Special Concentrically Braced Frames Research Articles

Collapse performance of seismically isolated buildings designed by the procedures of ASCE/SEI 7

This article presents an analytical study of the seismic collapse performance of seismically isolated buildings and comparable non-isolated buildings. The study is based on archetypical 6-story perimeter frame seismically isolated buildings designed with special concentrically braced frames (SCBF), ordinary concentrically braced frames (OCBF) and special moment resisting frames (SMF) for a location in California using the minimum criteria of ASCE/SEI 7-10 and ASCE/SEI 7-16 and also using a number of enhanced designs. The isolation system consists of triple Friction Pendulum (FP) isolators with stiffening behavior at large displacement. Additionally, double concave sliding isolators are considered and designed per minimum criteria of ASCE/SEI 7 and without a displacement restrainer, a practice permitted by the standards. Non-isolated structures, also with braced and moment frame configurations, are designed using the minimum criteria of ASCE/SEI 7 and studied. The study concludes that seismically isolated buildings designed by the minimum criteria of ASCE/SEI 7 of either 2010 or 2016 may have unacceptable probability of collapse in the Risk-Targeted Maximum Considered Earthquake (MCER). The probability of collapse in the MCER becomes acceptable when they are designed with enhanced criteria of RI=1.0 and with isolators having a displacement capacity at initiation of stiffening equal to 1.5 times the demand in the MCER. It is also observed that designs that meet the minimum criteria of ASCE/SEI 7 of either 2010 or 2016 and without any displacement restrainer have unacceptably high probabilities of collapse.

Cyclic Performance of Braces with Different Support Connections in Special Concentrically Braced Frames

Gusset plate connections between the steel braces and the supporting frame members play an important role in the performance of special concentrically braced frames (SCBFs) under earthquake loading conditions. Extensive studies have been conducted on SCBFs in which the gusset plate connections are designed to ensure the out-of-plane buckling of steel braces. However, research on the cyclic behavior of gusset plate connections allowing the in-plane buckling of braces is very limited. An experimental investigation has been carried out in this study to investigate the cyclic performance of the in-plane buckling of gusset-brace assemblies. Tests showed that the gusset plate connections detailed for in-plane buckling of braces provided performance at par with those detailed for the out-of-plane deformation arrangement. A numerical comparative study on three types of connection arrangements has also been conducted, namely, a) out-of-plane buckling of braces using gusset plates, b) in-plane buckling of braces using knife plates, and c) direct connection of braces without using any gusset plates. Braces made of hollow steel sections having constant slenderness ratio and width-to-thickness ratio are used in all the numerical models. The main parameters compared are the energy dissipation capacity, displacement ductility, patterns of failure, and the sequence of yielding in the components. Both test and analysis results are used to quantify the performances of gusset plate connections in order to achieve an efficient and reliable concentrically braced frame systems.

Earthquake-Induced Collapse Risk and Loss Assessment of Steel Concentrically Braced Frames

This paper quantifies the collapse risk and earthquake-induced losses for a wide range of archetype buildings with special concentrically braced frames (SCBFs). The collapse risk and expected economic losses associated with repair, demolition and collapse are computed based on a performance-based earthquake engineering framework developed within the Pacific Earthquake Engineering Research Center. It is shown that the collapse risk of the steel SCBF archetypes may be significantly overestimated when the influence of the gravity framing system on the lateral frame strength and stiffness is ignored. It is also found that the building-specific earthquake loss assessment is significantly overestimated at low probability of occurrence seismic events (i.e., 2% probability of occurrence in 50 years) when the gravity framing system is not modeled explicitly as part of the nonlinear building model. For frequent and design-basis seismic events (i.e., 50 and 10% probability of exceedance over 50 years of building life expectancy), acceleration-sensitive nonstructural component repairs govern the building losses regardless of the employed nonlinear building model representation. For the same seismic events, steel brace flexural buckling contributes to structural repair losses.

Seismic response of concentrically braced frames with staggered braces in split-x configurations

Special concentrically braced frames (SCBFs) are used in high-seismic regions to provide adequate lateral strength, stiffness, and ductility to a structural system. Although these frames help the structure to survive the lateral seismic actions, high uplifting/settling forces in the bracing columns may, sometimes, damage the structural integrity. This detrimental effect can be mitigated by distributing the concentrated uplifting forces among the columns using braces in staggered arrangement. Thus, the primary aim of this study is to understand the seismic response of SCBFs with staggered braces arranged in split-X configurations as compared to the conventional single-stack braced frame. Six different brace arrangements are considered for a five-story steel braced frame. Nonlinear static analyses are conducted to evaluate the lateral strength, ductility, and yielding mechanism of these study frames. In addition, nonlinear dynamic analyses have been carried out to assess the inter-story drift response and the occurrence of soft-story mechanisms in the braced frames. Analysis results highlighted the performance of staggered braced frames nearly at par with the conventional single stack arrangement. Axial force demand at the column bases are also remarkably reduced in case of staggered braces. Hence, to improve the performance of staggered braced frame a revised design method was implemented which resulted in improved lateral strength, ductility, and yield mechanism.

Seismic performance of all-steel buckling-controlled braces with various cross-sections

An all-steel buckling-controlled brace (BCB) with two different configurations is studied and its behavior is compared with the conventional braces in terms of energy dissipation and ductility capacities. A parametric study was first conducted on an ensemble of all-steel BCBs in a general purpose finite-element (FE) software in order to study the influential parameters of these braces. The select types of BCBs were then experimentally investigated. Finally, seismic performance of buckling-controlled braced frames (BCBFs) was compared with that of special concentrically braced frames (SCBFs) as well as that of buckling-restrained braced frames (BRBFs). The study concludes that (1) the BCB with round-in-square tube section has stable hysteretic behavior either when thickness ratio of the outer tube to inner tube is greater than one or when an enhanced gusset plate is employed. Furthermore, due to much increased compressive strength in square in round BCBs, it is necessary to utilize an enhanced gusset plate in order to achieve ductile behavior; (2) BCBs have a stable and symmetrical hysteretic behavior in tension and compression with little post-yielding strength decrease or increase, avoiding the significant unbalanced force on the brace-intersected beams in SCBFs and BRBFs; (3) BCBFs are capable of sustaining larger story drift ratio response without considerable strength loss in comparison with SCBFs; (4) Inelastic deformation demand distributes throughout the height of BCBF floors, preventing the occurrence of weak story often observed in SCBFs.

Seismic performance of concentrically braced frames with and without brace buckling

This paper presents an analytical study on seismic performance of special concentrically braced frames (SCBFs) with and without brace buckling. It demonstrates that the collapse prevention goal needs attention in modern buildings using SCBFs as main seismic-force-resisting systems, and an simple and low-cost solution is possible to achieve this goal. Typical 6-story buildings using two-story X-braced frames with strong and weak braced-intersected beams and inverted V braced frames with and without brace buckling were subjected to a set of 15 earthquake ground motions, and resulting seismic responses were discussed in terms of seismic strength and deformation demands on braces, beams and columns. The study finds that the braces in SCBFs often fracture prior to 2% story drift ratio response, particularly in the popular two-story X-braced frames with weak beams, mainly due to premature yielding of the beams. The buckling-controlled braced frames (BCBFs) are shown to be a cost-effective system to improve the seismic performance of SCBFs. The analysis indicates that the buckling-controlled braces may substantially reduce story drift response, eliminate weak beam yielding, and prevent braces from fracturing.

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

Drift Concentration of a Three-Story Special Concentrically Braced Frame with Strongback under Earthquake Loading

In conventional special concentrically braced frame (SCBF) structures, the buckling of the braces leads to severe reduction in system strength and stiffness. Therefore soft story mechanisms followed by large permanent deformation are commonly observed in SCBF structures. The strongback system using additional structural components along the height of the building to resist local deformation is able to improve the distribution of the drift. This research conducted case studies to investigate the effects of strength and stiffness of the strongback system on the behavior of typical three-story SCBF system. The primary variables to be investigated are stiffness factor α (strongback stiffness/SCBF stiffness) and strength factor β (strongback strength/SCBF strength). We conducted nonlinear dynamic analyses to evaluate the effectiveness of α and β on structural demand parameters including maximum drift ratio and drift concentration factor (DCF). Analyses results show that although strongback system with excessively high α and β will reduce maximum drift ratio and DCF of SCBF systems, it is noneconomic. On the other hand, strongback systems with low α and β (α<0.0048 and β<0.054) have only little effects on improving structural behavior. The case studies suggested that 0.0096≤α≤0.0168 and 0.081≤β≤0.134 accounting for both efficiency and economics can be used in the design of the selected SCBF systems.

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.

Establishing and Developing the Weak-Axis Strength of Plates Subjected to Applied Loads

When a plate is subjected to applied loads and significant out-of-plane deformation, the demand on the connection may exceed that derived from calculations due to the applied loads only. Where inelastic behavior is acceptable, the intent may be to ensure ductile behavior. For plates with fillet-welded edge connections, this can be accomplished by sizing the fillet welds to develop the strength of the plate. One specific example of this arises when a brace in a special concentrically braced frame (SCBF) is subject to compression and buckles out-of-plane. Bending of the gusset plate may demand more of the gusset plate edge connection than the calculated forces that result on the gusset edge due to the brace force specified in Section F2.6c.2 of AISC 341-16 (AISC, 2016). If not accounted for in the weld size, the uncalculated weak-axis moment on the welds from out-of-plane bending of the gusset plate might cause rupture of the fillet welds to govern the behavior of the system. The method provided in this paper is suitable to determine the minimum size of fillet welds necessary to prevent weld rupture as out-of-plane deformations occur. It can be used for fillet-welded gusset plate edges in SCBFs to satisfy the exception provided in Section F2.6c.4 of AISC 341-16.

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....

Optimum range of slenderness ratio of hollow steel square braces for special concentrically braced frames

Past studies have shown that the slenderness ratio and the width-to-thickness ratio of braces are primarily responsible for achieving the desired cyclic response of special concentrically braced frames. An increase in brace slenderness ratio results in a reduction in its energy dissipation capacity along with a simultaneous increase in the ductility nearing its fracture. Since both energy dissipation capacity and ductility of braces are essential parameters in quantifying the seismic performance of special concentric braced frames, there is a need of establishing the optimum range of brace slenderness ratio and width-to-thickness ratio. Hence, an extensive finite element parametric study has been conducted on a wide range of values of these parameters on braces of hollow steel square section using a commercial software ABAQUS. The finite element models account for the inelastic hysteretic characteristics and the fracture behavior of braces. The results of simulation models matched very well with the past experimental results with respect to the performance points, namely, global buckling, local buckling, fracture initiation, complete fracture, and ductility. Finally, a relationship between the lower limit of slenderness ratio and the width-to-thickness of square braces has been established based on the simulation results.

A simplified methodology to determine damping for special concentrically-braced frames

There are two approaches most widely used in the literature to determine damping: steady-state harmonic excitation and the earthquake excitation approach. Using these approaches, different expressions have been developed to determine damping of structures based on various hysteretic models and ductility levels. In this paper, a simplified methodology is proposed for the determination of the damping of SDOF systems, in particular, special, concentrically-braced frames. This new method, based on revised effective mass, uses both of the above approaches to determine the damping. Using this method, damping is determined for special inverted V and X braced frames. Two expressions are proposed for determining the damping for special, concentrically-braced frames in terms of ductility and slenderness ratio.

Limiting values of slenderness ratio for circular braces of concentrically braced frames

Special concentrically braced frames (SCBFs) are commonly used to resist lateral forces in the structures located in high-seismic regions. Steel braces of SCBFs are expected to undergo large inelastic axial deformations in order to provide an adequate level of structural ductility and hysteretic energy dissipation under cyclic loading. The energy dissipation capacity and ductility of SCBFs largely depend on the slenderness ratio and width-to-thickness ratio of braces. The main objective of this study is to find an optimum range of these parameters for braces of hollow circular steel (HCS) sections in order to achieve the enhanced seismic performance of SCBFs. A finite element (FE) study has been conducted on a wide range of values of these parameters using a software package ABAQUS. The FE models account for the inelastic hysteretic characteristics and the fracture behaviour of braces. The results of simulation models matched very well with the past experimental results with respect to the performance points, namely, global buckling, local buckling, fracture initiation, and complete fracture. Finally, a relationship has been established between the slenderness ratio and the width-to-thickness of HCS braces based on the simulation results.

쉬어탭 공유 접합부를 갖는 비내진중심가새골조의 내진거동

Special concentrically braced frames(SCBFs) have distinctive advantages in considerable seismic performance, which make engineers widely use SCBFs as lateral-load resisting systems in buildings and have researchers to develop SCBFs design methods. Compared to the extensive research of SCBF, comparatively little information is currently available on the performance of SCBFs designed and constructed before the early 1990`s. Prior to 1988, concentrically braced frames(CBFs) design requirements were substantially less restrictive. As a result, many existing structures designed to these requirements may not ensure ductility and pose a significant concern in current buildings. In this study, these older frames are referred as non-seismic braced frames(NCBFs). In order to investigate the seismic behavior of NCBFs, finite-element(FE) models of SCBF and NCBF were suggested and verified using case investigation of NCBF conducted on the University of Washington. Using these models, the seismic behavior of NCBF with shared welding shear tab, which is the representative of the types of connections, was established and compared with the seismic performance of SCBF.

Evaluation of Seismic Behavior in Non-Seismic Concentrically Braced Frames

SCBFs (Special Concentrically Braced Frames) have been praised for considerable strength, high lateral stiffness, economic feasibility and good performance of seismic loads, which make engineers widely use SCBF as lateral-load resisting systems in buildings and have researchers develop design requirements. Compared to the extensive research of SCBF, comparatively little information is currently available on the performance of SCBFs designed and constructed before the early 1990’s. Before the early 1990’s, CBFs (Concentrically Braced Frames) design requirements were substantially less restrictive. As a result, many existing structures designed to these requirements may not exhibit desirable behavior in a seismic event. Existing CBFs that do not meet modern SCBF criteria shall be referred to herein as NCBFs (Non-Seismic Concentrically Braced Frames). In this study, FE (Finite Element) analysis was carried out. The detailed inelastic FE models were verified, which are based on the preceding research. Finally, these models were compared to evaluate the seismic performance of NCBFs.

Direct displacement-based design of special concentrically-braced frames in near-fault regions

The effect of near-fault earthquakes on the effective period of special concentrically-braced frames is studied in this paper. The effective period is one of the most important parameters in direct displacement-based design. Given the effective period, the design base shear of structure can be obtained. Since the braced frames have high stiffness, an appropriate category of near-fault earthquakes for structures with high stiffness is established in this paper and the relations are presented to determine the pulse period and peak ground velocity. Sixteen near-fault accelerograms are simulated by the proposed methodology and applied to 16 one-storey special concentrically-braced frames. The results show that the behaviour of frames subjected to earthquakes with magnitude 6. The effective period depends on pulse period, peak ground velocity, ductility and design displacement. Two expressions are suggested to determine the effective period of two types of special concentrically-braced frames (X-braced and Inverted V-braced frame) according to these parameters.

Computational Approach for Collapse Assessment of Concentrically Braced Frames in Seismic Regions

This paper proposes a computational approach for the collapse assessment of concentrically braced frames (CBFs) subjected to earthquakes. Empirical formulations for modeling the postbuckling behavior and fracture of three main steel brace shapes that are commonly used in CBFs are developed. These formulations are based on extensive calibrations of a fiber-based steel brace model with available information from a recently developed steel brace database. As part of the same computational approach, the representation of strength and stiffness deterioration associated with plastic hinging in steel columns and gusset-plate beam-to-column connections is considered. Through a case study of a 12-story Special Concentrically Braced Frame (SCBF), the influence of classical damping on the collapse capacity of CBFs is investigated. It is demonstrated that when SCBFs attain a negative stiffness during an earthquake, their collapse capacity can be significantly overestimated, if viscous damping is based on a commonly employed Rayleigh assumption with initial stiffness approximation. It is shown that sidesway collapse of CBFs should be traced based on a combination of criteria that associate large story drift ratios and the story shear resistance of a CBF at the corresponding story drift ratios. © 2014 American Society of Civil Engineers.

Special Issue on Computational Simulation in Structural Engineering

Special Issue on Computational Simulation in Structural Engineering

Seismic Design and Hybrid Tests of a Full-Scale Three-Story Concentrically Braced Frame using In-Plane Buckling Braces

This research investigates the brace-to-gusset connection designs to allow the braces buckle in the plane (IP) of the frame. In order to study the performance of the IP buckling brace connections with different design details, five 3,026 mm–long A36 H 175 × 175 × 7.5 × 11 mm braces were tested using cyclically increasing axial displacements. All specimens failed at an average axial strain less than 0.025 due to the brace fracture at the mid-length where severe local buckling had occurred. Pseudo-dynamic tests on a three-story special concentrically braced frame (SCBF) using the proposed brace end connection details for six A36 H 150 × 150 × 7 × 10 mm braces were conducted using the PGA = 597 cm/s2 LA03 record to confirm with the component tests. The knife plates and IP buckling braces sustained a peak 0.049 rad interstory drift under the design base earthquake without fracture. The highly nonlinear responses were satisfactorily predicted by OpenSees. Recommendations on the seismic design of the IP buckling brace connections are provided.