SAC Steel Project Research Articles

Seismic Performance of Vertical Irregular Steel Frame Structure under Mainshock-Aftershock

Abstract: The majority of the structures are involved with architectural importance and it is highly impossible to achieve uniform structural properties in all directions. Hence earthquake resistant design codes considered it as irregular frames based on relative difference in the story properties. In many cases these irregularities are responsible for structural collapse of buildings under earthquake ground motions. The seismic response of buildings with irregular distribution of stiffness along the height may be differentfrom that ofregular building. Also, past earthquakes showed that structure may be subjected to sequence of ground motions but current codes do not have guidelines for such cases. It is considered that aftershock do not cause any more damage to damaged structure by the preceding mainshock ground motion. In this study steel moment resisting frame buildings are evaluated to understand the seismic response of vertical stiffness irregular frames subjected to mainshock-aftershock ground motions. The 9-story steel moment resisting frame building situated in Los Angeles is used in this study was originally developed as part of the SAC steel project. In this study soft and stiff storey case was considered at three different locations along the height, i.e., at bottom storey, mid-height storey and top storey. The single modification factor is used for irregularity. For comparison purpose dynamic properties of regular and irregular frames are kept same. Two sets of 6 records were selected representing a seismic hazard level of 2% and 10% probability of exceedance in 50 years respectively as mainshock. Two sets of 30 mainshockaftershock ground motion are considered for the study. These ground motions were developed using randomized approach. A non-linear time history analysis of regular and irregular building is carried out separately under mainshock and mainshockaftershock. The effect of building irregularity was studied for single storey modification at bottom storey, mid-storey and top storey with comparison to regular building. The comparison of a regular and irregular building is carried out in terms of maximum roof displacement and interstorey drift ratio. Also, comparison of frames under mainshock and mainshock-aftershock is done.

Selection and modification of ground motion records using a weighted scaling method based on the Newmark-Hall target spectrum

For the problem of the selection and modification of ground motions in nonlinear time-history analysis, a weighted scaling method based on the Newmark-Hall target spectrum (WNM) was developed in this study. The average Newmark-Hall spectra of the three ground motion sets developed in the SAC Steel Project are defined as the target spectra, which represent 50 %, 10 %, and 2 % probabilities exceeded in 50 years, respectively. The matching errors (SSE) and scaling factors (SF) were calculated by the weighted least-squares method to match with the Newmark-Hall target spectrum. The 9- and 20-story steel-moment-resisting frame structures were used as examples. Their mean structural responses obtained by the WNM were compared with the results of the method using the Newmark-Hall target spectrum without weight in scaling records (NM). Both the WNM and WN can control the relative errors of the prediction of the mean structural response within ± 20 % as compared with the benchmark results when 7 or 10 records are selected, if the SSE and SF used to scale and rank a record are both obtained by the same scaling method. Both the WNM and NM have better capacity for reducing the variability of structural responses than the acceleration target spectrum method. Especially for the 20-story structure with long fundamental period in regions with high earthquake intensity, this advantage of the WNM is more prominent than that of the WN. In addition, if the record has a large forward and back displacement peak, the steel-moment-resisting frame structures would produce a large offset at the low half of the building, and the panel zones are critical components for the structural deformation.

Comparison of Scaling Ground Motions Using Arithmetic with Logarithm Values for Spectral Matching Procedure

In recent studies, spectral matching is the most commonly proposed method for selecting earthquake records for time-history analysis of structures. However, until now, there have been no serious investigations of the effects of coordinate values on the scaling of ground motions. This paper investigated the influence of using arithmetic and logarithmic values of response spectra in spectral matching procedures (i.e., ASM and LSM methods) on the results of nonlinear structural time-history analysis. Steel moment resisting frame structures of the 3-, 9-, and 20-stories, which represent low-, medium-, and high-rise buildings, respectively, were used as examples. Structural benchmark responses were determined by calculating the arithmetic mean and median of peak interstory drift ratio (PIDR) demands based on the three record sets developed by the American SAC Steel Project. The three record sets represent seismic hazard levels with 50%, 10%, and 2% probabilities exceeded in 50 years, and their average acceleration spectra were also taken as the target spectrum. Moreover, another 40 record components for selection were scaled both by ASM and LSM methods. The seven components whose spectra were best compatible with the target spectra were selected for the structural time-history analysis. The scale factors obtained by the LSM method are nearly larger than that of the ASM method, and their ranking and selection of records are different. The estimation accuracies of structural mean (median) responses by both methods can be controlled within an engineering acceptable range (±20%), but the LSM method may cause larger structural responses than the ASM method. The LSM method has a better capacity for reducing the variability of structural responses than the ASM method, and this advantage is more significant for longer-period structures (e.g., 20-story structure) with more severe nonlinear responses.

Vibration control for torsionally irregular buildings by integrated control system

Torsion irregularity increases the risk of building failure during a strong dynamic excitation that is generated by earthquakes or wind gusts. To enhance the safety and performance of torsional irregular buildings, a newly developed Integrated Control System (ICS) is proposed in this research. The new control approach was applied to a two-way eccentric Benchmark 9-story steel building, constructed for the SAC steel project in California, where each floor is represented by two translational and one rotational degree of freedom. The performance and effectiveness of the ICS were examined and compared with a Tuned Mass Dampers (TMDs) approach by subjecting the building to real earthquake excitations of N-S and E-W components of El Centro in 1940, Loma Prieta in 1989, and Kocaeli, Turkey in 1999. Results showed that the ICS was effectively mitigating the lateral and coupling vibrations by the new design configuration and arrangement for both tuning and detuning cases. The ICS is also more robust in restricting the inter-story drift ratio as compared with TMDs.

Ductility demands and reduction factors for 3D steel structures with pinned and semi-rigid connections

A numerical investigation regarding local (uL) and story (uS) ductility demand evaluation of steel buildings with perimeter moment resisting frames (PMRF) and interior gravity frames (IGF), is conducted in this study. The interior connections are modeled, firstly as perfectly pinned (PP), and then as semi-rigid (SR). Three models used in the SAC steel project, representing steel buildings of low-, mid-, and high-rise, are considered. The story ductility reduction factor (RuS) as well as the ratio (QGL) of RuS to uL are calculated. uL and uS, and consequently structural damage, at the PMRF are significant reduced when the usually neglected effect of SR connections is considered; average reductions larger than 40% are observed implying that the behavior of the models with SR connections is superior and that the ductility detailing of the PMRF doesn\'t need to be so stringent when SR connections are considered. RuS is approximately constant through height for low-rise buildings, but for the others it tends to increase with the story number contradicting the same proportion reduction assumed in the Equivalent Static Lateral Method (ESLM). It is implicitly assumed in IBC Code that the overall ductility reduction factor for ductile moment resisting frames is about 4; the results of this study show that this value is non-conservative for low-rise buildings but conservative for mid- and high-rise buildings implying that the ESLM fails evaluating the inelastic interstory demands. If local ductility capacity is stated as the basis for design, a value of 0.4 for QGL seems to be reasonable for low- and medium-rise buildings.

Energy Dissipation and Local, Story, and Global Ductility Reduction Factors in Steel Frames under Vibrations Produced by Earthquakes

Ductility plays a central role in seismic analysis and design of steel buildings. A numerical investigation regarding the evaluation of energy dissipation, ductility, and ductility reduction factors for local, story, and global structural levels is conducted. Some steel buildings and strong motions, which were part of the SAC Steel Project, are used. Bending local ductility capacity (µLϕ) of beams can reach values of up to 20, as shown in experimental investigations. The values are larger for medium than for low‐rise buildings, reflecting the effect of the structural complexity on µLϕ. Most of the dissipated energy occurs on beams; however, resultant stresses at columns are also significantly reduced by beam yielding. A value of 1/3 is proposed for the ratio of global to local ductility; thus, if local ductility capacity is stated as the basis for the design, global ductility capacity can be calculated by using this ratio. It is implicitly assumed in seismic codes that the magnitude of the global ductility reduction factor is about 4; according to the results found in this paper, it is not justified; a value of 3 is observed to be more reasonable. According to the well‐known ratio of the ductility reduction factor to ductility, this ratio should be unity for the models under consideration; the results of this study indicate that, for global response parameters, a value of 3/4 is more appropriate and that, for local response parameters, values larger than 2 can be reached; the implication of this is that using simplified methods like the static equivalent lateral force may result in nonconservative designs from a global structural point of view, but in conservative designs from a local point of view. A value of 8 is proposed for the ratio of the global ductility reduction factor to the global normalized energy.

Response modification factor for steel moment-resisting frames by different pushover analysis methods

The earthquake loads imposed to the structures are generally much more than what they are designed for. This reduction of design loads by seismic codes is through the application of response modification factor (R-factor). During moderate to severe earthquakes, structures usually behave inelastically, and therefore inelastic analysis is required for design. Inelastic dynamic analysis is time consuming and interpretation of its results demands high level of expertise. Pushover analysis, recently commonly used, is however, a simple way of estimating inelastic response of structures. Despite its capabilities, conventional pushover analysis (CPA) does not account for higher mode effects and member stiffness changes. Adaptive pushover analysis (APA) method however, overcomes these drawbacks. This research deals with derivation and comparison of some seismic demand parameters such as ductility based reduction factor, Rμ, overstrength factor, Ω, and in particular, response modification factor, R, from capacity curves obtained from different methods of APA and CPA. Three steel moment-resisting frames of 3, 9 and 20 stories adopted from SAC steel project are analyzed. In pushover analyses for each frame, eight different constant as well as adaptive lateral load patterns are used. Among the main conclusions drawn is that the maximum relative difference for response modification factors was about 16% obtained by the methods of conventional and adaptive pushover analyses.

GENERATION OF UNIFORM HAZARD EARTHQUAKE ACCELEROGRAMS AND NEAR-FIELD GROUND MOTIONS

The sets of records developed for the SAC Steel Project are classified according to the level of seismic hazard and specific geographic region, and have been used extensively for structural response and seismic hazard analyses. This study presents a parametric analysis of these record data sets for generation of uniform hazard earthquake and near-field records. The record parameters define far-field characteristics such as power spectral density and envelope function, and near-field effects such as acceleration pulse, power spectral density and envelope function. The proposed method for generation of near-field records uses the decomposing capabilities of wavelet transform on earthquake records. A set of uniform hazard earthquake accelerograms and near-field ground motions is generated based on the record parameters. The generated uniform hazard earthquake accelerograms representing a uniform level of seismic hazard for a particular geographic region involves seismic hazard studies, calibrated attenuation relationships, and local site amplification models. In order to assess the reliability and efficiency of the presented method, the statistical response spectra obtained from the generated accelerograms have been compared with those from the actual records. The obtained results showed that there is a good compatibility between the response spectra of the generated and actual records in the most of the frequencies.

Control of structures under uniform hazard earthquake excitation via wavelet analysis and pattern search method

A new methodology for optimal control of structures under uniform hazard earthquake excitation has been presented in this study. The proposed method for generation of uniform hazard earthquake accelerograms is based on the sets of ground motions developed for the SAC Steel Project. The proposed controller employs the decomposing capabilities of wavelet analysis on uniform hazard earthquake accelerograms and the ability of the pattern search method to optimize a cost function to control response of structure. The performance and effectiveness of the presented method are applied to both single degree of freedom and multiple degree of freedom systems for the response control of uniform hazard earthquake-excited systems. The results are compared with the linear quadratic regulator control algorithm; performance of the proposed control system has been found to be better than the linear quadratic regulator controller. Copyright © 2012 John Wiley & Sons, Ltd.

A Record-Based Method for the Generation of Tridirectional Uniform Hazard-Response Spectra and Ground Motions Using the Hilbert-Huang Transform

A new method for the construction of tridirectional uniform hazard- response spectra and ground motions for a given site is proposed. The rupture directivity of near-site earthquakes is considered. This method uses historical records without scaling from a region close to the site and a simulation method recently developed for nonstationary random processes using the Hilbert-Huang Transform. The simulation method can reproduce the ground-motion intensity and spectral content variation with time and is thus suitable for near-fault ground motions with long- period pulses. An example is given of a site at Los Angeles city hall. The results are compared with those of existing methods such as the U.S. Geological Survey National Seismic Hazard Mapping Project and sac Steel Project (formed by Structural Engineers Association of California [seaoc], Applied Technology Council [atc], and Consortium of Universities for Research in Earthquake Engineering [curee]). The advantages and limitations of the proposed method are also discussed.

SEISMIC BEHAVIOUR OF PERIMETER AND SPATIAL STEEL FRAMES

The present study deals with the seismic performance of partial perimeter and spatial moment resisting frames (MRFs) for low-to-medium rise buildings. It seeks to establish perimeter configuration systems and hence the lack of redundancy can detrimentally affect the seismic response of framed buildings. The paper tackles this key issue by com-paring the performance of a set of perimeter and spatial MRFs, which were “consistently designed”. The starting point is the set of low-(three-storey) and medium-rise (nine-storey) perimeter frames designed within the SAC Steel Project for the Los Angeles, Seattle and Boston seismic zones. Extensive design analyses (static and multi-modal) of the perimeter frame buildings and consistent design of spatial frame systems, as an alternative to the perimeter configuration, were conducted within this analytical study. The objectives of the consistent design are two-fold, i.e. obtaining fundamental periods similar to those of the perimeter frames, i.e. same lateral stiffness under design horizon-tal loads, and supplying similar yield strength. The seismic behaviour of perimeter and spatial configuration structures was evaluated by means of push-over non-linear static analyses and inelastic dynamic analyses (non linear time histories). Comparisons be-tween analysis results were developed in a well defined framework since a clear scheme to define and evaluate relevant limit states is suggested. The failure modes, either local or global, were computed and correlated to design choices, particularly those concerning the strength requirements (column overstrength factors) and stiffness (elastic stability indexes). The inelastic response exhibited by the sample MRFs under severe ground motions was assessed in a detailed fashion. Conclusions are drawn in terms of local and global performance, namely global and inter-storey drifts, beam and column plas-tic rotations, hysteretic energy. The finding is that the seismic response of perimeter and spatial MRFs is fairly similar. Therefore, an equivalent behaviour between the two configurations can be obtained if the design is “consistent”.

Prequalification of Steel Moment-Frame Connection Performance

Welded-flange/bolted-web connections were damaged during the Northridge earthquake, and the SAC Steel Project was funded by the Federal Emergency Management Agency to find solutions to the problems caused by this damage. The Connection Performance Technical Advisory Panel was one of several groups completing the project research, and this group examined all connection performance issues. Many connections were evaluated, and some were prequalified for future use without detailed testing or evaluation. This prequalification criteria and the methods used to satisfy the criteria are described. Some connections that satisfy the prequalification criteria are summarized and the general basis for the prequalification limits are noted. Prequalification requires an adequate experimental database for understanding and evaluating all yield mechanisms and failure modes for each connection type. The concept of the yield mechanisms and failure modes, and the application of the experimental data toward meeting the prequalification requirements are described. A wide range of connections including fully bolted connections, welded connections, and a range of partial-strength and partial-stiffness connections are included.

Post-earthquake Evaluation and Repair of Welded Steel Moment-Frame Buildings

In the wake of a potentially damaging earthquake, every affected welded steel moment-frame building should be assessed to determine whether it poses a safety risk. A straightforward, multi-step process has been developed to streamline this formidable task. This paper provides an overview of this procedure and repair techniques as published in FEMA-352, Recommended Post-earthquake Evaluation and Repair Criteria for Welded Steel Moment-Frame Buildings. The evaluation procedure, developed as part of the SAC Steel Project, begins with screening to rapidly identify those buildings unlikely to have been damaged. Subsequent steps help identify buildings that have sustained sufficient structural damage to compromise future performance and to determine appropriate actions regarding building occupancy and repair.

Connection Performance for Seismic Design of Steel Moment Frames

Welded-flange-bolted-web connections were damaged during the Northridge Earthquake, and the SAC Steel Project was started to find solutions to the problems caused by this damage. The Connection Performance Team was one of several groups completing the project research, and this group examined all connection performance issues. A number of connections were evaluated, and three strategies were used to improve the seismic performance of different connections. An understanding of all yield mechanisms and failure modes for each connection type is required for applicaiton of these improvement strategies. Variations in yield mechanisms and failure modes obtained for different connection types are described, and the application of these modes and mechanisms to improve the seismic performance of three different connections is summarized. The three connections are the welded-flange-welded-web, reduced-beam-seciton, and bolted-flange-plate connections. These connections are all capable of providing good seismic performance with large plastic rotational capacity, but each connection requires evaluation of different modes and mechanisms and different balance requirements for good seismic performance. The strategies and procedures used for these connections are typical of those used for many other connections.

General Issues Influencing Connection Performance

Welded-flange--bolted-web connections were damaged during the 1994 Northridge earthquake. The SAC Steel Project was founded to address the problems associated with this damage. The Connection Performance Team completed research on the behavior of a number of connection types. During this research, some issues were found to influence the behavior of many different connection types. This paper summarizes these general issues and describes their impact on connection performance. These general issues include: process and toughness, web attachment, panel zone yielding, beam depth and frame geometry, yield stress of the steel, geometry and column orientation, effect of composite slabs, local slenderness and unsupported length, and temperature and strain rate.

Behavior of Extended End-Plate Moment Connections Subject to Cyclic Loading

Extended end-plate moment connections are one alternative to fully welded connections that has been considered for use in seismic force resisting moment frames. As a part of the SAC Steel Project, a research program to investigate the behavior and design of extended end-plate moment connections under cyclic loading was conducted at Virginia Polytechnic Institute and State University. Six bare steel beam-to-column connection specimens and one composite slab beam-to-column connection specimen were tested. An overview of the design, fabrication, and testing of the specimens is presented. The test results show that extended end-plate moment connections can be designed to provide the strength, stiffness, and ductility required for use in seismic force resisting moment frames. The effects of the composite slab are discussed, and it is recommended that the effects of the slab be considered in the design of beam-to-column extended end-plate moment connections.

Annual limit-state frequencies for partially-inspected earthquake-damaged buildings

A procedure is presented for estimating an “annual limit-state frequency”, that is, the mean annual frequency of exceeding a (e.g. seismic-drift) limit state, for a partially-inspected earthquake-damaged SMRF building with fractured beam-column connections. Typically, inspection of all the moment-resisting connections in a building for fractures is prohibitively expensive. Motivated by this reality, the proposed procedure (developed as part of the SAC Steel Project) accounts for the uncertainty, due to incomplete inspection, in the total number and locations of fractured connections. Since the procedure can also take into account the aftershock ground motion hazard, an annual limit-state frequency estimated for a damaged building can serve as a basis for deciding, for example, whether to permit occupancy soon after the main-shock. Conversely, the estimated annual limit-state frequency can be used to guide inspection decisions, such as whether to inspect more connections and thereby reduce the uncertainty in the state of damage.

Summary Interpretation of SAC Survey Data on Damaged Welded Steel Moment Frames

This paper summarizes the review and interpretation of data collected on welded steel moment frames (WSMFs) that were damaged in the January 17, 1994, Northridge Earthquake. The study was part of the Phase I, Task 2 SAC Steel Project funded by the Federal Emergency Management Agency. The data assessment included 89 WSMFs surveyed by structural engineers, 150 buildings covered under a survey of owners, and ground motion contour maps with the geographic distribution of all WSMF buildings surveyed. Data collected from interviews with the structural engineers familiar with the buildings and their locations were used to independently validate the database and provide guidance on its interpretation. General conclusions on the significant trends in the data are presented, along with suggested future (Phase II) data gathering and statistical analysis.

National Program to Improve Seismic Performance of Steel Frame Buildings

This paper reviews the performance of welded steel moment frame buildings during the Northridge earthquake. Some of the studies being undertaken in the United States as part of the FEMA-funded SAC Steel Project are described. The intent of these studies is to devise improved methods for designing new steel frame structures; for inspecting, evaluating, and repairing seismic damage to these types of structures following a major earthquake; and for inspecting, evaluating, and retrofitting existing at-risk steel frame buildings. General observations resulting from these studies are highlighted and the overall format for the new design provisions is presented.

Summary of SAC Case Study Building Analyses

Summarized in this paper are the major findings from analytical studies of nine steel moment frame buildings conducted under Phase 1 of the SAC Steel Project. The buildings range in height from two to seventeen stories and most of them experienced damage to welded beam-column connections during the Northridge earthquake of 1994. Elastic response spectrum, inelastic static pushover, and elastic and inelastic time-history analyses were conducted using ground motion data representative of the Northridge earthquake to establish the loading/deformation demands that the buildings experienced. The primary performance indices obtained from the analyses were demand-to-capacity ratios, interstory drift ratios, and inelastic hinge rotations. Maximum ratios of elastic member force demands to plastic strengths ranged between 1.0 and 2.0; maximum inelastic hinge rotations were 0.005–0.010 rad; and maximum interstory drift ratios were from 1 to 2%. These damage indices increased by 50%–150% under more severe ground motions recorded during the Northridge earthquake at the Sylmar site. Accuracy of the analyses is shown to be sensitive to a number of modeling parameters including finite joint size, joint panel behavior, composite beam action, strain hardening, second-order (P-Δ) effects, and three-dimensional response. Overall, there was only modest correlation between the frame performance indices and the observed connection damage, due largely to the fact that significant aspects of the connection fracture behavior are not captured in the frame analyses.