Seismic Force-resisting Systems Research Articles

Effect of Preliminary Selection of RC Shear Walls\u2019 Ductility Level on Material Quantities

According to the National Building Code of Canada, the seismic force resisting systems (SFRS) of reinforced concrete (RC) buildings are classified based on their ductility level as being ductile, moderately ductile and conventional construction systems. The selection of the ductility level of an SFRS at the conceptual design phase is primarily governed by the seismicity at the building location, the building dynamic characteristics, and the height limitations specified by the design code. The selected ductility level affects the design loads, the cross-sections and reinforcement of the SFRS components, and hence the overall construction cost. This paper aims to evaluate the effect of the wall’s selected ductility level on the quantities of its constituent materials as well as the rebar detailing. Four multi-storey RC shear wall buildings with different heights located in three different cities in Canada; Toronto, Montreal, and Vancouver, were selected to represent three different seismic hazard zones (low, medium, and high). For each building height and location, the walls were designed using the dynamic analysis procedure of the National Building Code of Canada to reach different ductility levels. The construction material quantity estimates were evaluated and compared to a reference case for each building height, seismic hazard and ductility level. The effect of ductility level on the bars detailing is also investigated. This paper helps the structural engineers to select the cost-effective and constructible RC shear wall system at the conceptual design phase before reaching the detailed design phase.

Applying the Cyclic Void Growth Model to Assess the Ultralow Cycle Fatigue Life of Steel Castings

Steel castings have become an attractive option in the development of new seismic force-resisting systems because of their geometric freedom and ability to control material properties. Despite, in most past applications, being used as elements that remain elastic during seismic events, recent research has led to the development and validation of new systems that rely on their yielding response. As such, further study of their ultralow cycle fatigue (ULCF) life is essential to understanding their performance as yielding fuses. Fatigue laws are calibrated herein for multiple heats of cast steel using an existing law proposed for rolled steel. These laws are used to predict the ductile failure of steel castings in full-scale experiments. The concept of a characteristic cyclic coupon test to assess the castings’ ULCF life at the foundry is also discussed. It is found that the ULCF model provides good predictions of the onset of failure in full-scale steel castings. Finally, the initial investigation into relating the full-scale ULCF life to the steel microstructure and small-scale fatigue test results has shown promising results.

Assessment of design parameters influencing seismic collapse performance of buckling-restrained braced frames

Buckling Restrained Braced Frames (BRBFs) are widely used as seismic force-resisting systems due to their significant ductility and energy dissipation capacity. However, owing to their modest overstrength and relatively low post-yield stiffness, BRBFs subjected to seismic loading may be susceptible to concentrations of story drift and global instability triggered by P-∆ effects. Due to the use of simplistic methods that are based on elastic stability, current code-based design provisions do not address seismic stability rigorously and do not consider the unique inelastic characteristics of different systems. Supplemental strategies may be used to prevent undesirable seismic response of BRBFs, such as story drift concentration and large residual drift. This study employed the FEMA P695 framework to evaluate the response of BRBFs designed according to current codes in the United States and to study the effect on seismic stability of three additional parameters: BRBF column orientation, gravity column continuity, and dual systems. Results from nonlinear static and dynamic analyses provide insight into seismic behavior, and collapse performance evaluation quantifies the relative performance of various BRBF designs and supplemental strategies for enhancing seismic stability.

Behavior of cross-laminated timber diaphragm connections with self-tapping screws

Monotonic and cyclic tests were carried out to determine strength and stiffness characteristics of 2.44 m (8 ft) long shear connections with 8 mm and 10 mm diameter self-tapping screws. The goal of this research is to compare test values of cross-laminated timber (CLT) diaphragm connections in seismic force-resisting systems to the design values calculated from formulas in the National Design Specification for Wood Construction (USA) and the Eurocode. Understanding and quantifying the behavior of these shear connections will provide structural engineers with increased confidence in designing these components, especially with regard to the seismic force-resisting systems. Ratios of the experimental yield strength (from the yield point on the load-deflection curve) to factored design strength were in the range of 2.1–6.1. In the ASCE 41-13 acceptance criteria analysis, the m-factors for the Life Safety performance level in cyclic tests ranged from 1.6 to 1.8 for surface spline connections and from 0.9 to 1.7 for cyclic half-lap connections. The half-lap connections with a unique combination of angled and vertical screws performed exceptionally well with both high, linear elastic initial stiffness and ductile, post-peak behavior.

Seismic Performance Evaluation of Steel Ordinary Moment Frames

Steel ordinary moment frames (OMF) are seismic force-resisting systems that can be used in buildings. In current seismic design and detailing provisions, such as the American Society of Civil Engineers ASCE/SEI 7-10 (2010) , American Institute of Steel Construction ANSI/AISC 341-10 (2010), and ANSI/AISC 358-10 (2010) , less stringent design and detailing requirements are specified for steel OMFs compared with those for steel special- and intermediate-moment frames. The strong-column weak-beam (SC/WB) requirement is not enforced for steel OMF connections. In the present study, the seismic performance evaluation is conducted for steel OMFs designed according to current seismic design and detailing provisions considering different combinations of gravity, seismic, and wind loads, as well as wind drift limits. Based on the results of seismic performance evaluation, permissible structural heights for steel OMFs are also proposed.

Seismic risk assessment of reinforced masonry structural wall systems using multivariate data analysis

In contrast to the single design objective associated with force-based design approaches of different seismic force resisting systems (SFRSs), performance-based seismic design (PBSD) allows the selection of more than one design/performance objective. Each performance objective is linked to a tolerable risk level associated with the considered SFRS experiencing a specific damage at a specified seismic hazard level (presented in the form of fragility curves). Similar to other SFRSs, damage of reinforced masonry structural wall (RMSW) SFRSs can be linked to their lateral displacements/drifts. As such, this study focuses on using a multivariate data analysis techniques, to develop a RMSW load-displacement (backbone) model. The backbone model is calibrated using a database of RMSW experimental test results, and the model is further utilized to investigate the influence of the wall geometrical and mechanical characteristics on altering the displacement of RMSW corresponding to different performance levels. These analysis results are subsequently used to develop RMSW seismic fragility bands to facilitate visualizing the risk (losses) associated with the resulting range of displacement (drift) demands, when the different RMSW ranges of characteristics are considered. The developed fragility bands are compared to the individual fragility curves currently adopted by the Federal Emergency Management Agency, FEMA P-58 pre-standards, Seismic Performance Assessment of Buildings. This comparison shows that the developed fragility bands significantly deviate from the individual fragility curves currently adopted by FEMA P-58 at each damage state. The developed backbone model and fragility-bands are expected to not only facilitate adoption of the RMSW SFRSs in the next generation of PBSD codes, but also to equip researchers and designers with a clear understanding of the different aspects governing RMSW systems seismic performance and the associated risk.

Verification of the Seismic Performance of a Rigidly Connected Modular System Depending on the Shape and Size of the Ceiling Bracket.

Modular systems have been mostly researched in relatively low-rise structures but, lately, their applications to mid- to high-rise structures began to be reviewed, and research interest in new modularization subjects has increased. The application of modular systems to mid- to high-rise structures requires the structural stability of the frame and connections that consist of units, and the evaluation of the stiffness of structures that are combined in units. However, the combination of general units causes loss of the cross-section of columns or beams, resulting in low seismic performance and hindering installation works in the field. In addition, the evaluation of a frame considering such a cross-sectional loss is not easy. Therefore, it is necessary to develop a joint that is stable and easy to install. In the study, a rigidly connected modular system was proposed as a moment-resisting frame for a unit modular system, and their joints were developed and their performances were compared. The proposed system changed the ceiling beam into a bracket type to fasten bolts. It can be merged with other seismic force-resisting systems. To verify the seismic performance of the proposed system, a cyclic loading test was conducted, and the rigidly connected joint performance and integrated behavior at the joint of modular units were investigated. From the experimental results, the maximum resisting force of the proposed connection exceeded the theoretical parameters, indicating that a rigid joint structural performance could be secured.

Influential properties of hysteretic energy dissipating devices on collapse capacities of frames

Various advantages of hysteretic energy dissipating devices (HEDDs) have been recognized and their applications are getting popular. In parallel with this, current seismic codes prescribe a design procedure for structures employing HEDDs. According to one of seismic codes, ASCE/SEI 7, the minimum seismic base shear resisted by its seismic force-resisting systems (SFRSs) shall not be <75% of the total seismic base shear which is carried by a structure. This minimum base shear requirement for SFRSs is implicitly intended to provide safety in the event of malfunction of HEDDs. Although there are rigorous requirements for damped structures, further studies are still required to investigation of the collapse capacities of damped structures which have increasingly paid attention to earthquake engineering communities. In order to address this, this study carried out incremental dynamic analyses to evaluate the collapse performance of the 3- and 6-story prototype steel special moment-resisting frames (SMRFs) with and without HEDDs of which the strength capacities are different. The analysis results show that the collapse margin ratios of the prototype frames depend on the amount of base shear carried by their HEDDs and uncertainties on design requirements. The prototype structures with larger seismic base shear carried by HEDDs result in small variation in collapse capacity distribution and satisfy the minimum collapse margin ratio regardless of uncertainties on design requirements.

Experimental Evaluation of the System-Level Seismic Performance and Robustness of an Asymmetrical Reinforced Concrete Block Building

AbstractIn recent years, research interests in studying the response of different seismic force-resisting systems have been shifting from component-level to system-level studies. Building on the existing knowledge of component-level performance of reinforced masonry shear walls (RMSW), the current study evaluates some similarities and discrepancies between RMSW system-level and component-level responses under seismic loading. The study also focuses on evaluating the system-level seismic robustness of a RMSW building by quantifying key relevant robustness indicators proposed in the literature. To meet the study objectives, an experimental asymmetrical two-story reduced-scale RMSW building was tested to failure under simulated seismic loading. Subsequently, the study first presents a brief summary of the experimental program followed by a discussion of the damage sequence and the load-displacement hysteretic behavior of the RMSW building. In general, the experimental results demonstrated the impact of both ...

Evaluation of Seismic Performance Factors in High Rise Steel Buildings with Dual Lateral Systems Consisting of Buckling Restrained Braced Frames and Intermediate Moment Frames

Dual lateral systems are commonly used in high rise buildings for various architectural reasons. Buckling Restrained Braced Frame (BRBF) is an emerging seismic force-resisting system that is currently being used either as a single seismic force-resisting system or in combination with other seismic force resisting systems. American Society of Civil Engineers (ASCE) provides distinct directions regarding the estimation of Seismic Performance Factors such as, Seismic Response Modification Factor (R), Over Strength Factor (Ωo) and Deflection Amplification Factor (Cd) when Eccentrically Braced Frames (EBF) and Special Moment Resisting Frames (SMRF) are used in combination. But ASCE gives no clear suggestion about what should be the values for R, Ωo and Cd when stiffer BRBF systems are used in conjunction with softer Intermediate Moment Frame (IMF) Systems. Since, these seismic parameters (R, Ωo and Cd) vary significantly for BRBF systems in comparison with IMF systems, it is essential to estimate and evaluate the correct values for them. In conventional practice, ASCE suggests that when a stiffer lateral system is used in conjunction with a softer system in a dual configuration, the lowest Response Modification Factor (R) pertaining to the softer system shall be used.

Performance Evaluation of Seismic Force–Resisting Systems for Low-Rise Steel Buildings in Canada

Steel is one of the most popular seismic force–resisting systems (SFRS) in use worldwide. In Canada, several SFRS have been prequalified for use in the national and provincial building codes. The design of each SFRS has been covered comprehensively in literature. However, no guidance has been provided in selecting the optimum system for a project. In this paper, a prototype building located in Vancouver, Canada, was designed nine times to utilize each of the prequalified SFRS. Detailed seismic hazard and finite element models were developed for each system. The performance in terms of initial construction and life-cycle cost was used to rank each SFRS. The result of this analysis shows that the eccentrically braced configuration has the lowest material usage and life cycle maintenance cost; it is therefore the most economic system in this study. The presented methodology is transparent and can be easily adopted by engineers to select the most economic seismic system for projects with different configurations and geometries than those given in this research. Furthermore, this system introduces a metric with which to estimate the life-cycle costs of a structure taking into account seismic damage over the service life.

QUANTIFYING GLOBAL SEISMIC PERFORMANCE FACTORS IN DUAL SYSTEMS WITH BUCKLING RESTRAINED BRACED FRAMES

Dual structural systems are commonly used in high rise buildings for various architectural reasons. Buckling Restrained Braced Frame (BRBF) is an emerging seismic force-resisting system that is currently being permitted by American Society of Civil Engineers (ASCE) to be used either as a single seismic force-resisting system or in combination with other seismic force resisting systems. In conventional practice, ASCE suggests that while using BRBF in conjunction with other lateral force resisting systems in a dual configuration, the lowest Response Modification Factor (R) pertaining to the softer system shall be used. This may result in significant overdesigning of structures as higher contribution from the BRBF system are often remain unutilized. This research aims at developing a methodology for calculating modified Response Modification Factor, R for structures where dual system occurs horizontally. This research investigates the effect of using the newly suggested Response Modification Factor (R) for dual systems, where a BRBF system is combined with an Intermediate Moment Frame (IMF). The study aims at proposing an innovative way of calculating Response Modification Coefficient (R), Over-strength Factor (Ωo) and Deflection Amplification Factor (Cd) pertaining to the dual system. A wide variety of archetype sets are designed following FEMA guidelines with modified R as trial values for different seismic zones. To validate the trial values for R, system over-strength and period-based ductility, nonlinear 3D static (pushover) analyses were performed. The nonlinear models directly simulate essential deterioration modes that contribute to collapse behavior. Afterwards, for collapse assessment, nonlinear incremental dynamic analyses are conducted.

Experimental evaluation of the seismic performance of reinforced concrete structural walls with different end configurations

The Canterbury Earthquake Royal Commission report (2013) showed that cantilever reinforced concrete (RC) walls failed at a lower ductility capacity than expected due to a plasticity concentration region within a very limited height near the location of the primary cracks at the base of the walls. The New Zealand Standards (NZS 3101) (2006) [2] and the Canadian design standards (CSA A23.3-14) (2014), adopt the same capacity design approaches for RC walls design, with both standards specifying a minimum vertical reinforcement ratios (ρv%) of 0.25% for RC walls. Subsequently, the current study was conducted to study the seismic performance of RC walls with different vertical reinforcement ratios and cross sectional configurations. In this paper, six half-scaled RC structural walls were constructed and tested under quasi-static displacement controlled cyclic loading. The walls had three different cross sectional configurations; rectangular, flanged and boundary elements and were tested with specific design characteristics selected to evaluate and compare the wall ductility capabilities. In this respect, wall ductility can be defined as the ability of the walls to undergo inelastic deformations with no/low strength degradation, which is essential in Seismic Force Resisting Systems (SFRS) as it is not economically feasible to design SFRS to behave elastically under seismic loadings. So the ductility quantification of the structural walls used were ductility ratio between the intended displacements with the yield displacement. Based on the test results, the ultimate drift at 20% ultimate strength degradation varied between 0.9% and 1.6% and the ultimate level displacement ductility (μΔ0.8u), ranged approximately between 4.0 and 6.0. Although the flanged walls and the walls with boundary elements were designed to develop almost the same capacity as that of the rectangular walls, the seismic performance of the former wall type was found to be superior to that of their rectangular counterparts with respect to both the ultimate displacement capacity and ductility level. Moreover, using the flanges and the boundary elements walls resulted in approximately 30% reduction of the vertical reinforcement compared to that of the rectangular walls when designed to resist the same lateral loads while carrying identical gravity loads. In addition to gaining insights on the response of walls with boundary elements, the results indicated that structural walls with low vertical reinforcement ratio can experience reduced ductility as indicated in the Canterbury Commission Report.

Seismic axial loads in steel moment resisting frames

During the 1994 Northridge Earthquake, many buildings with modern steel moment resisting frames (SMRFs) suffered from connection failures. One year later, similar damage has occurred in the 1995 Kobe earthquake in Japan. The unexpected seismic response of SMRFs resulted in comprehensive analytical and theoretical investigations and major changes in steel building design have been implemented consequently. One of the requirements in the subsequent seismic design codes is the stability check of the columns. Column yielding in a seismic force resisting systems (SFRSs) is not the desired damage mode and might result in column rupture or global buckling and threaten life safety. This study focuses on exploring the seismic axial loads for columns in SMRFs under strong ground motions. For this purpose, the increase in axial loads in low-, medium-, and high-rise SMRFs are investigated at the maximum lateral load level and the corresponding lateral displacement. The results are presented in terms of PHRs, average system overstrength factors (Ω0) of all columns in the frames under the selected ground motions, the distribution of Ω0 in the individual columns in the frame, and axial load levels in columns. The results indicate that axial load level remains below 0.4 in the columns for low- and medium-rise frames, whereas it may get as high as 0.95 in highrise frames.

Shake Table Seismic Performance Assessment of Lightly Reinforced Concrete Block Shear Walls

AbstractThere is a need to develop a reinforced masonry (RM) seismic performance database (SPD) of experimental results in order to facilitate adoption of RM seismic force-resisting systems (SFRS) in the next generation of performance-based seismic design (PBSD) codes. As a contribution to this SPD, and within a larger ongoing research program, this paper reports on recent shake table tests performed on lightly reinforced masonry shear walls. The test walls covered a range of design parameters to facilitate benchmarking, further performance investigation, and calibration of numerical models as well as future development of fragility curves within the context of PBSD. The design parameters of the walls were selected to meet the requirements of the lightly reinforced conventional construction category. This particular RM wall category is not permitted in moderate and high seismic zones in the Canada based on Canadian masonry design code. Within the context of PBSD, the wall seismic performances, in terms of...

Behavior of Post-Tensioning Strand Systems Subjected to Inelastic Cyclic Loading

AbstractPost-tensioning (PT) strands have been employed in a number of self-centering seismic force-resisting systems as part of the restoring force mechanism that eliminates residual building drif...

Performance-Based Seismic Design of RC SMF Using Target Drift and Yield Mechanism as Performance Criteria

Reinforced concrete special moment frames (RC SMF) have been widely used as part of seismic force-resisting systems. Design methodologies and systematic procedures of RC SMF are needed which require no or little iteration after initial design in order to meet the targeted design objectives. This paper presents the first time application of the Performance-Based Plastic Design (PBPD) approach to RC SMF. The PBPD method uses pre-selected target drift and yield mechanisms as key performance objectives. These two design parameters are directly related to the degree and distribution of structural damage, respectively. Four baseline RC SMF (4, 8, 12 and 20-story) as used in the FEMA P695 were selected for this study. Those frames were redesigned by the PBPD approach. The baseline code designed frames and the PBPD frames were subjected to extensive inelastic pushover and time-history analyses. The seismic responses of the study frames met the targeted performance criteria with considerable improvement over the corresponding baseline code designed frames.

Comparative Study of Codes for Seismic Design of Structures

Abstract This paper presents a comparative evaluation among some international, European and American, seismic design standards. The study considers the criteria for the analysis of conventional (residential and commercial) buildings. The study is focused on some critical topics: definition of the recurrence periods for establishing the seismic input; definition of the seismic zonation and shape of the design response spectra; consideration of local soil conditions; definition of the seismic force-resisting systems and respective response modification coefficients; definition of the allowable procedures for the seismic analysis. A model for a standard reinforced concrete building (“Model Building”) has been developed to permit the comparison among codes. This building has been modelled with two different computer programs, SAP2000 and SOFiSTiK and subjected to seismic input according to the several seismic codes. The obtained results compared are leading to some important conclusions.

Comparative study of codes for the seismic design of structures

A general evaluation of some points of the South American seismic codes is presented herein, comparing them among themselves and with the American Standard ASCE/SEI 7/10 and with the European Standard Eurocode 8. The study is focused in design criteria for buildings. The Western border of South America is one of the most seismically active regions of the World. It corresponds to the confluence of the South American and Nazca plates. This region corresponds roughly to the vicinity of the Andes Mountains. This seismicity diminishes in the direction of the comparatively seismically quieter Eastern South American areas. The South American countries located in its Western Border possess standards for seismic design since some decades ago, being the Brazilian Standard for seismic design only recently published. This study is focused in some critical topics: definition of the recurrence periods for establishing the seismic input; definition of the seismic zonation and design ground motion values; definition of the shape of the design response spectra; consideration of soil amplification, soil liquefaction and soil-structure interaction; classification of the structures in different importance levels; definition of the seismic force-resisting systems and respective response modification coefficients; consideration of structural irregularities and definition of the allowable procedures for the seismic analyses. A simple building structure is analyzed considering the criteria of the several standards and obtained results are compared.

Damage states and fragility functions for link beams in eccentrically braced frames

Steel Eccentrically Braced Frames (EBFs) are used in seismic force-resisting systems of buildings and infrastructure constructed in regions of moderate and high seismic hazard. Future performance assessment of such structures, which will focus on repair cost and business interruption rather than component checking per ASCE 41, will utilize fragility functions that relate the probability of exceeding one or more damage thresholds to an efficient response (demand) parameter. Fragility functions are developed for link beams of EBFs to enable future performance assessment of buildings and other structures incorporating these structural components. Data are presented for shear- and flexure-critical link beams. The functions are developed by review and statistical evaluation of data from tests. Link plastic rotation is used as the demand parameter. Experimental data are interpreted using damage states and methods of repairs. Observations from experimental programs are used to identify the most appropriate damage states and their corresponding methods of repair. Damage states are characterized in most instances by direct indicators of damage to steel components such as web and flange local buckling, and fracture. Each of these damage states is linked with one of four methods of repair in increasing order of repair cost, namely, cosmetic repair, concrete replacement, heat straightening, and link replacement. An ordered sequence of fragility functions is provided for EBF link beams.