were able to accurately capture the complex interactions between the frame components, i.e. the columns, beams, brace–gusset–plate​ connections and beam-to-column connections, and hence replicate the overall behaviour of CBFs, specifically, two-storey chevron braced frames. The influence of cross-section and member slenderness was evaluated and the importance of considering both in the development of cross-section slenderness limits was highlighted. The combined member- and structure-level simulation approach is proposed as an accurate and efficient means of assessing the seismic performance of CBFs.

Dual system design for a low-ductility concentrically braced frame with a reserve moment frame

To address shortcomings of the moderate-seismic design philosophy currently implemented in the United States, this paper proposes design criteria for a Concentrically Braced Dual Frame (CBDF). In contrast to existing low-ductility Concentrically Braced Frame (CBF) systems, such as R=3 CBF (R3CBF) and Ordinary Concentrically Braced Frame (OCBF) systems, the proposed CBDF design approach explicitly considers the strength and integrity of the post-elastic, degraded (reserve) system. Specifically, the CBDF is defined to consist of a stiff low-ductility primary concentrically braced frame (PCBF) supplemented by a flexible, distributed reserve moment-resisting frame (RMRF). Relative to the R3CBF and OCBF systems, the proposed CBDF system achieves superior collapse performance without the need for costly ductility detailing requirements or substantial changes to traditional seismic design provisions. Consequently, the CBDF system is proposed as a practical, reliable, and cost-effective design alternative to R3CBF and OCBF systems.

Structural performance of concentrically and eccentrically braced frame

Indonesia is prone to earthquakes. Therefore, the building needs to be designed to withstand earthquake lateral force. One of the lateral-resistant elements is a steel frame system with bracing. There are two types of bracing based on its configuration, that is Concentrically Braced Frame (CBF) and Eccentrically Braced Frame (EBF). The objective of this research is to examine the capability (lateral resistance, plastic hinge mechanism, and ductility factor) of CBF and EBF. Structural analysis is performed using SAP2000 v.18. Pushover analysis is performed on Moment Resisting Frame (MRF), CBF, and EBF with various link lengths e = 0.4 m, 0.6 m, 0.8 m, and 1.0 m). The increase in lateral resistance resulting from the use of CBF is 74% and for EBF is 52% compared to MRF. The plastic hinge mechanism for CBF is marked by the formation of plastic hinges on bracing, while EBF is marked by the formation of plastic hinges on the links then beams and columns. The ductility factor for CBF is 45.75% smaller than MRF. While for EBF, the ductility factor is 18.20% smaller than MRF but 33.68% larger than CBF. Therefore, EBF is more ductile than CBF.

Utilizing I‐shaped shear links as dampers to improve the behavior of concentrically braced frames

SummaryDuring the past few decades, concentrically braced frames (CBFs) have become a commonly used load‐resistant bearing system in steel structures, to overcome the post‐earthquake performance issues frequently associated with conventional seismic‐resisting structural systems. CBF systems have high lateral stiffness and lateral strength; however, they have low ductility and a low seismic energy dissipating capacity. Under seismic loading, the diagonal members of a CBF system are susceptible to buckling, causing hysteresis in the compression zone and severely reducing energy absorption. Although the use of steel dampers to enhance the behavior of CBF systems can prevent the buckling of CBF members and improve the hysteretic behavior of braces, their use imposes additional costs on structures. Therefore, in this paper, an I‐shaped steel damper with a shear yield mechanism is introduced which is economical, has a simple construction, and can be easily replaced after an earthquake. The proposed damper is evaluated numerically and parametrically. The results indicate that the proposed damper acts as a ductile fuse that prevents buckling of the diagonal members of a CBF system. It was found that both the web and flange plates contribute to the shear resistance, with the flange carrying approximately 15% of the shear force, as expressed in the equations presented. The overstrength obtained for the damper was found to be greater than the value of 1.5 proposed by the AISC. Equations are presented for the design of the proposed damper and the brace elements connected to the damper.

Component-based model for bolted brace connections in conventional concentrically braced frames

In low and moderate seismic regions, low-ductility concentrically braced frames (CBFs) are widely used as the seismic force-resisting system for steel structures. The capacity-based design method is not required for such systems, i.e. no individual component in the lateral load carrying path is explicitly designated to sustain plastic deformations under seismic loading. Such CBFs are referred to as conventional CBFs (CCBFs) in this paper. Prior studies have revealed that, in CCBFs, the brace-to-gusset connections are inherently weaker in tension than the adjoining braces and gusset plates. Therefore, the accurate numerical modelling of the brace connections is critical for the reliable seismic evaluation of CCBFs. However, few research publications address the inelastic bolted brace connection modelling necessary for the structural analyses of these braced frame systems. In this paper, an efficient inelastic numerical modelling method, comprising the component-based modelling concept, is proposed for bolted brace connections. The accuracy of the numerical model is validated through comparison with laboratory test results of full-scale I-shape brace connection specimens. Eight single-storey CCBFs with the symmetric diagonal bracing configuration were designed and modeled. The nonlinear static and dynamic analyses revealed that: 1) although the buckling of the middle column at small storey drifts resulted in substantial lateral strength deterioration, a secondary seismic mechanism provided stable resistance to prevent collapse; 2) when loaded in tension, the brace connections deformed more than the braces; 3) stronger brace connections resulted in higher structural lateral stiffness and triggered earlier buckling of the middle column; 4) stronger brace connections possessed higher frictional energy-dissipating capacity which reduced the maximum storey drift.

Shake Table Testing of Self‐Centring Concentrically Braced Frames

AbstractThe self‐centring system presented in this paper is a novel damage control technique designed to improve the resilience of concentrically braced frames (CBF) under seismic action. Namely, traditional CBFs can undergo large residual drifts following an earthquake event which can limit the opportunity for cost‐effective repair of the structure. Additionally, the gusset plates connecting the brace members to beams and/or columns can experience substantial rotations as a result of the compression buckling of the bracing members. Through the utilisation of post‐tensioning strands placed between flanges of beams, the novel self‐centring concentrically braced frame (SC‐CBF) system can return the frame to its original position after significant inelastic deformations experienced during large earthquakes, resulting in minimum residual drifts.In this paper, shake table testing of the aforementioned SC‐CBF system subjected to realistic earthquake loading is presented. The research is carried out as part of the H2020 “Seismology and earthquake engineering research infrastructure alliance for Europe” SERA project. Four sets of bracing configurations, incorporating varying square hollow section (SHS) braces and gusset plates were utilised in the shake table testing. Uniaxial loading with varying shake table accelerations was executed and the structural response evaluated using data from strain gauges (SG), load cells (LC), displacement transducers and accelerometers. The measured results provide information on the important parameters such as the tensile and compressive strength of the braces, post‐buckling capacity, gusset plate strains and post‐tensioning force. These findings are then presented and the crucial local and global response performance emphasised.

Experimental Evaluation of Ductility of Bracing Members

AbstractIn earthquake‐prone region, concentrically braced frames (CBFs) are used as lateral load resistance systems and prevents damage in structures. Special concentrically braced frames (SCBFs) are special type of CBFs system which is designed for higher level of displacement ductility and energy dissipation. In case of SCBF system, the connections are designed to undergo inelasticity which helps in dissipating the energy of the system. Two types of the connections that are used in the SCBF system are out‐of‐plane buckling (OOPB) connection and in‐plane buckling (IPB) connection. The latter is a newly developed connection for the SCBF system to prevent out‐of‐plane buckling of the braces which minimizes the damage of the non‐structural components of the building. In the present study, a state‐of‐the‐art comparison of the IPB and OOPB brace system were done keeping slenderness ratio and compactness ratio constant. Large‐scale tests were conducted on the two type of connections. Elliptical clearance of eight‐time the thickness of the gusset plate was used for the OOPB braced system. Linear clearance of three times the thickness of the knife plate was used for the IPB braced system. The parameters that were compared to measure the efficiency of the connections were displacement ductility, cumulative energy ductility and failure mechanism. It is found that the OOPB brace system performs better as compared to the IPB brace system keeping all other parameters constant. The reasons for the poor performance of the IPB brace system were discussed.

Utilizing an I-shaped shear link as a damper to improve the behaviour of a concentrically braced frame

Over the past few decades, concentrically braced frames (CBFs) have become a commonly used load-resistant bearing system in steel structures. CBF systems have high lateral stiffness and lateral strength; however, they have low ductility and a low seismic energy dissipating capacity. Under seismic loading, the diagonal elements of a CBF system are susceptible to buckling, causing hysteresis in the compression zone and severely reducing energy absorption. Although steel dampers improve the hysteretic behaviour of the braces, they impose additional costs on structures. Therefore, in this study, an I-shaped steel damper with a shear yield mechanism was introduced; this is an economical solution, has a simple construction, and can be easily replaced after an earthquake. The proposed damper was evaluated numerically and parametrically. Results indicate that the proposed damper acts as a ductile fuse that prevents buckling of the diagonal element of a CBF system. It was found that both the web and flange plates contribute to the shear resistance, with the flange carrying approximately 15% of the shear force, as expressed in the presented equations. Additionally, overstrength obtained for the damper was greater than 1.5 (proposed by AISC). Necessary relations are represented to predict and design the damper and the elements outside it.

Performance-based seismic retrofit of RC structures using concentric braced frames equipped with friction dampers and disc springs

In this study, concentric braced frame installed with a friction damper-disc-springs is proposed to enhance the seismic performance of RC framed structures. The structural configuration of the retrofit is outlined and the analytical model is explained. A performance-based seismic retrofit procedure combined with a genetic algorithm (GA) optimization is proposed to obtain the optimum design variables of the retrofit. 2D and 3D structural models are used as case studies. The effectiveness of the proposed retrofit is assessed through non-linear time-history response analysis (NLTHA), fragility analysis, and seismic life cycle cost (LCC) estimation. To ensure that concrete stresses after retrofit are within acceptable limits, finite element method is used. The results show that the proposed retrofit technique is effective in reducing the maximum inter-story drift ratio and in eliminating the residual drift of the model structures significantly. In addition, the probabilities of exceeding different limit states and the total seismic LCC are significantly reduced compared to the un-retrofitted cases.

Conventional I-shape brace member bolted connections under seismic loading: Laboratory study

In many regions of the world, a conventional low-ductility concentrically braced frame (CBF) is commonly chosen as the seismic force resisting system. The energy dissipation is not restricted to yielding and buckling of the braces, as would be required for ductile braced frames; rather, energy dissipation is assumed to occur through localised yielding and friction in connections, as well as limited member yielding. The objective of this study was to characterize by means of laboratory testing the inelastic response of full-scale conventional I-shape braces and their bolted connections under reversed-cyclic seismic loading. Six brace specimens comprising two commonly used bolted connection types, flange plate and flange angle, were designed following the Type CC provisions prescribed by CSA S16. The brace specimens achieved storey drift ratios of 1%–2%, due to inelastic deformations in both the braces and the connections, even though capacity based design provisions were not incorporated in their design. Energy dissipation resulted from overall and local brace buckling, gusset plate buckling, tension yielding of flange plates, angles and gusset plates, block shear failure of brace webs and angles, bolt fracture and fracture of the gusset plates, along with bolt slip. However, the observed performance of these braces and connections in this laboratory based study is not necessarily guaranteed to be replicated for all conventional CBF designs due to the conservative nature and variability in accuracy of the existing CSA S16 design equations that were relied upon to design and detail the test specimens.

Applicability of the direct displacement-based design procedure to concentrically braced frames with setbacks

ABSTRACT The objective of this paper is to investigate and develop seismic design guidelines for multi-storey vertical irregular concentrically braced frames (CBFs). The work develops a direct displacement-based design (DDBD) procedure for irregular CBFs associated with steps in building plan area. In this procedure, design displacements considered are decided upon the code and material drift limits, then the strength required to achieve this displacement is calculated and finally all structural elements are designed. A case study of a 12-storey CBF structure with vertical irregularity is designed using the developed DDBD procedure. The configuration of the vertical irregularity assessed is in the form of setbacks up the vertical axis of the building where the frames have more bays at the base of the building than at the top. Non-linear time history analysis (NLTHA) using seven different accelerograms with displacement response spectra matching the design displacement spectrum are used to record the behaviour of the irregular CBF structure when subjected to real earthquakes. It is found that the design displacements and storey drifts from the DDBD procedure for the case study matched relatively well with those recorded through the NLTHA analyses and a new DDBD procedure for CBFs with vertical irregularity is validated.

The influence of the axial restraint on the overstrength of short links

Eccentrically braced frames (EBFs) constitute a good structural solution located between Moment Resisting Frames (MRFs) and Concentrically Braced Frames (CBFs) because they exhibit an adequate lateral stiffness joined with a high ductility capacity, especially for what concerns short and very short links. Link design plays an important role in the effective dissipation capacity of the structure that is well designed only if the link members are involved in plastic range while the other members remain elastic. A proper design of the non-dissipative members needs to account for the link overstrength that results to be very high, especially for very short links.In this paper, the influence of the axial restraints on the evaluation of the link overstrength and ultimate rotation is investigated. A wide parametric analysis is carried out based on a set of shapes selected in the European Standards. FEM simulations are used to develop the parametric analysis after a calibration carried out on experimental tests selected from a literature review. Empirical formulations are also proposed with the scope of providing a more accurate evaluation of the overstrength factor for short and very short links in the framework of EU “I shaped” standard profiles.

Dynamic tests of the collapse-prevention performance of a low-ductility low-rise steel concentrically braced frame

In past earthquakes, low-ductility steel concentrically braced frame (CBF) buildings have been observed to retain reserve capacity after brace fracture. The failure mechanisms and earthquake ground motion-sensitive characteristics of the reserve system of low-ductility CBFs are not understood due to limited dynamic experimental data. In this study, a series of shaking table tests is presented to examine the reserve capacity of a scaled two-bay, three-story, low-ductility CBF model in which a pair of square hollow section braces are arranged in a chevron pattern. The model structure was repeatedly subjected to two unidirectional ground motions with successively increasing magnitudes in a stiff soil site and a soft soil site. The experimental phenomena of elastic response, brace buckling behavior, brace reengagement behavior, reserve system response and collapse state were observed. The braces of the low-ductility CBFs were vulnerable to local buckling under dynamic loads and resulted in a soft story where braces failed, while braces and frames in the other stories had no further damage. The reserve stiffness, rather than energy dissipation or reserve strength, was the most important factor in the collapse-prevention capacity for the reserve system of the tested low-ductility CBF, and the lateral stiffness of the columns was the main source of reserve stiffness. Flexible reserve systems with small lateral stiffness were never significantly excited at their natural periods by hard soil site-specific ground motions, while with such systems, a greater risk of potential collapse appears to be posed due to earthquakes at soft soil sites than at hard soil sites for the test model.

Nonlinear Analysis of Eccentrically Braced Frames against Progressive Collapse

Eccentrically braced frame (EBF) incorporates the advantages of lateral resisting systems from moment resisting frame (MRF) and concentrically braced frame (CBF) into a single structural system. In order to investigate the effects of brace and link length in resisting progressive collapse, a finite element model of 10-story planar steel frame were built and analyzed with the SAP2000 software through nonlinear static analysis and nonlinear dynamic analysis respectively. The results show that brace can obviously improve the performance of the structure no matter which kind of brace is adopted. The inverted V-shaped (IV) eccentrically braced frame has the highest capacity against progressive collapse and the Y-shaped eccentrically braced frame has the highest energy absorption capacity of the structure. Increasing the link length leads to an increase in deformation and energy absorption of the structure. The results of nonlinear static analysis could reflect the structural performance basically.

Improving the performance of concentrically braced frame utilizing an innovative shear damper

Concentrically braced structures are one of the most common resistant systems in steel structures due to their high stiffness and lateral strength. However, when these structures are subjected to moderate and severe earthquakes, their compression member begins to buckle. Also, buckling of compression members in Concentrically Braced Frames (CBFs) decreases ductility and degradation of the hysteresis curve. Numerous studies have been performed to improve the behavior of CBFs. Using steel dampers is considered one of the appropriate ways to improve the behavior of CBFs. Although steel dampers improve the hysteretic behavior of the CBFs, they impose additional costs on structures. Therefore, in this paper, a steel damper with a shear yield mechanism is introduced, which in addition to being economical, has the ease of construction and replacement after an earthquake. The numerical results indicate an enhancement of the hysteretic behavior of the CBFs utilizing the proposed damper. The main goal of equipping CBFs with the proposed damper is to control the possible damage in the damper and to ensure the elastic behavior of other structural components. The results indicate that the proposed damper can act as a structural fuse by dissipating more than 99% of the system energy and concentrating most of the plasticity and damage, which effectively protect the main structural components from damage while being subjected to seismic excitations. Necessary relations are presented for designing this damper. Besides, a simple method is presented for estimating the nonlinear behavior of the braces equipped with the proposed damper without any need for finite element modeling.

Parametric study on the I-shape brace connection of conventional concentrically braced frames

Concentrically braced frames (CBFs), designed using the conventional linear elastic method without seismic proportioning and detailing requirements, are referred to as Conventional CBFs (CCBFs) in this study. They are widely used in moderate and low seismic areas in North America due to the ease of design and economy. Without a code specified dedicated fuse member to dissipate earthquake induced energy, or a prescribed yield/failure hierarchy, the brace connection of a CCBF is usually the weakest link in the lateral load-carrying path and prone to fracture. The brace connection is therefore determinant for the structural seismic performance. In this paper, a parametric study based on a validated numerical simulation procedure was carried out on a typical I-shape brace connection, i.e. the flange plate connection. Three key design parameters, namely, the gusset plate thickness, the flange lap plate thickness, and the web lap plate thickness, were varied to study their effects on both the compressive and tensile behaviour of the brace and connection assembly. Various possible failure modes were revealed both in compression and in tension. The results showed that the brace end restraint provided by flange plate connections in CCBFs was significant; the pinned-end assumption would lead to conservative estimation of the brace buckling resistance, which might trigger detrimental gusset plate buckling. The tensile overstrength of the flange lap plate, due to the presence of transverse tensile stress along the net section, was quantified using the von Mises criterion. Design recommendations are proposed with regards to attaining better deformation capacity.

Numerical investigation into I-shape brace connections of conventional concentrically braced frames

In many low and moderate seismic regions, a low-ductility concentrically braced frame (CBF) is used as the seismic force resisting system for steel structures. The design of such CBFs is straightforward: all members and connections are designed based on the seismic force demand obtained through linear elastic structural analysis; capacity-based design and additional seismic detailing are not required. There is no designated energy-dissipating fuse in the lateral load-carrying path. Such CBFs are referred to as Conventional CBFs (CCBFs) in this study. In CCBFs, the brace-to-gusset connections are inherently weaker in tension than the adjoining gusset plates and braces. This occurs because both the gusset plates and the braces are selected on the basis of their respective compressive buckling resistances, and hence, typically have a much greater resistance in tension. Described herein is a numerical study of the popular flange plate bolted I-shape brace connection configuration. A high-fidelity finite element (FE) simulation procedure was developed and validated against laboratory test results. The resulting numerical models were created with the objective of improving our understanding of the inelastic response of these brace connections. The force transfer mechanism within the two branches of the connection was characterized. Significant nonuniform shear distribution was found to exist within the bolt group in the flange branch, which may be detrimental to the safe functioning of these bolts in the seismic design context. The loading eccentricity on the weld group was quantified. Recommendations on how to avoid brittle bolt shear rupture and premature weld fracture are proposed.

Hybrid braced frame with buckling-restrained and strong braces to mitigate soft story

The steel concentric braced frames (CBFs) are the common structural system for buildings. Unlike high strength and stiffness, brace buckling in CBFs results in high nonlinear behavior, which causes damage concentration in one or a few stories during a severe earthquake. This paper proposes a hybrid CBF bracing system to reduce soft-story mechanisms and improve the structural behavior of the CBF system. The proposed system is composed of a buckling restrained brace (BRB) or an energy dissipation device and a strong brace (SB) in one span. The strong braces are designed to have an elastic performance during an earthquake. The direction of BRB and SB are changed, inversely, story by story in a zigzag form. This configuration leads to paradoxical results: stiffness and ductility. This improves both the soft story mechanisms and the buckling capacity of CBFs. In this study, a set of 6-story structural models with different bracing patterns including inverted-V, multi-X, zipper, hexa (hexagonal bracing configuration), and strongback braced frames were evaluated through nonlinear static (pushover) and dynamic analyses (IDA). The collapse performance of structural models was assessed by comparing the seismic behavior of conventional CBFs with similar bracing configuration based on FEMA P695 criteria. From the evaluation of collapse fragility curves, the hybrid bracing system had more safety margin against collapse during severe earthquakes than conventional CBFs.