Welded Steel Moment Frame Buildings Research Articles

A Model of Steel Frame for Simulating Connection Fractures

Following the 1994 Northridge earthquake, widespread damages were discovered in welded steel moment frame buildings. In order to accurately simulate the typical seismic damage of welded steel moment frame structures, a new simplified model is proposed for performing seismic evaluation of welded steel moment frame structures. In this model, the slabs effect is considered, as well as the effects of the slip between slabs and steel beams, deformation of panel zone and connection fractures. Fracture toughness demands were evaluated in terms of the mode I stress intensity factor. The model was employed in simulation of seismic damage of Blue Cross Building which experienced fractured connections in the Northridge earthquake. It indicates that the model can accurately predict the earthquake response of welded steel moment frame structures and estimate the level of damage. The approach proposed in this paper has important meaning to the research on seismic damage of steel frame which may experience fractured connections.

ASCE-41 and FEMA-351 Evaluation of E-Defense Collapse Test

A welded steel moment-frame building is used to assess performance-based engineering guidelines. The full-scale four-story building was shaken to collapse on the E-Defense shake table in Japan. The collapse mode was a side-sway mechanism in the first story, which occurred in spite of a strong-column and weak-beam design. Computer analyses were conducted to simulate the building response during the experiment. The building was then evaluated using the Seismic Rehabilitation of Existing Buildings (ASCE-41) and Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings (FEMA-351) for the collapse prevention performance level via linear and nonlinear procedures. The guidelines had mixed results regarding the characterization of collapse, and no single approach was superior. They mostly erred on the safe side by predicting collapse at shaking intensities less than that in the experiment. Recommendations are made for guideline improvements.

Relative Performance of Kobe and Northridge WSMF Buildings

Seismic behaviors of a five-story welded steel moment-frame (WSMF) office building in Kobe, Japan, and a six-story WSMF office building in Northridge, California, are compared. Both experienced earthquake damage (1995 Kobe and 1994 Northridge earthquakes, respectively). Computer models of the buildings are formulated, having the ability to simulate damage in terms of fractured moment connections. Analyses are conducted to assess building response during the earthquakes. The calibrated models are then analyzed using a suite of earthquake records to compare building performance under consistent demands. The Kobe building is found to be more rugged than the Northridge building. Analysis suggests it would experience much less damage than the Northridge building from shaking equivalent to 2,500-year earthquake for a generic Los Angeles site. Superior performance of the Kobe building is attributed to its relatively greater stiffness and strength. The results provide insight into the difference in seismic fragility expected for this class of mid-rise WSMF buildings in Japan and the United States.

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.

Earthquake Loss Estimation Methods for Welded Steel Moment-Frame Buildings

This paper describes procedures that may be used by experienced structural engineers to develop earthquake damage and related loss functions for welded steel moment-frame (WSMF) buildings. The damage and loss functions are based on and compatible with the loss estimation methods of HAZUS, a technology developed by Federal Emergency Management Agency (FEMA) for assessing regional impacts of earthquakes. The loss estimation procedures were developed by the SAC Steel Program as described in SAC Joint Venture Topical Report SAC/BD-99/13. These procedures form the basis for Appendix B of FEMA-351, Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings. The procedures for developing damage and loss functions for WSMF building response are general in nature and applicable to WSMF buildings designed to different seismic criteria and having different connection details. Default values of damage and loss function parameters are provided for typical 3-story, 9-story, and 20-story WSMF buildings, designed for Los Angeles, Seattle, or Boston seismic criteria and having pre-Northridge, post-Northridge, or damaged pre-Northridge connection conditions.

Evaluating and Upgrading Welded Steel Moment-Frame Buildings Using FEMA-351

In July 2000, the SAC Joint Venture (a joint venture of the Structural Engineers Association of California, the Applied Technology Council, and California Universities for Research in Earthquake Engineering) prepared a series of recommendations regarding welded steel moment-frame design, evaluation, and upgrade procedures. FEMA-351, Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings, was developed to evaluate the probable performance of existing steel moment-frame buildings in future earthquakes and to provide guidance or upgrading these buildings. The procedures introduced in FEMA-351 allow the determination of the level of confidence a structure will be able to achieve based on a specified performance objective, using simplified analytical methods. Simplified procedures for estimating the probable post-earthquake repair costs and nonstructural damage, based on the losses incurred in the 1994 Northridge earthquake, are presented as well. This paper provides a brief chapter-by-chapter overview of the information contained in FEMA-351 and emphasizes the performance evaluation procedures by stepping through the process using an example building.

Northridge Welded Steel Moment-Frame Damage Data and Its Use for Rapid Loss Estimation

FEMA-351, Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings, offers two methods for estimating seismic losses in pre-Northridge WSMFs: detailed and rapid. The rapid method uses empirical relationships between seismic demand parameters and either damage levels or repair costs. The relationships are based on actual damage data collected after the 1994 Northridge earthquake. This paper summarizes the Northridge data, explains the FEMA-351 rapid method loss functions, and comments on the nature of the damage data and its application to loss estimation. Use of the loss functions can be enhanced by understanding their inherent assumptions and uncertainties and by considering how the underlying data was collected and interpreted in the years following the 1994 earthquake.

Overview of the U.S. Program for Reduction of Earthquake Hazards in Steel Moment-Frame Structures

Considerable research has been conducted worldwide to assess the unexpected damage to welded steel moment-frame buildings during the 1989 Loma Prieta, 1994 Northridge, and 1995 Hyogo-ken Nanbu earthquakes, as well as to find effective and economical remedies that can be incorporated into analysis, design, and construction practices. A major six-year program has been undertaken with the sponsorship of the U.S. Federal Emergency Management Agency (FEMA) to synthesize and interpret the results of this research, and to conduct additional investigations to develop reliable, practical, and cost-effective guidelines for the design and construction of new steel moment-frame structures, as well as for the inspection, evaluation and repair or upgrading of existing ones. Topics investigated as part of this program include (1) performance of steel buildings in past earthquakes; (2) material properties and fracture issues; (3) joining and inspection; (4) connection performance; (5) system performance; (6) performance prediction and evaluation; and (7) social, economic, and political impacts. The project utilizes a performance-based engineering framework and addresses issues pertaining to various types of steel moment-resisting frames including those utilizing welded, bolted, and partially restrained connections. The guidelines are applicable to regions of low, medium, and high seismicity throughout the United States. This paper reviews the overall organization and management of this program of research, guideline development, training and peer evaluation, the scope of the investigations undertaken, and the general organization and contents of the guidelines developed.

Overview of the FEMA/SAC program for reduction of earthquake hazards in steel moment frame structures

Considerable research has been conducted worldwide to assess the unexpected damage to welded steel moment-frame buildings during the 1989 Loma Prieta, 1994 Northridge and 1995 Hyogo-ken Nanbu earthquakes, as well as to find effective and economical remedies that can be incorporated into analysis, design, and construction practices. A major 6-year program was undertaken sponsored by the US Federal Emergency Management Agency (FEMA) synthesized and interpreted the results of this research, and conducted additional investigations to develop reliable, practical and cost-effective guidelines for the design and construction of new steel moment-frame structures, as well as for the inspection, evaluation and repair or upgrading of existing ones. Topics investigated as part of this program included: (1) performance of steel buildings in past earthquakes; (2) material properties and fracture issues; (3) joining and inspection; (4) connection performance; (5) system performance; (6) performance prediction and evaluation; and (7) social, economic and political impacts. The project utilized a performance-based engineering framework and addresses issues pertaining to various types of steel moment-resisting frames including those utilizing welded, bolted, and partially restrained connections. Published late in 2000 by FEMA, the guidelines are applicable to regions of low, medium and high seismicity throughout the US. This paper reviews the overall organization and management of this program of research, guideline development, training and peer evaluation, the scope of the investigations undertaken, and the general organization and contents of the guidelines developed.

The W1 Issue. I: Extent of Weld Fracturing during Northridge Earthquake

Ultrasonic indications in the roots of complete penetration welds (W1's) were the most common of the unexpected conditions identified after the 1994 Northridge earthquake in welded steel moment frame (WSMF) buildings. Widely treated by the engineering profession as earthquake-caused damage requiring repair, and characterized as such by the SAC, W1's have not previously been subjected to systematic study either to factually establish their ostensible relationship to earthquake ground motions or to debunk it. In this first part of a two-part paper, the conclusions from several studies that were generally directed toward accurately characterizing W1's, identifying their origins, clarifying the extent and distribution of earthquake damage to WSMF buildings caused by the Northridge event, and developing recommendations for cost-efficient post-earthquake damage surveying of WSMF buildings are presented. In a companion paper, the reliability of ultrasonic testing (UT) of WSMF buildings during construction is also investigated as a possible explanation for the widespread presence of W1's in existing WSMF buildings. The studies found that W1's are not related to earthquake ground motion, that the extent of Northridge earthquake damage to WSMF buildings is substantially less than previously published accounts suggest, and that UT as normally implemented by testing lab personnel is an unreliable inspection technique for the roots of complete penetration T-joints with backing.

The W1 Issue. II: UT Reliability for Inspection of T-Joints with Backing

Ultrasonic indications in the roots of complete penetration welds (W1's) were the most common of the unexpected conditions identified after the 1994 Northridge earthquake in welded steel moment frame (WSMF) buildings. Widely treated by the engineering profession as earthquake-caused damage requiring repair, and characterized as such by the SAC, W1's have not previously been subjected to systematic study either to factually establish their ostensible relationship to earthquake ground motions or to debunk it. In this paper, the reliability of ultrasonic testing (UT) of full-penetration welded joints with backing in WSMF buildings is investigated. A study involving verification of the findings of UT via destructive investigation, a study of the influence of operator variability, and studies delineating some of the limitations of ultrasonics are described. In the first part of this two-part paper, several studies that were generally directed toward accurately characterizing W1's, identifying their origins, clarifying the extent and distribution of earthquake damage to WSMF buildings caused by the Northridge event, and developing recommendations for cost-efficient post-earthquake damage surveying of WSMF buildings were presented. Those studies found that W1's are not related to earthquake ground motion, and that the extent of Northridge earthquake damage to WSMF buildings is substantially less than previously published accounts suggest. In the present paper, it is shown that, as normally implemented by testing lab personnel, UT is an unreliable inspection technique for complete penetration T-joints with backing, which provides a plausible explanation for the widespread occurrence of W1's in existing buildings.

Cyclic Testing of Simple Connections Including Effects of Slab

The problems with welded moment connections in the Northridge earthquake of 1994 gave researchers incentive to examine the actual contribution of the simple, or shear, connections to the lateral resistance of welded steel moment-frame buildings. With the composite action of the floor slab, these connections may have more lateral resistance than traditionally assumed; thus, they may be used in the repair or retrofit of damaged welded steel moment-frame buildings or used more actively in new construction. To evaluate these possibilities, and to establish the strength, stiffness, and ductility of these connections under cyclic loading, two series of eight full-scale, cyclic tests on shear connections with floor slabs were planned. The first test series has demonstrated the cyclic behavior of the simple connections and has shown that these “pinned” connections have significant moment capacities, both on their own and with the contribution of the floor slab. This paper summarizes the results of the first eight tests.

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.

A connection model for the seismic analysis of welded steel moment frames

An analytical approach is described for performing seismic evaluation of welded steel moment frame buildings constructed prior to the 1994 Northridge earthquake. The approach is based on an analytical model that captures the possibility of connection fracture and the subsequent loss of strength and stiffness. This connection model was developed and calibrated on the basis of laboratory experiments. It was employed in the seismic analysis of a 13-story building which experienced fractured connections in the Northridge earthquake and for which there are seismograph records and a damage survey. Results from several analyses indicate that the inelastic and fracture models are capable of predicting the behavior of steel moment frame buildings and of estimating the level of damage.

Probable maximum loss estimation in earthquakes: an application to welded steel moment frames

An approach to the evaluation of the damageability of buildings and the determination of probable maximum loss (PML) values is presented and applied to welded steel moment frame buildings. PML is defined as the loss that has a given (usually 10%) probability of exceedance in a specified number of years from earthquake ground shaking. A Markov Model developed is used in a Monte Carlo simulation to determine damage statistics, from which PML values are determined. Four sites within the Los Angeles area are considered, as are a range of building damageabilities from very low to very high. Specific assessments are made for welded steel moment frame buildings representative of the post-Northridge understanding of damageability. PML values are shown to be very sensitive to the probability of loss exceedance, the time period considered, the number of buildings in the portfolio, and the lower bounds of earthquake magnitude and ground motions considered. The number of buildings considered in the PML assessment is shown to be very important as a means of reducing PML values. The use of MMI ground motion characterization is assessed and found to be wanting. A comparison of PML results with other damageability measures (both average and 10% exceedance) for four sites, including different ground motions and earthquakes, and a maximum probable loss associated with the closest serious earthquake, indicates that these other measures are not good estimators of the full probabilistic PML that considers in a consistent manner both site ground motion and building performance uncertainty. © 1997 John Wiley & Sons, Ltd.