ML20151H147
| ML20151H147 | |
| Person / Time | |
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| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 04/12/1988 |
| From: | Roeder C WASHINGTON, UNIV. OF, SEATTLE, WA |
| To: | |
| Shared Package | |
| ML20151H012 | List:
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| References | |
| NUDOCS 8808010160 | |
| Download: ML20151H147 (5) | |
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OVERV1EW OF EARTHOUAKE HAZARDS REDUCTION IN PUGET SOUND
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AND PORTt.AND AREAS THROUGH IMPROVED BUILDING PRACTICES by Charles W. Roeder, Professor of Civil Engineering, University of Washington INTRODUCTION Any discussion of seismic hazard reduction in_ building design for the Pacific Northwest region must consider the basic concepts for seismic structural design, how these concepts relate to the regional selsmicity, and how they are employed by the local practicing profession. The discus-slon must also include consideration of the local history, since many variations in practice have occurred over time. Finally, the wide range of building types and ages must be included. This paper will attempt to join these different topics 'o provide a coherent picture of the existing ha:ards and the improvements which have been made in recent years. The discussion will focus on the Puget Sound region because it is the most heavily populated area of Westem Washington and Oregon and the author is most familiar with this part of the region. However, the general comments should apply to most parts of the general region.
SEIShtC DESIGN CONCEPTS The primary design concept applied to the seismic design of buildings in the United States is based on the Uniform Building Code and the SEAOC recommendations. With this method, the structure is designed to remain elastic for a modest lateralload distribution. The magnitude of the lateral loads depend upon the regional seismicity, the soll conditions, the importance of the structure, the type of structure, and the dynamic properties of the building. The lateral forces are relatively small, and so this part of the design essentially assures that the structure re-mains serviceable during small frequent earthquakes. Much larger forces must be expected in a major earthquake, but it is not economical to design most building structures to resist these larger forces. The overall safety of the structure during a major earthquake is assured by using a structural system which is very ductile. The ductility results in much c;naller lateral forces at the cost of large permanent deformations of the structure during a major earthquake. The ductility permits the structure to dissipate large quantities of energy, and this dissipated energy dampens the dynamic response. However, it requires that the building be able to sustain large cyclic, inelastic deformations while supporting the gravity loads. The specifications are frequently ambiguous about how the ductility requirement is satisfied, but it is usually met by seioction of a well behaved structural system and good connection detailing.
Some variations of the above procedure should be noted. Some structural engineering firms have become proficient at linear elastic dynamic analysis. They obtain an appropriate ground acceleration for their bullding site, and use linear elastic dynamic analysis methods to generate a response spectra or a time dependent response for the structure. They may use these computed results in several ways. The computed forces and displacements may be examined to verify that the structure can support the gravity loads and sustain the required displacements wilhout yleiding and without failure. This method has been used to justify the design and construction of new or unusual structt.ral sys* ems, and it has also been used to justify the use of connections with questionable ductility. The computed linear Glastlc response is sometimes adjusted to account for invastic behavior. The computed forces are decreased and the deflections are increased by appropriate ductility factors. The structure must then be designed to remain elastic at these reduced forces and it must also be designed to develop the required ductility.
Other variations in the seismic 00 sign procedure have been employed. Base isolation is receiving increasing acceptance in the United States. Isolators are inserted between the foundation and the structure. The isolators enange the dynamic procerties of the structure and t
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sometimes add additional damping to reduce the dynamic response. This method is sometimes
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proposed as a method which can reduce the design requirements of the basic structure, and more important it may insure the serviceability of the structure even during extreme earthquakes.
These later methods are based on rational principles. They clearly have a range of validity, and they have received some acceptance t'y the profession. There also appears to be some rational concems with the use of these methods. Some engineers are rationally concemed that these methods may be misused, or that these methods required greater knowledge of seismicity than truly exists. However, these methods clearly have a range of validity, and they have received some acceptance by the profession. The methods also require very specific information related to the earthquake acceleration and relatively sophisticated analysis techniques.
REGIONAL SEISMICITY A elementary knowledge of the seismicity of the Pacific Northwest region is necessary tn eval-uate the selsmic risk potential. The seismicity of the Puget Sound Region is quite different 'han the California experience. The earthquakes tend to have a relatively deep focus (typicall/15 to 35 miles), and they are associated with local movements at the junction of two major faults rather than movement along a long seismic fault. Portland and other cities outside the Puget i
Sound Region are more distant from major fault locations. Ground shaking at these locations is sometimes caused by movements along smaller inland faulte, and smaller accelerations are usually expected for these cities than for the Puget Sound Region. As a result of these differences, the ground motion expected for a regional earthquake is somewhat dif,% rent than that expected in Califomia. The lack of long fault lines has led most engineers to believe that the maximum plausible earthquake has a magnitude of approximately 7.5 in the Puget Sound area.
The very deep focus of the earthquake tends 'o attenuate the peak acceleration in ground motion records, but it also tends to modify the predominate period of the acceleration record to that of the soil deposit. Thus, the properties of the soil are extremely important throughout the region.
Three major soil categories may be noted throughout the Pacific Northwest. In the mountains and other isolated locations, bedrock lies at or very near the surface. Large portions of the populated regions have very deep soil deposits over the bedrock. These soil deposits tend to be very stiff and strong due to overconsolidation which occurred in recent glacial periods. The third major category is the very soft recent deposits which can be seen in many deltas anri river basins. Probably the majority of the major buildings in the region are situated on the deep glacial deposits, but some significant structures are situated over the softer more recent river deposits. The location of the structure with respect to these deposits may have a large impact on me damage potential for the structure. The deep glacial deposits have fendamental periods in the j
range of.6 to.8 seconds, and as a result one must expect that buildings of intermediate height 4
(5 to 10 stories) have the greatest potential for severe damage during a major earthquake.
Oulte different characteristics must be expected in regions over bedrock or more recent soil deposits.
Recent developments in the regional selsmicity may further complicate the issue. It has been postulated that a major earthquake with inagnitude greater than 8.0 has a small probability of occurrence in the subduction zone off the Pacific Coast. This possibility severely complicates the evaluation of the potential seismic hazard for the region, since the peak acceleration, pre-i dominate period and duration of shaking could be very different for such an earthquake.
K! STORY OF SEISMIC DESIGN FOR THE REG ON The first settlers arrived in the Puget Sound region approximately 140 years ago. Seattle was developit Yo a significant city by the early 1900's, in the early years of recorded history,
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there is considerable evidence of seismic activity, but there were no provisions for earthquakes in the design of buildings during thH period. Many of these buildings are stillin service and are
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regarded as historic structures. Seismic design provisions were introduced in California in the 1930's, and these provisions ultimately became the provisions of the Uniform Building Code.
Seattle and the Puget Sound region was considered as Seismic Zone 1 until after the 1949 Olympia Earthquake. This effectively means that no buildings in the reglon built before 1950 had any consideration of seismic design. After 1950, the city of Seattle adopted the major portions of the Uniform Building Code for engineered structures and the seismic zone rating was upgraded. However, many other cities in the state of Washington did not adopt this specification until the late 1960's, and many buildings were built in these communities during this period.
During this same period, significant changes were occurring within the Uniform Building Code.
Strength and stiffness requirements have modified considerably in the past 30 years.
Many restrictions have been Inserted to assure the ductility of structures and components. The specification has become more rational in that il now considers the soll conditions and im-portance of the structure. Therefore, many buildings which were designed to satisfy the Uni-i form Building Code during this period would not meet the present specifications.
Changes have also occurred within the engineering profession during this period. Most structural engineering firms in this region had little if any expertise in structural dynamics as recently as 10 to 20 years ago. Thus, any buildings designed by these firms were designed with code based 3tatic design concepts. They did not always understand the importance of ductility nor did they understand the dynamic amplification that can occur with seismic excitations. Some of the buildings designed by these engineers may have potential problems even though they legally and technically satisfied the code at the time of construction. Today nearly all the regional firms have a few engineers who are famitiar with the concepts of dynamic response. The major firms in the region usually have one or two people that are highly skilled in this area. These major firms design most of the major buildings, and a number of these recent buildings were designed by alternate defgn concepts. That is, a linear elastic dynamic analysis may be performed to verify the performance of the structure, or the design forces may be determined by a linear elastic er modified response spectra. Some unusual and daring design concepts were used in some of these bulidings. While these firms sometimes use sophisticated elastic analys!s con-cepts, few if any of these firms employ inelastic analysis in their designs.
Further, ductility often does not appear to be in the forefront of their thinking. Small or Intermediate sized buildings (buildings 10 to 15 stories or less) are frequently desicoed by smaller engineering firms, and appear to be designed by the usual code based static design methods. One may logi-cally ask if all of these smaller firms have a sound understanding of the unwritten ductility requirements of the code, and if these buildings all satisfy both the spirit and legal require-ments of the code. This distinction may be quite important when it is recalled that local soil conditions suggest that these small to intermediate sized buildings are prime candidates for damage durir,g future earthquakes.
The construction methods have also changed during this period. A number cf small or inter-mediate sized buildings were constructed in the region prior to 1950. Most of these buildings have light structural frames with considerable mass contributed by unreinforced masonrf, heavy plaster, and ornate architectural fixtures.
Some of these buildings are still in tervice and are in need of renovation. These buildings typically have light structural frames which cannot possibly satisfy the present seismic desgn code. At the same time, many of these structures have survived two major earthquakes, They probably survived these past earth-quakes because of the stiffness and resistance provided by the unreinforced masonry and j
nonstructural elements. These elements are relatively brittle and design codes oo not provide a method for incorporating this strength or for estimating the degree of deterioration. As a result, this causes a serious dilemma to structural engineers, developers, and government agencies. It is generally impractical or even impossible to bring the building up to existing code standards, and the engineer is concemed that he will be legally responsible if a failure occurs 4'
with a building that is rehabilitated to less than present design standards. Government agencies
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are hesitant to assume the responsibility for legal walvers to the existing design provisions. On the other hand, nearly everyone agrees that the buildings are in need of repair, and that seismic upgrading is required. Many of these older buildings are of the intermediate height which is most susceptible to damage in the region. The balance between the opposing concerns is dif-ficult to achieve.
Many buildings built since 1950 have similar problems. They were built with little or no con-sideration of selsmic design or they were built to standards well below those required today, in addition the canstruction methods changed considerably during this period. Heavy masonry and plaster walls were replaced by light partitions and glass curtain walls. The weight and mass of the structure was reduced dramatically, but the strength and stiffness provided by these non-structural components was well below that found in older structures. The 1965 Pugtt Sound Earthquake resulted in significant damage to a number of these newer buildings. In addition, these buildings do not have the psychological advantage of having withstood the 1949 Olympia i
Earthquake. Thus, upgrading of these somewhat newer buikiings is also a question of some concern.
Most homes and many other small one story buildings in the regions are built without any en.
gineering design. These buildings are generally wood frame buildings which behave well if they are properly attached to their foundations, have good connection between members, and have chimneys which are reinforced and attached to the building. The building codes usually have minimum requirements for these details. However, the public awareness of the seismic problem in the region is not great, and the inspection requirements for the construction of these small buildings is minimal. As a result, one must suspect that there are a number of potential hazards with these structures.
SEISMIC HAZARD POTENTIAL This has been a brief and simplified discussion of the seismic design practice in building design t
in the Pacific Northwest with particular emphasis on the Puget Sound region. The evaluation of the hazard potential is a highly uncertain process because of the many variables and uncertaintles involved. However, it appears that a few important observations can be made -
- 1. The design specifications in the region have changed significantly in the last 25 years.
Today they appear to be at a level consistent with other seismic areas of this country. However, many of the buildings in this regions were built with no consideration of seismic design or by standards well below the present level. The appears to be considerable potential for damagq in these buildings.
- 2. The structural engineering profession has become much more aware of the seismic design problem and the special requirements of des:gning for dynamic loads. Some of the more i
sophisticated firms use lineer elastic dynamic analysis to help them in their designs. Essen-j l
tlally alllocal firms use linear elastic static analysis methods in their design. There does not appear to be any usage of inelastic response calculations by the local profession. Therefore, it is not clear that the profession is appropriately concerned with inelastic behavior and ductility
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requirements, it is not clear that some of the new and daring structural designs used in the region are justifled with this present state of practice.
- 3. The many older and substandard buildings raise serious concerns for rehabilitation of buildings in the region. Many of these older buildings are of intermediate size which appear to be most susceptible to damage with local soil conditions. Rehabilitation is needed and required, j.
but the complex technical and legal issues make it difficult to achieve.
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- 4. The present codes and design practice appear to be in line with recent seismic history.
This recent history has suggested that the magnitude of the maximum earthquake is limited by local geological conditions. The amplitude of the acceleration, the duration of shaking, and the predominant period of the acceleration record are strongly influenced by the deep focus of these earthquakes and the local soll deposits. However, recent research has suggested that a much larger magnitude earthquake could occur off the Pacific Coast. This much larger earthquake could change the characteristics of ground motion and greatly increase the damage potential for the region.
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