Earthquake Spectra

The U.S. Geological Survey (USGS) recently completed new probabilistic seismic hazard maps for the United States, including Alaska and Hawaii. These hazard maps form the basis of the probabilistic component of the design maps used in the 1997 edition of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, prepared by the Building Seismic Safety Council and published by FEMA. The hazard maps depict peak horizontal ground acceleration and spectral response at 0.2, 0.3, and 1.0 sec periods, with 10%, 5%, and 2% probabilities of exceedance in 50 years, corresponding to return times of about 500, 1000, and 2500 years, respectively. In this paper we outline the methodology used to construct the hazard maps. There are three basic components to the maps. First, we use spatially smoothed historic seismicity as one portion of the hazard calculation. In this model, we apply the general observation that moderate and large earthquakes tend to occur near areas of previous small or moderate events, with some notable exceptions. Second, we consider large background source zones based on broad geologic criteria to quantify hazard in areas with little or no historic seismicity, but with the potential for generating large events. Third, we include the hazard from specific fault sources. We use about 450 faults in the western United States (WUS) and derive recurrence times from either geologic slip rates or the dating of pre-historic earthquakes from trenching of faults or other paleoseismic methods. Recurrence estimates for large earthquakes in New Madrid and Charleston, South Carolina, were taken from recent paleoliquefaction studies. We used logic trees to incorporate different seismicity models, fault recurrence models, Cascadia great earthquake scenarios, and ground-motion attenuation relations. We present disaggregation plots showing the contribution to hazard at four cities from potential earthquakes with various magnitudes and distances.

The 1997 NEHRP Recommended Provisions for Seismic Regulations for New Buildings use a design procedure that is based on spectral response acceleration rather than the traditional peak ground acceleration, peak ground velocity, or zone factors. The spectral response accelerations are obtained from maps prepared following the recommendations of the Building Seismic Safety Council's (BSSC) Seismic Design Procedures Group (SDPG). The SDPG-recommended maps, the Maximum Considered Earthquake (MCE) Ground Motion Maps, are based on the U.S. Geological Survey (USGS) probabilistic hazard maps with additional modifications incorporating deterministic ground motions in selected areas and the application of engineering judgement. The MCE ground motion maps included with the 1997 NEHRP Provisions also serve as the basis for the ground motion maps used in the seismic design portions of the 2000 International Building Code and the 2000 International Residential Code. Additionally the design maps prepared for the 1997 NEHRP Provisions, combined with selected USGS probabilistic maps, are used with the 1997 NEHRP Guidelines for the Seismic Rehabilitation of Buildings.

Recent code provisions for buildings and other structures (1994 and 1997 NEHRP Provisions, 1997 UBC) have adopted new site amplification factors and a new procedure for site classification. Two amplitude-dependent site amplification factors are specified: F _a for short periods and F _v for longer periods. Previous codes included only a long period factor S and did not provide for a short period amplification factor. The new site classification system is based on definitions of five site classes in terms of a representative average shear wave velocity to a depth of 30 m (V¯ _s ). This definition permits sites to be classified unambiguously. When the shear wave velocity is not available, other soil properties such as standard penetration resistance or undrained shear strength can be used. The new site classes denoted by letters A - E, replace site classes in previous codes denoted by S1 - S4. Site classes A and B correspond to hard rock and rock, Site Class C corresponds to soft rock and very stiff / very dense soil, and Site Classes D and E correspond to stiff soil and soft soil. A sixth site class, F, is defined for soils requiring site-specific evaluations. Both F _a and F _v are functions of the site class, and also of the level of seismic hazard on rock, defined by parameters such as A _a and A _v (1994 NEHRP Provisions), S _s and S _l (1997 NEHRP Provisions) or Z (1997 UBC). The values of F _a and F _v decrease as the seismic hazard on rock increases due to soil nonlinearity. The greatest impact of the new factors F _a and F _v as compared with the old S factors occurs in areas of low-to-medium seismic hazard. This paper summarizes the new site provisions, explains the basis for them, and discusses ongoing studies of site amplification in recent earthquakes that may influence future code developments.

The fault sources in the Project 97 probabilistic seismic hazard maps for the state of California were used to construct maps for defining near-source seismic coefficients, N _a and N _v , incorporated in the 1997 Uniform Building Code (ICBO 1997). The near-source factors are based on the distance from a known active fault that is classified as either Type A or Type B. To determine the near-source factor, four pieces of geologic information are required: (1) recognizing a fault and determining whether or not the fault has been active during the Holocene, (2) identifying the location of the fault at or beneath the ground surface, (3) estimating the slip rate of the fault, and (4) estimating the maximum earthquake magnitude for each fault segment. This paper describes the information used to produce the fault classifications and distances.

Currently the most widely accepted code regulations in the United States for seismic design of structures and nonstructural components are those found in the Uniform Building Code (UBC). The UBC seismic requirements were significantly revised in the 1997 edition. Among the issues addressed in the UBC revisions are near-source effects and ground acceleration dependent soil site amplification factors for both short- and long-period structures. Also, the design force levels in the 1997 UBC are based on strength design rather than allowable stress design, as had been used previously. Other significant changes include introduction of a redundancy/reliability factor, a more realistic consideration of story drift and deformation compatibility, and new equations for equivalent static forces for both structural and nonstructural components. This paper traces the recent history of the code development and describes the major elements of the 1997 UBC seismic provisions.

This paper follows the evolution of the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures from the development of ATC 3-06 to the 1997 edition of the document. The features of the 1997 NEHRP Provisions are described in detail. Complementary information about the NEHRP Provisions is found in several other papers in this volume of Earthquake Spectra. Subject areas covered elsewhere are specifically referenced in this paper.

With the publication of the 1997 Uniform Building Code (UBC) and the 1997 NEHRP Recommended Provisions for the Seismic Regulations for New Buildings and Other Structures, there has been a significant change in the earthquake design force provisions for buildings, structures, elements of structures and nonstructural components. Engineers and architects need to become informed regarding a variety of earthquake design force provisions, primarily those published in the UBC and those developed as part of the NEHRP Provisions. Both sources provide design force provisions for the building structural system and separate design force provisions for elements of structures and nonstructural components. This paper describes the development, evolution, and application of the earthquake design force provisions for elements of structures and nonstructural components.

The building code development process has traditionally given little effort to developing the seismic design process of nonbuilding structures. This has created some unique problems and challenges for the structural engineers that design these types of structures. The intended seismic performance requirements for “building” design are based on life safety and collapse prevention. Structural elements in buildings are allowed to yield as a method of seismic energy dissipation. The seismic performance of nonbuilding structures varies depending on the specific type of nonbuilding structure. Nonlinear behavior in some nonbuilding structures is unacceptable while other nonbuilding structures may be allowed to yield during an earthquake. Nonbuilding structures comprise a vast myriad of structures constructed of all types of materials, with markedly different dynamic characteristics, and with a wide range of performance requirements. This paper discusses the development of codes, design practices, and future of the seismic design criteria for nonbuilding structures.

U.S. seismic codes are undergoing profound changes as of this writing. Changes from the 1994 to the 1997 edition of the Uniform Building Code (UBC) (ICBO 1994, 1997) are many and far-reaching in their impact. The 1997 edition of the National Earthquake Hazards Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings (BSSC 1998) contains further evolutionary changes in seismic design requirements beyond those of the 1997 UBC. The latter document will form the basis of the seismic design provisions of the first edition of the International Building Code (IBC), to be published in the spring of 2000. This paper first discusses the major changes that have been made in the concrete-related provisions from the 1994 to the 1997 edition of the UBC. The paper gives background to these changes, provides essential details on them, and indicates how they have been or how they are going to be incorporated (at times with significant modifications) into the 1997 NEHRP Provisions and the 2000 IBC. The newly published ACI 318-99, Building Code Requirements for Structural Concrete (ACI 1999), is going to be adopted by reference into the 2000 IBC. This entails further changes in concrete-related provisions beyond the 1997 UBC. Some of the more important of these changes are discussed here. A small number of amendments and additions to the ACI 318-99 provisions are going to be included in the 2000 IBC. The more important of these are also outlined in this paper.

While there have been significant advances in the design and construction of composite steel-concrete building structures, their use in regions of high seismicity has been hindered by the lack of design criteria in building codes and specifications. This has prompted initiatives in the Building Seismic Safety Council and the American Institute of Steel Construction to develop seismic design provisions for composite structures. The 1997 edition of the AISC Seismic Provisions includes a new section with requirements for composite steel-concrete structures that are cross-referenced by the general seismic loading and design criteria in the 1997 NEHRP Provisions and the 2000 International Building Code (final draft). Intended to complement existing provisions for steel, reinforced concrete and composite structures in the AISC-LRFD Specification and the ACI 318 Building Code, these new provisions provide an important resource for seismic design of composite structural systems, members, and connections.

One of the important surprises of the Northridge earthquake of January 17, 1994, was the widespread and unanticipated brittle fracture of welded steel beam-to-column connections. Although no casualties or collapses occurred during the Northridge earthquake as a result of these connection failures, and many WSMF buildings were not damaged at all, a wide spectrum of brittle connection damage did occur, ranging from minor cracking to completely severed columns. This paper summarizes two of the most important documents that have been developed in response to the damage suffered to steel moment frame buildings in the Northridge earthquake. The first, FEMA 267, Interim Guidelines: Evaluation, Repair, Modification and Design of Welded Steel Moment Frame Structures, was generated from studies undertaken as part of a project initiated by the U.S. Federal Emergency Management Agency (FEMA) to reduce the earthquake hazards posed by steel moment-resisting frame buildings. The second document addressed in this paper is the 1997 edition of the American Institute of Steel Construction (AISC) Seismic Provisions for Structural Steel Buildings (commonly referred to as the AISC Seismic Provisions) that incorporates the new information generated by the FEMA-sponsored project and other investigations on the seismic performance of steel structures, and has been adopted by reference into the 2000 International Building Code (IBC).

The need for earthquake-resistant construction in areas of low-to-moderate seismicity has been recognized through the adoption of code requirements in the United States and other countries only in the past quarter century. This is largely a result of improved assessment of seismic hazard and examples of recent moderate earthquakes in regions of both moderate and high seismicity, including the San Fernando (1971), Mexico City (1985), Loma Prieta (1989), and Northridge (1994) earthquakes. In addition, improved understanding and estimates of older earthquakes in the eastern United States such as Cape Ann (1755), La Malbaie, Quebec (1925), and Ossippe, New Hampshire (1940), as well as monitoring of micro-activity in source areas such as La Malbaie, have increased awareness of the earthquake potential in areas of low-to-moderate seismicity. Both the hazard and the risk in moderate seismic zones (MSZs) differ in scale and kind from those of the zones of high seismicity. Earthquake hazards mitigation measures for new and existing construction need to be adapted from those prevailing in regions of high seismicity in recognition of these differences. Site effects are likely to dominate the damage patterns from earthquakes, with some sites suffering no damage not far from others, on soft soil, suffering near collapse. A number of new seismic codes have been developed in the past quarter century in response to these differences, including the New York City (1995) and the Massachusetts State (1975) seismic codes. Over the same period, the national model building codes that apply to most areas of low-to-moderate seismicity in the United States, the Building Officials and Code Administrators (BOCA) Code and the Southern Standard Building Code (SSBC), have incorporated up-to-date seismic provisions. The seismic provisions of these codes have been largely inspired by the National Earthquake Hazard Reduction Program (NEHRP) recommendations. Through adoption of these national codes, many state and local authorities in areas of low-to-moderate seismicity now have reasonably comprehensive seismic design provisions. This paper will review the background and history leading up to the MSZ codes, discuss their content, and propose directions for future development.

Based on the conclusion that the primary barrier to widespread seismic rehabilitation of buildings in the United States was the lack of a consensus-backed, nationally applicable, professionally accepted rehabilitation standard, the Federal Emergency Management Agency supported the development of the NEHRP Guidelines and Commentary for the Seismic Rehabilitation of Buildings (FEMA 273 and 274). A six-year effort by a team of experienced professional practitioners and university researchers who were motivated to produce a standard that specifically addressed the differences in designing for seismic resistance in new buildings, as opposed to existing buildings, resulted in the NEHRP Guidelines and Commentary for the Seismic Rehabilitation of Buildings. These NEHRP Guidelines will provide the tools for design professionals of varying expertise in seismic design to design economical and appropriate seismic rehabilitation for buildings of essentially any size, commonly used building material and configuration.

The Applied Technology Council (ATC), with funding from the California Seismic Safety Commission developed the document, Seismic Evaluation and Retrofit of Concrete Buildings, commonly referred to as ATC 40. This two-volume, 612-page report provides a recommended procedure for the seismic evaluation and retrofit of concrete buildings. Although the focus is specifically on concrete buildings, the document provides information on emerging techniques applicable to most building types. This paper provides an introduction and overview of the document. The conceptual basis of the procedures is performance-based design using nonlinear static structural analysis. The ATC 40 document comprises a practical guide to the entire evaluation and retrofit process. Topics include performance objectives, seismic hazard, determination of deficiencies, retrofit strategies, quality assurance procedures, nonlinear static analysis using the capacity spectrum method, modeling recommendations, foundation effects, and response limits.

Efforts at improving earthquake recovery policies have been hampered by a lack of criteria and standards for evaluating and repairing damaged buildings. The Applied Technology Council has developed a performance-based methodology for the evaluation of earthquake-damaged concrete wall buildings and masonry wall buildings, recently published as FEMA 306/307/308. The methodology provides a way to quantify damage in terms of loss of seismic performance capability. It also provides guidelines for remedial measures to restore or improve seismic performance capability. In this methodology, the expected future seismic performance of a building is evaluated in its pre-event, damaged, and repaired conditions. Following the nonlinear static analysis procedure, displacement demands and capacities of the structure are used as indices of seismic performance. Identifying the governing mechanism of nonlinear deformation and the behavior mode of a structure and its components is shown to be a necessary first step towards evaluating expected seismic performance, interpreting indications of damage, and assessing their significance. The methodology provides a technical resource for understanding how buildings respond seismically on both global and component levels, and gives a basis for formulating post-earthquake policies.

Caltrans' Seismic Design Criteria (SDC) has been adopted as the minimum seismic standard for ordinary bridges on California's highways. The SDC is a compilation of new and existing seismic criteria that had been previously documented in a variety of Caltrans documents. The SDC extends the capacity design philosophy introduced in the 1980 Caltrans Bridge Design Specifications. The most significant departure from the previous procedure is that ductile members are now designed by comparing the displacement demand to the displacement capacity. The demands are generated by a linear elastic analysis, and the capacities are determined from a curvature analysis that incorporates the nonlinear behavior of the structural elements. The demand/capacity methodology supplants the previous method based on reducing the elastic dynamic forces by a force reduction factor. In this paper, the significant features of Caltrans' SDC are described.