Special Concentrically Braced Frames Research Articles

Evaluation of New Seismic provisions for special concentrically braced frames in AISC 341-22

The seismic provisions for special concentrically braced frame (SCBF) had expected the properly detailed braces to provide inelastic cyclic deformation of 10 - 20 ductility without fracture during the design earthquake ground motions over 30 years prior to 2022. The new seismic provisions for SCBF in AISC 341–22 eliminates the clear and quantifiable statements on the expected capacity and demand of the braces, and indicates that the diagonal braces can sustain large inelastic cyclic deformations without experiencing premature failures if they are properly detailed for ductility as prescribed in the provisions. The appearance of such word as “large inelastic deformation” and “premature failure” causes a concern about the uncertainty. One might wonder why a range of amount of inelastic cyclic deformation capacity for a properly detailed brace cannot be defined after more than 40 years of extensive research on SCBF. This paper presents a study using available data accumulated over more than 40 years to investigate how much inelastic cyclic deformations are needed for braces to avoid fracture under the design earthquake, and concludes that the SCBFs with properly detailed square HSS have insufficient inelastic cyclic deformations to avoid experiencing premature failures that lead to structural collapse under the design earthquake.

Inelastic Deformation and Local Slenderness Requirements for Rectangular HSS Braces

Rectangular hollow structural sections (HSS) are commonly used as bracing in concentrically braced frames (CBFs) designed and detailed for seismic design requirements. As the primary yielding component in CBFs, braces are expected to sustain large inelastic axial deformation during earthquake loading. It is well known that their deformation capacity depends on the width-to-thickness ratio (local slenderness). Until 2013, HSS sections were produced to meet ASTM standard A500/A500M; after 2013, the ASTM 1085 specification was implemented, and since that time, HSS sections have also been produced to meet ASTM A1085/A1085M standards. Where the ASTM A500/A500M specification requires minimum yield and tensile stress values as well as a minimum elongation and tolerances on the wall thickness, the ASTM A1085/A1085M specification also requires a minimum Charpy V-notch (CVN) toughness, a maximum yield stress of the steel, and tighter tolerances on the wall thickness and radius of curvature of the corner. These requirements offer a more reliable brace for CBFs in seismic regions. Yet there has been limited research investigating the cyclic axial response of these members. A research study was undertaken to investigate the response of ASTM A1085/A1085M tubes using the response of ASTM A500C tubes as their reference. Forty-one brace specimens were tested under cyclic inelastic axial deformation. Comparison of the data shows that most of the ASTM A500/A500 M Grade C and ASTM A1085/1085M braces meet the respective requirements of their respective ASTM standard and that the differences between the performance of ASTM A500/A500M and ASTM A1085/1085M braces are not dramatic. In addition, the study investigated the impact of width-to-thickness ratio, global slenderness, and displacement history on the response of the braces. The data show that the current AISC 341-22 high-ductility slenderness limit for special concentrically braced frame (SCBF) braces is slightly conservative. However, the data suggest that the moderate ductility slenderness limit used for ordinary concentrically braced frame (OCBF) braces is significantly more conservative than required for consistent seismic safety. Further research is required to determine appropriate limits; this paper provides some initial recommendations based on this dataset.

A new friction–slip brace damper to improve seismic performance of braced frames

Global buckling in compression braces results in compression strength degradation, compression stiffness degradation, brace local buckling, and finally brace fracture. Consequently, the braces cannot achieve high energy dissipation capacity. The current study proposes a novel friction−slip damper employed in the middle of square−Hollow Structural Section (square−HSS) and H−shaped braces to prevent the strength and stiffness degradation of braces and improve their seismic responses. In these braces, slip prevents the global buckling of brace and friction dissipates a large portion of energy. To assess the performance of proposed damper, the hysteresis behaviors of square−HSS and H−shaped braces with the proposed friction−slip damper were compared with those of conventional braces both in bracing elements and in diagonal and chevron Special Concentrically Braced Frames (SCBFs) through modeling the numerical models in ABAQUS finite element software validated with two experimental studies. The study findings showed that in the models equipped with the friction−slip damper, the braces experienced no global buckling, local buckling, yielding, plastic hinge, damage, and fracture. Additionally, the obtained results indicated that no degradation was observed in the strength and stiffness of the models with the friction−slip damper than the conventional models. Consequently, a 186.3% increase in energy dissipation capacity was experienced by installing the proposed damper in the middle of braces. Moreover, the results showed that the beams of chevron SCBFs with the friction−slip damper experienced no unbalanced force and thus, no deflection resulting in the stiffness and strength degradation in the tension and compression braces was observed in the chevron beams.

Experimental validation of locally annealed braces towards ductile and efficient concentrically braced frames

In special concentrically braced frames (SCBFs), brace yielding and buckling are the preferred mechanisms of seismic energy dissipation, and mid-length brace fracture is the preferred failure mode. However, due in part to material overstrength and limited ductility typical of cold formed square steel braces commonly used in the USA, achieving the desired seismic performance requires careful attention to detailing which affects the perceived efficiency of this system. This paper presents the results of experimental testing to validate a recently proposed simple ductile fuse to improve the efficiency and ductility of SCBFs. The fuse is created by selectively reducing the brace material strength via annealing the steel. This in turn reduces connection force demands, increases brace material work hardening rate leading to improved brace ductility, and improves normalized brace compressive strength due to residual stress relief. Detailed material testing on ASTM A500 steel from square HSS tubing was conducted to characterize the mechanical property changes from the annealing process and FE parametric studies were conducted to study how changes in material properties affect brace ductility. Finally, the results of 8 full-scale brace tests and detailed post-test FE analysis using calibrated constitutive models are presented to validate the proposed concept. Based on the results of the study, preliminary design recommendations are proposed, and future research needs are identified and discussed.

Earthquake-induced loss assessment of steel dual concentrically braced structures subjected to near-field ground motions

Steel structures with dual systems, including special moment-resisting frames (SMRFs) and special concentrically braced frames (SCBFs), have found extensive applications in seismically active regions due to their considerable lateral stiffness and ductility. However, the seismic performance of such system has not been investigated subjected to near-field ground motion records. To address this research gap, this paper evaluates collapse risk and earthquake-induced loss of multi-story steel structures with dual concentrically braced frames. A comprehensive seismic performance assessment is conducted on selected five-, 10-, and 15-story structures subjected to near-field ground motions (NF-GMs) with and without pulses based on the methodology provided in FEMA P-58. The results are also used to assess the effect of NF-GMs with pulses on the seismic performance measures, such as life-cycle cost and repair time of the studied structures at different seismic intensity levels. Finally, the contribution rate of collapse and demolition of structures, and contribution of structural and nonstructural members in the repair cost and time of the steel structures are estimated and compared subjected to NF-GMs. It is concluded that collapse and demolition loss play significant roles for NF-GMs with and without pulses, respectively, while the seismic collapse behavior is governed by the ratio of the pulse period of the GM to the period of the first mode of the structure.

Quantification of optimal target reliability for seismic design: Methodology and application to steel buildings

The reliability levels against which seismic standards are calibrated are currently quantified based on accepted practice, which may not lead to optimal designs that minimize the lifecycle cost. Deviation from optimal design is even more pronounced in countries whose regulations are adopted from the few leading codes. A major challenge in the quantification of optimal target reliability for the countries with such adopted codes is the lack of risk models that are suited for the local construction industry and design practices. This paper presents a holistic risk-based framework to quantify the optimal target reliability for seismic design through the notion of minimum lifecycle cost. To enable the implementation of the framework in countries with adopted provisions, the present paper proposes an approach to adapt the available risk models to the local conditions of those countries. The proposed framework is showcased through the National Building Code of Iran, which is adopted from the codes of the United States, using a case study of five residential steel building archetypes. The archetypes have various structural systems and comprise intermediate moment-resisting frame (IMF) buildings of various heights and special concentrically braced frame (SCBF) buildings. The results reveal 50-year optimal target reliability indices of 2.5, 2.0, and 1.9 for low-, mid-, and high-rise IMF buildings and 2.1 for mid-rise SCBF buildings.

The Analysis of Mechanical Behavior of Special Concentrically Braced Single-Story Frames With Ductile Casting Steel Connectors

Background: The special concentrically braced frame (SCBF) is an aseismic structure, but its bracing system exhibits brittle failure and premature buckling connected with the weld fracture of the gusset plate and the post-buckling of the braces; thus, maximizing the role of energy dissipation is difficult. Methods: Here, this paper proposes a system of special concentrically braced single-story frames with ductile casting steel connectors. The large inelastic deformation of the bracing system is mainly concentrated in the ductile casting connectors under the earthquake, and the degree of buckling can be reduced. The finite element model of a single-story frame with special central support and casting steel connectors was established by ABAQUS. Monodirectional loading simulations were conducted on 12 groups of specimens with different parameters, then the deformation trend, stress distribution, initial stiffness, and ductility of the specimens were analyzed. Results: The seismic behavior of the frames was evaluated by the variable parameters of the width-thickness ratio of the energy dissipation plate and stiffener, the length of the energy dissipation segment of the casting connectors and the overstrength coefficient. The casting steel connector conducts the main connectors that exhibit good ductile and initial stiffness. Conclusion: The study results provided direct evidence that the seismic performance of specimens is closely associated with the length of the energy dissipation segment of the casting connector and the overstrength coefficient of axial force. Also, the stress distribution, initial stiffness, and ductility of the members can be independently controlled by reasonably designing the casting connector.

Experimental evaluation of composite and non‐composite columns and braces in special concentrically braced frames

SummaryHollow structural section (HSS) and concrete‐filled tube (CFT) cross‐sections have been widely employed in the columns and braces of special concentrically braced frames (SCBFs). Square‐HSS cross‐section widely used in multistory frames is filled with concrete and converted to square‐CFT cross‐section to enhance the behavior of this cross‐section. However, some investigations indicated that circular‐HSS cross‐section filled with concrete (circular‐CFT) showed better behavior in comparison with square‐CFT cross‐section due to more uniform and larger concrete confinement in circular‐CFT cross‐section. The current study was experimentally undertaken to evaluate (1) the seismic performance and the global and local hysteresis responses of HSS and CFT members with various cross‐section shapes from initial elastic range to collapse in the system level of multistory SCBFs, (2) the behavioral differences between square cross‐section and circular cross‐section, and (3) the behavioral differences between HSS cross‐sections and CFT cross‐sections employed in the columns and braces of SCBFs. Four full‐scale one‐bay, two‐story SCBFs with four various cross‐sections, namely, square‐HSS, circular‐HSS, square‐CFT, and circular‐CFT, for columns and braces were subjected to cyclic lateral loading. Evaluating base shear–roof drift hysteretic loops of SCBF specimens demonstrated that SCBF specimens with CFT columns and braces (CFT‐SCBFs) experienced respectively around 107%, 58%, 28%, and 152% higher stiffness, post‐yielding and post‐buckling strengths, ductility, and energy dissipation capacity than SCBF specimens with HSS columns and braces (HSS‐SCBF). In addition, the experimental observations indicated that CFT braces experienced local buckling initiation, crack initiation, and fracture at respectively 2.22, 2.35, and 2.32 times of roof drifts of those exhibited by HSS braces. Moreover, assessing braces with various cross‐sections indicated that CFT braces showed an increase in compression strength, post‐buckling strength, compression axial deformation, and out‐of‐plane buckling approximately by 83%, 152%, 127%, and 100%, respectively, in comparison with HSS braces. Finally, square‐HSS/CFT braces sustained rupture propagation better than circular‐HSS/CFT braces.

Impact of unique practice in concentrically braced frames on their seismic performance

The seismic provisions for special concentrically braced frame (SCBF) expect the braces capable of providing deformation of 10–20 ductility without fracture in order to withstand design earthquake ground motions. Seismic evaluation studies of the design provisions for SCBF, however, are substantially different from those for any other seismic-force-resisting system (SFRS) in three aspects: (a) the original design provision for a SCBF set a b/t ratio limit on braces to prevent fracture, but such b/t ratio limit is proven incapable of preventing brace fracture; (b) SCBF is the only SFRS in which the brace fracture termination of energy-dissipation mechanism is considered normal instead of the limit state of seismic design; and (c) the collapse probability evaluation and development of design parameters (R, Ω0, Cd) are formulated explicitly including potential but uncertain overstrength factors inherently existing in typical steel buildings. This paper describes a comprehensive inquiry into the origin and evolution of the unique practice in SCBF over a span of 30 years (1992–2022) and its impact on seismic performance and potential hazard of steel buildings with SCBF as a designated SFRS in steel buildings. The important conclusions include (a) seismic braces have fracture ductility range of 5–10; (b) the seismic design provisions and performance evaluation on SCBF have been based on post brace-fracture analysis of buildings with SCBF that is extremely unreliable and often overly optimistic; and (c) The unique seismic design practice of SCBF has a significant impact on seismic performance of steel buildings with SCBF.

Comparative seismic behavior assessment of a new damper-equipped and conventional chevron-braced frames

This paper presents the results of experimental studies performed on three 1/3-scale steel building frames for comparative assessment of the cyclic behavior of a Chevron frame equipped with a newly-developed damper, THDF (Torsional Hysteretic Damper for Frames) in relation to a Chevron special concentrically braced frame (SCBF) and an eccentrically braced frame with a vertical H-link. THDF is a damper with a displacement-dependent post-elastic stiffness, developed for Chevron frames. First, the SCBF was designed as a full-scale frame according to the requirements of AISC 360–16 and AISC 341–16, and scaled down to 1/3-scale according to similitude requirements due to limitations of the testing facilities. The other two frames are designed as scaled frames by adjusting their braces and energy dissipation elements so that the frames have similar levels of yield force. The frames are subjected to quasi-static cyclic tests of increasing amplitude until failure. In the case of SCBF and H-link frames, the force levels observed in the tests were higher than predicted due to oversight in the fabrication of the test specimens, which may not be the case in practice. In the case of the H-link frame, this led to early failure of the connection before the full plasticization of H-link. SCBF was tested up to low-cycle fatigue failure of both braces. The THDF-equipped frame showed stable force-displacement loops with little variation in force levels and a better distribution of energy among the stories.

Effect of modeling of inherent damping on the response and collapse performance of seismically isolated buildings

AbstractThis paper investigates the effect of inherent damping modeling on the computed seismic response and collapse performance of selected seismically isolated buildings. The analyzed seismically isolated buildings were designed by the procedures of the ASCE/SEI 7–16 standard. The structure is a six‐story perimeter frame building designed with special moment resisting frames or with special concentrically braced frames (SCBF) for a location in California. Three different seismic isolation systems are considered: (i) triple friction pendulum (TFP) bearings without moat walls, (ii) TFP bearings with moat walls (double concave [DC] friction pendulum bearings with moat walls have effectively the same ultimate behavior), and (iii) DC friction pendulum bearings without moat wall. The superstructure inherent damping schemes considered are (i) zero damping, (ii) modal damping, (iii) Zareian‐Medina damping, (iv) added virtual viscous dampers with and without force‐caps, and (v) added virtual viscous dampers with the same damping constant value. The response parameters computed are peak floor accelerations, peak story drift ratios, peak residual story drift ratios, peak isolator horizontal displacement, and floor acceleration spectra. Also, the probability of collapse in the maximum considered earthquake (MCER) is computed. It is shown that the modeling approach for the inherent damping has minor effects on the computed responses and the collapse probability of the studied seismically isolated buildings, except for the peak floor acceleration and the floor response spectra for periods below one second. It is suggested that a convenient way to model inherent damping is to use virtual viscous dampers with all having the same damping constant.

Evaluation of seismic performance factors for dual steel SMF-SCBF systems using FEMA P695 methodology

The present study evaluates the seismic performance factors of typical buildings with dual lateral force resisting system (LFRS) consisting of special moment frames (SMFs) combined with special concentrically braced frames (SCBFs) using FEMA P695 methodology. For this purpose, six building archetypes with 2-, 4-, 8-, 12-, 16- and 20-stories, in two short- and long-period performance groups were analyzed by the response spectrum method and designed based on the requirements of ASCE 7–16 and AISC 341–16. Validated nonlinear models were developed for each building in the OpenSees software platform. Then, the static and dynamic behavior of each building was investigated up to the collapse stage by nonlinear static analyses and nonlinear time history analyses under 44 far-field ground motions of FEMA P695. In this investigation, the peak and residual drift ratios, hysteresis behavior of the stories during the earthquake, collapse fragility curves, and seismic performance parameters (R, Ω, and Cd) were obtained and discussed. The results of nonlinear analyses demonstrated that code-compliant long period dual SMF-SCBFs (16 stories or higher) performed poorly and can be severely damaged in strong earthquakes.

Determination of Seismic Performance Factors for Buildings with Concentrically Braced Frame Systems under the Excitation of Near- and Far-fault Records

Performance coefficients are widely used in seismic design codes to achieve performance objectives. The values of these coefficients have significant importance in achieving pre-specified performance goals. Studies have shown that near-fault earthquakes decrease the ductility and increase the risk of failure in the structures; however, the current codes use the same performance coefficients to design structures against near- and far-fault records. In the present study, 1-, 5-, 10- and 15-story special concentrically braced frame (SCBF) structures designed in the region with high seismic hazard have been evaluated. Non-linear static, linear dynamic, and incremental non-linear dynamic analyses under the influence of two sets of near- and far-fault records extracted from FEMA-P695 have been used to calculate the performance coefficients. Furthermore, the fragility curves are calculated for three performance levels (IO, LS, CP) using a probabilistic assessment of the results derived from incremental dynamic analysis to investigate the relationship between obtained factors with the probability of exceedance from a specified level. According to the mean results of all records, the behavior factor for the steel special concentrically braced frame is 5.92. The mean behavior factor obtained for the near-fault records is 35% less than the far-fault records. Differences in the obtained behavior factor for structures under excitation of two types of earthquake records (near- and far-fault) are observed in the fragility curves related to the probability of exceedance from CP level. However, there is no significant correlation between the resulted behavior factors and the probability of exceeding IO and LS levels.

Demands on acceleration-sensitive nonstructural components in special concentrically braced frame and special moment frame buildings

This study investigates demands on acceleration-sensitive nonstructural components (NSCs) in code-compliant Special Concentrically Braced Frame (SCBF) and Special Moment Frame (SMF) structures. Previous studies have highlighted that the assumption of ASCE 7-16 for the ratio of Peak Floor Acceleration (PFA) to Peak Ground Acceleration (PGA) is overly conservative. In most previous studies, the results were based on generic analytical models with non-deteriorating structural behavior or recorded data from instrumented buildings subjected to ground motions that were significantly weaker than the design-level earthquakes. This study evaluates the acceleration demands on NSCs in 11 archetype buildings modeled using advanced techniques. The ground motions used elicit inelastic response in the frames at the level expected by ASCE 7-16 compliant designs. The study examines the effect of the ratio of NSC period to structural period, NSC damping ratio, and NSC vertical location on these demands. Results of this investigation are compared with the provisions of ASCE 7-16 and the recommendations of the recently completed effort by the Applied Technology Council 120 project (ATC-120), which are the basis for the seismic design requirements for nonstructural components in the forthcoming ASCE 7-22 standard. Equations for the PFA/PGA ratio and NSC amplification factor, which account for the dynamic characteristics of the buildings, are developed based on regression analysis of the generated data. Results of the analyses indicate that the formula recommended by ATC-120 for estimating PFA/PGA along the height of the structure, which includes building nonlinearity effects through a constant building ductility reduction factor, overestimates floor accelerations at the top floors but underestimates them in the lower floors of the archetypes considered. At the same time, results show that the NSC seismic design force equation recommended by ATC-120 is conservative for the SCBF archetypes, while it underestimates the NSC force demands in the SMFs.

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.

Seismic response of dual concentrically braced steel frames with various bracing configurations

Steel dual structures incorporating special concentrically braced frames (SCBFs) and special moment frames (SMFs) are, in general, effective for their high lateral stiffness and ductility. As such, ductile dual CBFs are often treated as a remedy for mitigating permanent drift demands along with the potential drift concentration in isolated stories. However, despite the benefits and interest, the effect of the bracing configuration on the seismic response of the dual concentrically braced systems, designed following the current capacity design approach stipulated in AISC Seismic Provisions, in particular, has not been thoroughly examined to date. The present study aims to shed light on the impact of the bracing scheme on the seismic behavior of dual SCBFs coupled with SMFs as backup frames. For this purpose, 4- and 10-story dual SCBFs with the commonly used chevron, split X, V, and cross X type bracing configurations are designed according to ASCE 7 and AISC 341. Then, the frames are subjected to a suite of ground motions. The nonlinear time-history analysis results are contrasted and discussed in terms of the peak and permanent drift demands, horizontal floor acceleration demands along with brace ductility demands.

Full-scale tests on two-story SCBFs with Through–Gusset Plate in brace-to-CFT column connections

In Special Concentrically Braced Frames (SCBFs) with Concrete-Filled Tube (CFT) columns, brace force must be transferred through beam–brace–column joint and thereafter distributed between steel tube and concrete infill of column. In the common practice, gusset plates are directly welded to the steel tube of CFT columns where the majority of force is transferred to only a part of steel tube placed adjacent to the gusset plate resulting in distortion and fracture. Accordingly, Through–Gusset Plate (TGP) passing through CFT column is proposed for connecting brace to CFT column and distributing the brace force between steel tube and concrete infill. Moreover, the gusset plates in the braced frames may encounter pinching forces due to frame action. The current study resolves this problem through proposing a new method for connecting the beam to the gusset plate. In addition, beam–to–column connections in SCBFs tolerate large rotation demands due to frame action. Consequently, fracture and damage may occur at beam–to–column connections. This research considers moment connections between the beam and the column using a Through–Plate (TP) to evaluate their effects on the behavior of SCBFs. The cyclic performance of the proposed connections is experimentally assessed through full-scale tests conducted on two one-bay, two-story SCBFs with CFT columns and braces. The results showed desirable behavior of TGP connections that remained undamaged. The proposed method for beam-to-gusset plate connections eliminated the pinching force on the gusset plates. In addition, beam-to-column moment connections using TPs resulted in delaying brace fracture and strength degradation.

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.

Seismic Collapse Risk Assessment of Braced Frames under Near-Fault Earthquakes

Special concentrically braced frames (SCBFs) located in regions close to earthquake faults may be subjected to near-fault ground motions, often characterized by pulses with long periods. These near-fault pulses could impose additional seismic demands on structures and increase the risk for structural collapse. Currently, there is limited research on the seismic collapse risk of SCBFs under near-fault earthquakes. This paper uses a general simulation-based framework to assess the seismic collapse risk of SCBFs under near-fault earthquakes. To quantify the large variability and uncertainty associated with the seismic hazard, a stochastic ground motion (SGM) model is used where the near-fault pulse characteristics are explicitly incorporated. The uncertainties in the SGM model parameters (including the near-fault pulse characteristics) are addressed through appropriate selection of probability distribution functions. To accurately predict the occurrence of collapse, numerical models capable of capturing the nonlinear and collapse behavior are established and used. Efficient stochastic simulation approaches are proposed to estimate the seismic collapse risk with or without considering the near-fault pulse. As an illustration, the seismic collapse risks of two SCBFs are investigated and compared. Probabilistic sensitivity analysis is also carried out to investigate the importance of uncertain model parameters within the SGM towards the seismic collapse risk.

SEISMIC COLUMN SPLICE DEMANDS IN BRACED FRAMES WITH BUCKLING CONTROLLED BRACES

Numerous experimental tests showed that ductile square hollow structural sections (HSS) are susceptible to premature fracture under design-level earthquake ground motions. In addition, recent research has highlighted the vulnerability of column splices to reach their full strength once ductile square HSS braces experience a fracture in braced frames. In this study, the performance of column splices in 5-and 13-story special concentrically braced frames (SCBFs), with X-bracing configuration, located at a distance permitted by the seismic design provisions (i.e., four feet over the concrete floor level), is evaluated under a large number of ground motion excitations. The performance of identical SCBFs with conventional HSS braces and innovative buckling controlled HSS braces is compared. Overall, results from this research indicate that the prevention of brace buckling in SCBFs yields a substantial reduction in the seismically induced bending demands on the column splices, leading to a large margin of safety to remain elastic.