Download The Anatomy of Seismically Designed Structures

The Anatomy of Seismically Designed Structures
adapted from the 2003 CUREE Calendar
illustrated essays by Robert Reitherman
© 2002 CUREE. All rights reserved.
Consortium of Universities for Research in Earthquake Engineering
CUREE
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The Anatomy of Seismically Designed Structures
Introduction
earlier times, the Maxwell-Mohr method (Maxwell, 1864, and Mohr, 1875),
Castigliano (1873), and Hardy Cross (1930) developed the basic understanding.
This year’s CUREE Calendar provides brief illustrated essays explaining in simple
terms the anatomy of seismically designed structures.
However, there are three key concepts that are inescapable in seismic design that
were only fully developed in recent years. First, earthquake ground motions generate inertial loads that rapidly change with time. Thus, calculations that include
a term labeled with a unit of time—seconds—are common in seismic analyses,
and these terms include periods of vibration or their inverse, frequencies; accelerations; velocities; and momentum. In many other structural engineering problems, e.g., calculations of gravity loads to design floor beams, no unit of time is
used. These time-related topics are the province of dynamics, and only beginning
in the 1960s did a course in structural dynamics become part of the standard civil
engineering curriculum in the USA. (Penzien, 2002) Second, probabilistic analysis is somewhat common in many engineering fields, but it is central to earthquake engineering, and it was also a 1960’s addition to the basic civil engineering
curriculum. (Benjamin and Cornell, 1970) The third fundamental earthquake engineering concept that distinguishes this field is that the earthquake loading can
be so severe that the materials must often be designed to behave inelastically.
Within the domain of Hooke’s Law, stress is proportional to strain, but beyond
that point, behavior becomes complex. Most of the analytical and experimental
work investigating inelastic behavior began approximately in the 1960s, (see for
example Newmark and Veletsos, 1960). The development of analysis and experimental methods to adequately account for inelastic behavior is still a growth industry.
The 206 bones and 639 muscles of the human body, along with the necessary
connecting material such as ligaments at the joints, make up the structural elements of the human being’s anatomy. Each of these elements has a particular role.
For example, the exertion of tensile forces in the biceps has the same action as the
tension in the top chord in a cantilever truss. In conjunction with the compression
member (humerus bone/bottom compression chord) it generates a moment, a rotation, that tends to hold up or raise your arm. If you wish to make your arm
swing downward and push on something, you generate tension in the muscle on
the underside of the cantilever, the triceps. These structural elements can be treated
as if they were components of “the human machine.” (Bridgman, 1939) Generations of medical students have learned about muscles, or myology, in the treatment of the classic anatomy textbook by Gray (1825-1861). In that work, terms
familiar to structural engineers such as “mechanics,” “lever arm,” and “parallelogram of forces” are used, and the structural actions are elegantly analyzed using
principles of statics to determine the forces the muscles exert. Osteology (study
of the bones) is an important aspect of anatomy. Bones can break, but doctors
probably spend more time learning about and dealing with the joints that connect
the bones in the overall skeleton. Structural
engineers similarly spend more time studying the joints in their seismically designed
structures than the members themselves,
(though they call this “detailing” while doctors call it syndesmology). In the discussion
here of the structural elements that make up
the anatomy of a seismically designed strucDiagram of tensile forces exerted by the biceps ture, connections will be repeatedly empha(bottom left) and triceps (bottom right).
sized.
Rarely is there an undergraduate civil engineering semester-long course devoted
to the subject of earthquake engineering that requires teaching the basic concepts
of dynamics, probability, and inelasticity, woven into the context of the kit of
parts, the physical elements, that are used to design structures that perform well in
earthquakes. (SJSU, 2002) At the master’s degree level (Master of Science in
Civil Engineering, or Master of Engineering degrees) many universities offer a
specialization in earthquake engineering. The entry level to the subject of earthquake engineering is usually encountered by engineering or architecture students
in their undergraduate years in occasional brief references to earthquakes during
classes on other subjects, such as introductory structures, introductory dynamics,
or a geotechnical engineering course. Thus, it may be helpful to offer in one place
short summaries of the basic seismic-load-resisting elements a practicing engineer uses in designing buildings, towers, bridges, and other structures,
illustration source: Bridgman, 1939, p. 42.
While the state of the art of seismic design has progressed greatly in the last half
of the twentieth century, some of the basic knowledge of earlier decades formed
the platform upon which it could be built. For example, trusses, and thus braced
frames, can be analyzed using the method of joints (Squire Whipple, 1847), sections (A. Ritter, 1862), and graphics (Clerk Maxwell, 1864). Though momentresisting frame analysis is now conducted more efficiently and accurately than in
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his or her head at the same time. Design and analysis are parallel, interacting
processes, not opposites. Thus the subheading above is labeled design “vis-àvis” analysis not design “versus” analysis.
Design vis-à-vis Analysis
The design aspect of these elements is featured here, rather than their analysis.
Conversely, most of the educational experience of an engineering student relates
to analysis, not design. For example, homework assignments and exam questions
frequently probe a student’s knowledge of how to analyze a truss or frame element, but less often deal with how to select the element and proceed through
design steps. Sometimes the terms “seismic analysis” and “seismic design” are
loosely used interchangeably. To be precise, however, there can be no seismic
analysis until some seismic design has already occurred. At a minimum, a preliminary or schematic seismic design must be produced. Perhaps this preliminary
seismic design is a hand-drawn sketch showing approximate dimensions, the overall
configuration of the structure, and notes on what the materials are. Perhaps it is a
stick diagram of a frame, with initial assumptions as to some typical member
sizes, allowing for a quick computer analysis. In many cases with buildings, a
relatively complete architectural preliminary design is the starting point. A structural designer typically proceeds through a number of iterations of design and
analysis—first proposing a solution and then testing that solution against code
provisions and engineering principles—to evolve a preliminary seismic design
concept into a finished design ready to be built. Good structural designers acquire
the knack or intuitive ability to efficiently apply analytical tools in resolving the
key design decisions that must be made first, which will then allow for an efficient
design process to refine other design decisions.
Architecture and Engineering
Some of the important decisions concerning the seismic design of a building, the
structural design that eventually appears on the structural or “S” sheets in the
construction drawing set, are made by the architect. Typically, the architect has a
prime contract with the owner. The structural engineer and other consultants,
such as mechanical or electrical engineers, landscape architects, and acoustical
experts, are subcontractors to the architect. Decisions as to the external shape of
the building and even the precise location of internal structural elements, and
determination of some of the structural materials, are typically made by the architect in his or her preliminary architectural design, often with only a few consultations with the structural engineer.
Many of us have had the experience as children of taking a clock apart to literally
“see what makes it tick.” Very few of us have any childhood memories of being
able to put the clock back together again. In learning a foreign language, it is
feasible for an intermediate-level student to learn how to diagram a complex sentence in detail, taking it apart to label each past perfect tense, each participial
phrase, and so on. However, it takes true mastery of the language to start with a
blank piece of paper and produce a complex, well-written sentence. Designing is
the process of writing down a statement, and analysis is the process of critiquing
that work to try to find errors in it or verify its accuracy. In that process of first
hypothesizing and then testing, it is often true that “it is easier to malign than
design.” Lateral force calculations are essential, but they follow the initial decisions as to the structure’s layout. “It has long been acknowledged that the seismic
resistance system of a structure is just as important, if not more important, than
the actual lateral design forces.” (Holmes, 1976, p.827) Though analytical tools
become more sophisticated every year, the basic design task of proposing a structural system and selecting materials and connections remains as demanding a task
as ever. The skillful structural designer keeps both modes of thought working in
Floor plan of Banco Central, 1972 Nicaragua Earthquake. Note the eccentric location of reinforced concrete walls. “Had a strong north-south component of shaking been present, considerably more damage would have occurred at the east end due to torsion induced by the elevator
cores.” (Wyllie, 1973, p.578)
That fait accompli is often the starting point for the engineer’s design work. This
is not to say that this common practice is efficient or desirable. “If we have a poor
configuration to start with, all the engineer can do is to provide a band-aid—
improve a basically poor solution as best he can. Conversely, if we start off with a
good configuration and a reasonable framing scheme, even a poor engineer can’t
harm its ultimate performance too much. This last statement is only slightly exaggerated. Much of the problem would be solved if all structures were of regular
shape, but economics of lot sizes and arrangements, various planning requirements for efficient use of space, and aesthetically pleasing proportions require the
structural engineer to provide for safe constructions of various shapes.”
(Degenkolb, 1977)
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ferentiated from the case that has only one of these. For purposes of tabulating
design coefficients, such as the R or response modification factor, the NEHRP
Provisions (BSSC, 2001, chapter 5) tabulates 67 combinations. In the Uniform
Building Code, K factors were assigned for many years to structures based primarily on the basis of a few kinds of vertical elements resisting lateral forces, for
example assigning a coefficient of 1.33 to shear wall/bearing wall systems and
half that value for “special” (meeting special seismic requirements) moment-resisting frames. (SEAOC, 1999, Appendix C)
Choices for the Lateral-force-resisting Elements of Structural Systems
There is an ever-increasing use of response modification techniques that change
the seismic demand on the lateral-force-resisting elements, but it is a rare structure that is not comprised of elements featured here. Such techniques are aimed at
changing the forces in the structure due to ground motion (e.g. seismic isolation)
or at changing the displacement within the structure due to the ground motion
(e.g. damping devices).
The taxonomy of the vertical elements in a seismically designed structure is quite
concise. The choices are limited to the following, with very few exceptions: braced
frames (vertically-oriented truss elements), moment-resistant frames (formerly
called rigid frames), and shear walls. Shear walls are so named because of the
loading they resist—they counteract the tendency of one story of a structure to
move sideways, or shear, vis-à-vis another—not because of their internal forces
or stresses, which in some cases are dominated by flexure, not shear.
The vertical elements need a “lid on the box,” a horizontal element to tie them
together and distribute forces in plan, and the diaphragm fills this role. While
Shear wall
The materials of which these elements can be made, again with very few exceptions, are limited to:
Steel (we include here other metals, such as aluminum)
Reinforced masonry
Reinforced concrete
Wood
1.
2.
3.
4.
Wood
Reinforced Concrete
Reinforced Masonry
Steel
Braced frame 5. Steel
6. Ecentrically
Braced Frame
7. Wood
(and other variants)
Some of these combinations of elements and materials are rare and are not treated
here with their own page of description. Braced frames can be made of reinforced
concrete, and occasionally such bracing in a bridge tower is seen, but most braced
frames have been made of steel or wood. Masonry forming a braced frame would
be an extremely unusual case. Moment-resistant frames, while theoretically possible in any material, can be excluded here with regard to masonry and wood.
Because the eccentrically braced frame is a new breed that should be distinguished
from other braced frames, it is treated here as its own type of element in its own
essay. The vertical cantilever or inverted pendulum, such as the ubiquitous example of a street light, or the hybrid case of the cable-plus-cantilever structure of
the telephone pole, are not treated here, though they are types of elements commonly encountered in bridge design. Other examples of types of elements that
resist earthquake loads that are outside the scope here include shells, arches, and
suspension bridges.
Moment frame 8. Reinforced Concrete
9. Steel
Diaphragms
Other taxonomies or classification systems are possible, and seismic codes usually divide up these vertical elements into more specific categories, so that design
provisions for a shear wall or moment-resisting frame with special ductile detailing is differentiated from one that has lesser detailing requirements, or the structural system that includes moment-resisting frames as well as shear walls is dif-
The basic choices for lateral-force-resisting elements.
illustration source: Arnold and Reitherman, 1982, p. 37
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10. Reinforced Concrete
11. Wood
there are historic buildings, especially in Europe, that have horizontally spanning masonry that serves as a diaphragm, masonry diaphragms may be neglected here in
the context of modern seismic design. Steel diaphragms in the form of trusswork are sometimes found in buildings, but this is more often restricted to industrial
structures or bridges, and that combination of material and type of element is also excluded here. Wood and concrete diaphragms make up the vast majority of
diaphragms encountered in seismic design. (Metal deck is somewhat common on roofs, but is not treated here).
The basic choices are very limited, though there is a vast number of ways this “kit of parts” can be used to assemble diverse structures. It is no surprise that earthquake
insurance rating schemes (described by Steinbrugge, 1982), or loss estimation methods (e.g., HAZUS, NIBS, 1999) are based on roughly ten basic structural system/
material combinations, with the addition of subclasses for ranges of number of stories. For most purposes, the matrix of elements and materials reduces our combinations to those listed on the prior page, each of which is featured for one of the twelve month pages.
The astute reader will have noted that the above list sums to eleven. Has the December page of this calendar been left blank?
No, there is a twelfth topic covered here, and it appears as an obvious addition when we realize that there are actually five “structural” materials, not just wood, steel,
concrete, and masonry. The fifth is the very ground on which the structure is founded. Increasingly, the fields of geotechnical engineering, foundation design, and the
earth sciences, employ sophisticated quantitative techniques that parallel the methods used in structural engineering, and in any seismic design problem, the “foundation for the foundation” must be considered along with the manmade materials that constitute the structure above. While the earth in which the foundation is
embedded is generally taken as a given and not “designed,” even that assumption is not always true, now that large-scale soil remediation techniques can be employed.
And a structural designer’s image of his or her structure cannot stop at the base and leave a blank spot on the conceptual sketch where the ground begins. Decisions
concerning framing plans and other aspects of the structure determine gravity and seismic loads on the foundation, which can be economically resisted only if soil
properties are taken into account in the early design states. The input motion to the structure from an earthquake is modified by local soil conditions and is an essential
ingredient that must be considered from the beginning. Soil-structure interaction is a major field in its own right that is devoted to the way the earthquake’s vibrations
through the ground are modified by the fact that from the earthquake’s point of view, there is a significant “outcropping,” i.e., a building, tunnel, bridge, or other
structure, built on or into the earth.
The state of development of the rational analysis of the earth that supports a structure is maturing today in many ways parallel with the way the properties of the
structural elements are considered in seismic design. A few of these commonalities are: how the materials dynamically respond to the earthquake based on frequencies and damping; the major distinction in inelastic behavior as compared to elastic behavior; estimation of how much displacement and deformation occurs as well
as the acceleration levels; how the varying amounts of knowledge about the material properties probabilistically affects the uncertainty of the resulting calculations;
consideration of the fact that the loading occurs over tens of seconds of rapidly changing movements during the earthquake, rather than in a static way, thus requiring
a consideration of the history of previous cycles of motion.
Non-seismic Factors in the Selection of Seismic-Force-Resisting Elements
Non-seismic considerations influence a structure’s seismic design in many ways. The following tabulation provides one simple, generalized way of looking at this
relationship. Space allows only one or two examples of seismic design implications with an example of a related non-seismic design factor. In some cases, the nonseismic influence is neutral or favorable, but in other cases, it introduces a major seismic design problem. Typically, the non-seismic design factors are set first during
the design process, and then the seismic design adjusts itself to those determinants. Just as engineering students are often unaware of the primary design role of the
architect in contractual terms, it is often a surprise to them that seismic design, at least for most buildings, and to a considerable extent even for bridges and industrial
structures, occurs within constraints already set by non-seismic considerations. One of the toughest tasks in structural engineering is to design a building to resist
strong earthquakes, when the earthquake that will test the construction is very uncertain as to when and where it will occur, its motions are very random, and the
capacity of the building to inelastically respond is known much less reliably than in the case of resistance to most other loads. Yet this tough seismic design problem
is only rarely the top priority, because the owner and architect are also confronted with issues that are judged by them as more immediate: efficiency of the functional
spaces, internal and external aesthetics, and compliance with non-seismic code and planning requirements.
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Seismic and Non-seismic Considerations for Selection of Type of Seismic Element and Material
Non-seismic Design Factor
Examples of Seismic Design Implications
Occupancy-related Configuration
The lobby level, usually the ground story, of a large public
building such as a hospital or office building, is often about
1 1/2 to 2 times the height of the other stories above and has
few or no walls.
Soft or weak stories: While the seismic loads are greatest at the bottom level, the floor plan imposes the
requirement that there be no solid interior or exterior walls, or walls spaced far apart, whereas in the above
stories, a shear wall system with cellular layout may be possible. This may create a lobby level weaker than the
story above, or shear walls that are discontinuous at the lobby level. Even with frame structures, tall stories can
be much more flexible (“softer”) than other levels.
Fire Protection
If Type I construction (the most fire resistant) is required,
reinforced masonry or reinforced concrete walls, or reinforced concrete or well-fire-protected steel columns, will be
required, and the diaphragm will be reinforced concrete.
Locations of structural materials and masses: These ramifications are not necessarily problematic from a
seismic design standpoint, but note that exposed steel or timber walls or floors may be eliminated as options,
and that buildings that have high fire protection levels are higher in mass than they would otherwise be. Fire
sprinkler water tanks are also significant concentrated masses in high-rise buildings.
Acoustics
If sound transmission through walls and floors must be
reduced to a very low level, then concrete floors and
concrete or masonry walls are often used.
Shear wall locations: Introduction of localized areas of stiffness due to concrete or masonry walls in a
woodframe building can be advantageous or disadvantageous, depending on the wall’s location. Walls that
cause eccentricity of center of mass from center of rigidity lead to torsion. Concrete topping on wood floors
increases their rigidity and mass.
Construction Cost
Low construction budgets are a fact of life.
Low structural engineering design budgets go along with low overall construction budgets, reducing the amount
of time that can be spent on design as well as minimizing or eliminating construction inspection by the designer.
Introducing a steel moment frame at strategic places in a woodframe building (e.g., garage openings, “soft front
walls” of retail buildings) may seem like a simple engineering feat, but may push the construction cost beyond
budget.
Appearance
Beauty is in the eye of the beholder, or as the late architect
George Simonds once said, “form follows fashion.”
Unfavorable configurations are produced as well as opportunities: Pilotis (stilts) made popular by modern
architecture can cause a soft story. If structural elements are featured architecturally (“structuralism”) then there
is wider latitude and more creativity in the seismic design, though detailing becomes more expensive to make
members, and especially connections, aesthetically pleasing.
Energy Conservation, HVAC System,
Environmental Concerns
Materials differ in thermal mass and insulation.
Selection of horizontal members, wall locations, materials: Large ducts may necessitate trusses in the suspended ceiling space, rather than beams, though beams would more easily allow the development of a momentresistant frame. Introducing high mass into wood residences for “thermal flywheel” reasons also introduces
high stiffness where concrete or masonry walls are located.
Gravity Load Resistance
With the exception of the eccentrically braced frame, all of
the structural elements used in seismic design were first
developed to resist gravity loads or wind, and then in the
20th century “retrofitted” to perform seismic roles.
Column or wall spacing is usually set by span limits for resisting gravity and to correspond to floor plan
requirements, and then seismic considerations are considered. The ability of steel frames in the decades prior to
the 1994 Northridge Earthquake to achieve longer economical spans allowed fewer columns to be placed farther
apart and also increased beam depths. This trend with regard to columns and beams negatively impacted the
ductility of steel moment-resisting frames.
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Seismic Provisions in Codes and Standards
Numerous prescriptive design requirements for all the various elements discussed
here are provided in codes and standards for buildings, bridges, and other kinds of
construction. No attempt has been made in these brief essays to cite the numerous
relevant documents or summarize their provisions. Oftentimes university architecture and engineering instructors in structures classes that are design-oriented
use code documents as basic texts to relate structural theory to structural practice.
The commentary portions of the NEHRP Provisions (BSSC, 2001), SEAOC blue
book (SEAOC, 1997), FEMA’s Seismic Rehabilitation Guidelines (ATC, 1997),
and Uniform Building Code and related International Building Code seismic provisions (ICBO, various) can be treated as, in effect, textbooks that help to bridge
between the analysis rules the codes mandate for checking designs, on the one
hand, and the concepts and procedures used to produce designs on the other. It
must be remembered, however, that a code should not be considered a complete
manual or “cookbook” but rather a standardized set of minimum legal requirements that must be met in the process of conceptualizing and engineering a safe
and efficient structure.
With regard to earthquakes, the case-by-case empirical approach is mismatched
against a phenomenon that is so uncertain and hard to predict. First, the earthquake shaking that was recorded in the last earthquake is not exactly the same as
what a given structure will experience in the future; secondly, the structure that
experienced an earthquake is not identical to the one that a structural engineer is
designing today. (Even if the “same” structure that was exposed to the last earthquake is to be exposed to an earthquake that could be made to be identical to the
last one, the structure will likely be different in its characteristics because of that
very experience of the first earthquake). The “recipe” of complete structures is
composed of many ingredients--many elements, layouts, details, sites. The longterm goal must be to evolve from an empirical basis to a sound theoretical foundation, being always open to new insights provided by practice, analysis, experimentation, and actual earthquakes.
While far from a comprehensive historical review, a brief list of key seismic design lessons provided by earthquakes is shown below. Because the essays that
follow on the anatomy of the elements of a seismically designed structure are
limited to American practice so they can fit a concise format, it is important to
note in passing the major contributions of researchers and engineers in other countries who have learned from their own earthquakes.
A Lesson Plan Provided By Earthquakes
As an aspect of the physical world becomes fully understood, science is able to
explain and predict a given phenomenon, and verified theory supplants or complements reliance on empirical data.
While the capability of engineering analysis today to predict structural and
geotechnical behavior in future earthquakes is remarkably advanced compared to
what it was only a few decades ago, there are other earthquakes that will inevitably occur that will provide more empirical data with which the theoretical basis of
seismic design will be advanced. The seismic design lesson plan provided to
engineers by actual earthquakes has yet to be completed.
Merely by mapping the wells that supplied drinking water in central London and
the locations of fatalities from an outbreak of cholera in 1854, Dr. John Snow was
able to identify the source of a cholera outbreak. (Gilbert, 1958) Snow realized
from the empirical data that for some reason, the water from one particular well
was the source of the disease, and after he had that pump’s handle removed, the
outbreak was controlled. This empirical approach to graphing (mapping in this
case) the data, and looking for a statistical pattern, could identify the geographic
source but not the causal mechanism. Snow’s work was 25 years before Louis
Pasteur experimented with the cholera virus in chickens and developed a vitiated
strain that could provide an immunity for an inoculated organism, all based on
Pasteur’s theory or understanding of the problem, which is the justly famous Germ
Theory of Disease. The applicability of that theory to other diseases, with several
common types of immunizations routinely saving millions of lives, was far beyond what was possible within the case-by-case empirical level of understanding.
Robert Reitherman
Acknowledgements
While any errors or omissions in these essays are solely my responsibility, the
helpful review comments provided by the following are gratefully acknowledged:
Gregg Brandow, Kelly Cobeen, Bill Holmes, and Andrew Whittaker.
6
The Instructiveness of Destructiveness
1908 Reggio-Messina, Italy Earthquake
A commission of engineers studying this earthquake proposed, in
essence, the equivalent static lateral force method, with a lateral
coefficient of 1/12 applied to the ground story mass and 1/8 to the
second story. (Freeman, 1932, p. 566)
1923 Tokyo or Great Kanto Earthquake
Reinforced concrete structures as tall as ten stories designed
by Tachu Naito, using structural response theories developed
by Riki Sano, Kyoji Suyehiro, and others in Japan in the
teens and twenties, performed well. (Reitherman, 1998)
illustration source: John. A Freeman
illustration source: EERC-NISEE
1933 Long Beach, California Earthquake
Unreinforced masonry’s poor performance led to the development
of modern earthquake resistive (reinforced) masonry, and this
earthquake also produced the first significant earthquake
accelerogram.
1971 San Fernando, California Earthquake
Reinforced concrete moment-resisting frame buildings and
bridges were especially hard hit, leading to rapid acceptance
of the need for detailing providing ductile behavior in
concrete. Pictured is the complete ground story collapse of
the Psychiatric Building at Olive View Hospital in the
1971 San Fernando Earthquake.
illustration source: Security Pacific National Bank Collection
Los Angeles Public Library
illustration source: Steinbrugge Collection, EERC-NISEE
1994 Northridge, California Earthquake
Compared to the vast population of structures in the Los Angeles
region the number of damaged wood and steel buildings was not
large. Nonetheless the damage was deemed by many owners,
engineers, and code officials to be excessive, and research
programs on these topics were launched.
illustration source: Robert Reitherman
1995 Kobe (Hyogo-Ken-Nanbu, Great Hanshin) Earthquake
This is one of the most instructive earthquakes to date for illustrating
the effects of a near-fault (kinchi jishin) earthquake, where the strongest
shaking (meizoseismal area) was a “bull’s eye” direct hit on a very
urbanized region. The top of this toppled building is at left, across the
street from its base.
illustration source: Charles A. Kircher
7
The familiar bridge or roof truss is loaded vertically by gravity and spans horizontally, while the braced frame is loaded primarily horizontally
by seismic inertia loads and in essence acts as a vertical cantilever. The basic unit of a truss and the source of its stability is the triangle, a
structural unit that resists structural loads via development of axial forces in its members. Pure truss action results if the forces in the members
are aligned with (concentric with) the centerlines of pinned joints. This distinguishes the concentric braced frame discussed here from the
eccentrically braced frame (discussed separately). The braced frame is a direct, economical, and elegant seismic solution -- virtually a fullscale diagram of the forces flowing through it. Directness of the path taken by forces in the members is also a potential seismic disadvantage:
When the strain in a braced frame member exceeds its elastic limit, there is no place for the system to “let off steam” safely unless special
seismic, i.e. inelastic, design features are incorporated, or special devices employed. “Buckling of beams and columns cannot represent
acceptable means of dissipating seismic energy as such response would endanger the gravity load carrying capacity of the structure. Hence
inelastic action under earthquakes must only take place in the diagonal bracing members and adequate detailing must be provided to ensure
that the braces can go through the expected inelastic demand without premature fracture.”
(Tremblay, 2001)
A basic seismic design principle is that the structure should gradually deform as the seismic
Seismic Retrofit of the Bennett Building
load increases into the inelastic range, allowing it to dissipate energy safely rather than illustration source: GSBS Architects, Reaveley Engineers
suddenly breaking. The title of a recent textbook points out this importance: Ductile Design
of Steel Structures. (Bruneau, Uang, and Whittaker, 1997) The non-ductile braced frame behaviors that must be prevented include the following:
(1) The diagonal delivers too much force to the connection at the beam-column joint and the connection breaks; (2) The brace or other member
buckles in compression; (3) A tension-only (e.g. tie-rod) diagonal stretches inelastically, seemingly a benign deformation, but on the next
repetition of a cycle when it is again loaded in tension, there is slack in the system and the frame must resist a“slamming” effect, and the hysteresis
loop is pinched; and (4) If diagonals frame into columns (as in a K brace) or beams (as in a V or chevron brace), the force delivered by the brace
damages the column or beam. In addition, modern seismic codes encourage redundancy. At a given story, on each line of bracing, diagonal
braces should share the lateral load exerted in a given direction by having some resist in tension while others take compression.
"The senses of the forces [tension or compression] in the
diagonals can be determined by first imagining them to be
removed and then ascertaining their role in preventing the
probable type of truss deformation that would occur." (Schodek, 1980, p. 126)
photo source: Godden Collection, EERC-NISEE
The fact that the diagonal is dedicated to a seismic role makes it an ideal place for application of innovative devices such as fluid dampers. The
recent retrofit of the Bennett Federal Building in Salt Lake City illustrates another technique, which was pioneered in Japan in the 1980s: “The
basic concept of the Unbonded Brace is the prevention of compression buckling of a central steel core by encasing it over its length in a steel
tube filled with concrete or mortar. A slip interface, or ‘unbonding’ layer, between the steel core and the surrounding concrete is provided to
ensure that compression and tension loads are carried only by the steel core…. inhibiting local buckling of the core as it yields in compression.”
(Brown, Aiken, and Iafarzadeh, 2001) The term used in an early paper by Watanabe et al., 1988 clearly describes the concept: “Brace Encased
in Buckling-Restraining Concrete and Steel Tube.”
Steel braced frames are often used in low-rise buildings, which, combined with the high stiffness of the bracing, tends to put these structures at the low-period end of the response spectrum. This
in turn usually means higher response (e.g., greater spectral acceleration) than in the long-period range, and these accelerations affect the structure as well as the equipment and contents. The
positive aspect to a stiff, low-period structure, however, is that it tends to protect built-in nonstructural components such as partitions from drift-induced damage. Steel braced frames are very
frequently used as the vertical structural elements providing lateral force resistance for bridges, electrical transmission towers, elevated water tanks, and non-building industrial structures.
Steel Braced Frames
CUREE
Trusswork has no equal in boldly expressing structural design. Famous non-seismic design examples where
the bracing is part of the "trademark" of a famous landmark include the Eiffel Tower in Paris and the George
Washington Bridge in New York. Seismic design examples, where the braced frames that resist earthquakes
are intentionally designed to provide a prominent architectural effect, are common in new buildings, and
braced frames are frequently expressed on the exterior of seismically retrofitted structures as an inexpensive
structural measure that also adds visual appeal.
CONSORTIUM of UNIVERSITIES for RESEARCH in EARTHQUAKE ENGINEERING
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It could be argued that some of the various wind design knee-bracing layouts for high-rise buildings
in the late 1800’s and early 1900’s were early examples of the eccentrically braced frame (EBF).
The EBF, however, is not merely any diagonally-braced frame in which both ends of the brace do
not connect at beam-column joints in a concentricity of center lines of all the structural members
involved. It is a framing system in which the eccentricity is intentionally planned, rather than
introduced as an accommodation to fenestration or other architectural layout considerations, and the
location of inelastic behavior is strategically and very explicitly designed to occur in beams in such
a way as to not threaten the gravity-load-resisting system.
Embarcadero IV Building, San Francisco, the first
large eccentrically braced frame structure
photo source: Andrew Merovich
To appreciate the degree of innovation represented by this invention, consider the other basic types
of lateral-force-resisting elements. There is only a limited menu from which the structural designer
can choose. Walls have been around for several thousand years for the purpose of enclosing space;
they were not invented to resist earthquakes, they were merely adapted for that purpose by earthquake
engineers who modified the materials and the detailing. Column-beam frames with joints that are
relatively rigid in resisting moments as the frame deforms under sidesway were constructed in metal
(iron, then steel) with various riveting and bolting techniques in Europe and the Eastern United States
without any seismic design in the 1800’s. The first building with extensively welded column-beam
joints, the Westinghouse factory in Sharon, Pennsylvania, built in 1926 (Condit, p.192) was
constructed without thought for earthquakes. Reinforced concrete frames were designed and
constructed as efficient vertical-load-resisting elements and only slowly “retrofitted” in the latter half
of the twentieth century to become reliable seismic elements. Earthquake engineers gradually
learned from earthquakes, such as the 1967 Caracas and 1971 San Fernando, how to modify nonseismic concrete frames to perform well as seismic load-resisting elements. Concentrically braced
frames of K, V or inverted V (chevron), X, or single diagonal configuration have been used for
centuries in timber, later in iron, and finally steel, in non-seismic regions. These braced frame layouts
were appropriated for seismic use and gradually modified. None of these three basic types of vertical
structural elements that resist horizontal seismic forces—walls, braces, frames—was invented
because of earthquakes, and all three have had to be modified by earthquake engineers to take into
account inelastic behavior and dynamic response. Starting with a “blank piece of paper,” and
devising a new structural element specifically with earthquakes in mind as was done with the
eccentrically braced frame by Fujimoto in Japan (Fujimoto et al., 1972) and Popov in the USA
(Popov, 1980), is thus very noteworthy.
Early prototype analytical EBF model "Eccentric
Seismic Bracing of Steel Frames," Egor Popov,
Proceedings of the Seventh World Conference on
Earthquake Engineering, 1980
Charles Roeder and an early physical EBF model
photo source: Egor Popov
Eccentrically Braced Frames
CUREE
Frames with moment-resisting joints advantageously respond to earthquakes in a flexible manner and have the
potential for high ductility, while braced frames have greater stiffness and reduced drift-induced nonstructural
damage. The eccentrically braced frame (EBF) offers some of the virtues of both. The development of this new
structural system, with earthquakes specifically in mind, is an example of the continuing trend toward energy
dissipation devices and strategies. H. Fujimoto tested eccentrically braced K braces in 1972, and in its current form
it can be dated to 1978 with the experimental work published by Egor Popov (1913 - 2001), Charles Roeder, and others.
CONSORTIUM of UNIVERSITIES for RESEARCH in EARTHQUAKE ENGINEERING
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The knee brace, which could be considered a hybrid of the braced frame and the moment-resistant frame, is sometimes used in timber structures,
but only for storage sheds and similar light mass (light earthquake load) cases where a large amount of flexibility does not cause drift-induced
damage to the nonstructural components that are present in most occupancies. Generally, wood posts and beams cannot be feasibly joined together
rigidly enough to create a joint that can resist rotation. The cabinetmaker’s rigid joinery on the scale of a chair or table tends to be overwhelmed
by the forces experienced by full-scale buildings or other structures. There is structural research interest in the subject of timber moment-resisting
frames, (Ceccotti and Karacabeyli, 1998), but such elements are to date rare. Thus, the two common options for vertical timber elements for
resisting lateral forces are shear walls (separately discussed) and braced frames.
It is difficult to make the connections in a wood braced frame as strong as the members. Unfortunately, a basic principle of seismic design calls
for just the opposite. It is desirable for connections to develop the full strength of the members so that a brittle failure at a joint cannot suddenly
occur. In addition, if the mode of failure of a connection is for the wood to suddenly split and lose its hold on the bolts or other hardware connecting
it to another member, ductility is compromised. Another seismic design principle is redundancy, but a timber braced frame tends to rely on a small
number of members and connecting bolts, whereas a single wood shear wall panel typically draws on the capacity of dozens of nails that each
tends to deform with great ductility. In referring to the timber braced frame research program conducted at the University of British Columbia
in collaboration with Forintek, Helmut Prion notes: “Because of their inherent stiffness and low ductility, braced frames are particularly vulnerable
in severe earthquakes. The only elements that can provide any significant amount of energy absorption are the connections, and therefore special
attention needs to be paid to connection detailing to assure ductile behaviour under large deformations.” (Prion et al., 1999, p. 64)
Light diagonal bracing inserted into woodframe walls
constitutes a "rule of thumb" braced frame, but is very
weak and brittle as compared to plywood or OSB
sheathed shear walls or engineered braced frames
utilizing bolted connections.
illustration source: Anderson, 1970
At one time woodframe walls that were composed of 2x4 (now 1.5 in. by 3.5 in.,
38 mm x 89 mm), or sometimes larger studs, often relied on diagonal wood bracing
for their earthquake resistance. After World War II, the advent of plywood
generally superseded these braces. Light braces built into the stud wall framing act essentially as small braced frames, but they
tend to quickly lose capacity in the inelastic range. Cut-in braces use pieces of wood the same depth as the wall studs and are inserted
between them so that the segments line up to form a diagonal. Let-in braces use one continuous piece of wood, typically a oneby (3/4 inch in thickness), notched into each stud. Where the ends of this diagonal lap the horizontal framing (plates) above and
below, there is room for only a few nails to provide this critical connection.
In this full-scale experiment with a specimen simulating a 7.5 m (25 ft)
high braced frame for a tall one-story building, connections were carefully designed as true pin joints to avoid secondary stresses.
illustration source: Yasamura, 1990
The flow of forces in a timber and a steel braced frame of the same layout are essentially the same under earthquake loads in the
elastic range, though the path of forces, the mode of behavior, and seismic design philosophy in timber and steel braced frame twins
can be quite different in the inelastic range. One indication of why there would be a difference is that grades of steel have welldefined yield points, whereas wood engineering researchers must debate at length and agree on an “effective” or reference yield
point for analytical purposes. The splitting, crushing, and breaking of a piece of wood in the range defined as inelastic does not
conform to a classic definition of ductility. As with the steel braced frame, a problem that timber braced frames must confront is
that the directness of the axial pathway of forces through the braced frame’s members does not automatically provide a place for
inelastic energy dissipation, though dampers and other innovative applications have been proposed. (Symans, et al., 2002)
Wood Braced Frames
CUREE
The wood members in braced frames are often large in cross section, either to handle their structural loads or
because fire provisions in building codes have required that members be nominally eight inches (200 mm) in
minimum dimension to qualify as Heavy Timber. Not everyone likes exposed concrete or steel; concrete block
is less than universally loved. But virtually everyone likes large timbers. Thus the wooden braced frames in
industrial buildings remodeled into shops, or in the restaurants or residences with dramatic interiors, have the
unique advantage of showing off a structural material that is also aesthetically appreciated.
CONSORTIUM of UNIVERSITIES for RESEARCH in EARTHQUAKE ENGINEERING
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The first large multi-story reinforced concrete frame building in the United States, using the Ransome method of reinforcement, was the Ingalls Building in
Cincinnati, Ohio built in 1903. By 1910, concrete construction had already become very popular. (Condit, p. 241) This element was invented long before modern
seismic design evolved. Most large concrete buildings designed to resist earthquakes in the first half of the 20th century in California, had a dual system with
a perimeter frame of concrete or steel and interior core shear walls of reinforced concrete, or at least masonry infill. As time went on, designs became more
adventuresome and started to rely primarily on concrete frames for lateral resistance, opening up more floor space.
Spirally reinforced column at left;
column with #3 (71 sq. mm) widely
spaced ties at right. Olive View
Hospital, 1971 San Fernando Earthquake.
source: Steinbrugge Collection,
EERC-NISEE
Earthquakes such as in 1957 in Mexico City, 1964 in Alaska, and 1967 in Caracas pointed out the potential for collapse of inadequately ductile concrete frames
in severe earthquakes. As far back as 1935, during the Helena, Montana Earthquakes, a multistory school with reinforced concrete frame-plus-hollow-tile walls
had collapsed. (Steinbrugge, 1970). The damage to the Olive View Hospital in the 1971 San Fernando Earthquake (Mahin et. al., 1976) clearly illustrated the
value of spiral or other close-confinement transverse reinforcing in columns as well as toughness throughout the beam-column joints. Confinement (hoops or
spirals in the column, stirrups in the beam near joints), continuity (rebar lap splice details and the presence of bars running through joints or anchored securely
in them with 135 degree bar bends rather than just right angles), and other ductile detailing had not kept pace with
construction practices. "From the perspective of reinforced concrete construction, the San Fernando earthquake
represents a turning point in structural engineering practice." (Wood, 1998, p. III-25) Significant research by the concrete
industry, university researchers, and others was necessary to establish a ductile design philosophy for reinforced concrete
moment-resistant frames in the years following 1971 and in forward-looking research of the 1960's. (see Blume,
Newmark, and Corning, 1961) By the time of the 1994 Northridge Earthquake, reinforced concrete moment-resistant
frames, if designed under recent codes and standards, performed well, even in the case of older bridges that had benefited
from 1990s vintage retrofits (Yashinsky, 1998).
In the Northridge Earthquake, eight large reinforced concrete parking garages collapsed completely or partially. (Holmes
and Somers, 1995, chapter 4) In these cases, many of the issues had to do with diaphragms, columns designed only to
resist gravity loads, or precast concrete beams rather than just the moment-resisting frames. Pre-cast concrete elements
in high seismic areas of the USA are not typically designed to have ductile connections and to provide the structure’s lateral-force-resisting capacity. There
are few generalizations that have no exceptions in the field of seismic design, and this is true with regard to the preceding statement. Research specifically
aimed at enabling precast concrete to constitute the structure’s ductile lateral-force-resisting system (PRESSS, Precast Seismic Structural Systems) has
indicated its feasibility (Sritharan, Priestley, Seible, and Igarashi, 2000).
An early researcher and practitioner aptly pointed out “the utility of both iron and béton [Fr., concrete] could be greatly increased for building purposes
through a properly adjusted combination of their special physical properties….” (Ward, 1883, p. 388) None of the other structural materials are made with
recipes individualized to achieve “a properly adjusted combination” that is as precisely fashioned to meet what is called for by the engineer's calculations.
To the contrary, all of the other structural materials come to the construction site as units of standardized dimensions with the same properties throughout—
pieces of steel of structural shapes and grades selected from a catalogue, sticks of timber of standardized dimensions and grades, blocks or bricks that
are the same standard product used for non-seismic purposes. The process of customizing reinforced concrete structures to better resist earthquakes has
not stopped but has instead entered a new phase in seismic design in recent years by taking advantage of the properties of fiber reinforced polymers,
especially for the retrofit of bridges.
Good performance of the well-confined concrete within the spiral reinforcement, Ruffner
Avenue I-118 Overcrossing, 1994 Northridge
Earthquake. (Spalling of concrete outside the
confinement does not significantly affect capacity and can be easily repaired).
photo source: Thomas Sardo, Caltrans
Reinforced Concrete Moment-Resisting Frames
CUREE
Reinforced concrete has been called “the supreme engineering material.” (Condit, p. 168) An engineer can
customize its properties to make it precisely conform to the requirements of calculations. Other advantages
are its durability, low cost, and fire resistance. Major figures in the development of Modern architecture, such
as Le Corbusier, Gropius, Wright, Nervi, and Maillart, produced innovative designs featuring reinforced
concrete moment-resisting frames. With the ductility possible today, this element is a frequent seismic design
choice for both bridges and buildings.
CONSORTIUM of UNIVERSITIES for RESEARCH in EARTHQUAKE ENGINEERING
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Moment-resisting steel frames originated as riveted members and connections comprised of numerous small pieces of steel such as
angles, which evolved to a relatively standard approach of using larger rolled shapes for columns and beams with rivets to connect
T-stubs and angles at their joints. (ATC, 1997, Section C 5.2) The entire steel skeleton was formerly encased in concrete fireproofing,
whereas lightweight fireproofing, which adds no strength or stiffness, is the rule today. “These buildings included many stiff, strong
unreinforced masonry walls and partitions. Structural engineers relied upon these walls and partitions to help resist wind and
earthquake loads…” and all the connections in the building were moment-resisting. Riveting was phased out as a construction
technology in the 1950s and 1960s, replaced by high-strength bolts (Roeder, 1998-a, p. III-694)
Roeder (1998-b, p. III-686) notes that by the 1970s, welded moment-resistant steel frames incorporated some design features that
were not seismically advantageous, such as less redundancy (not all of the column-beam joints were moment-resisting and
seismically detailed), and the columns were farther apart to reduce the number of labor-intensive connections and to provide for more
architectural flexibility. Those trends in turn meant that deeper beam sections were used where the moment-resistant connections
were located, and this reduced the ability of the joints to accommodate rotation in a ductile manner. A less desirable welding process
also became common. Configurations and materials in some cases went significantly beyond the experimental earthquake
engineering knowledge base built up by researchers such as Bruce Johnston, Glen Berg and others at the University of Michigan
beginning in the 1950s, Egor Popov, V.V. Bertero, Helmut Krawinkler, and others in the 1960s and 1970s at UC Berkeley, and John
Fisher, Le-Wu Lu and others at Lehigh University in the 1970s.
A 21-story building in the Pino Suarez Complex collapsed onto a similar,
14-story adjacent building in the 1985 Mexico City Earthquake.
photo source: Robert Reitherman
By the 1960s “engineers began to regard welded steel moment-frame buildings as being among the most ductile systems contained in the
building code. Many engineers believed that steel moment-frame buildings were essentially invulnerable to earthquake-induced structural
damage and thought that should such damage occur, it would be limited to ductile yielding of members and their connections.” (SAC, 2000,
p. 1-3) However, “one of the important overall surprises of the Northridge earthquake of January 17 1994, was the widespread and
unanticipated brittle fractures in welded steel beam to column connections.” (Mahin, Hamburger, and Malley, 1998, p. III-647) The collapse
of very large buildings in the Pino Suarez complex in Mexico City in the 1985 earthquake, while involving an unusual frame system with large
box columns and trusses, had earlier illustrated the possibility of brittle failure. Testing and analysis of improved beam-column connections
was conducted by the SAC Joint Venture and others following the Northridge Earthquake to return the steel moment-resisting frame to
“membership in good standing” in the club of seismically resistant structural elements. The resulting changes to the building code require
a more complicated approach to steel frame design than prior to Northridge, but one which offers significantly better performance.
Typical Northridge Earthquake fracture
illustration source: SAC Joint Venture
(FEMA, 2000)
As that research was underway, exactly a year after the Northridge Earthquake, the collapse of some steel frame buildings in Kobe, Japan,
(AIJ, 1995) illustrated that with a different design code and different detailing, there were also potential problems, even though the material
itself was the paragon of ductility. Any seismic element is a combination of its material and the precise way in which it is assembled. This
is true even in the case of a material such as steel, which has such a large amount of inherent ductility.
Steel Moment-Resisting Frames
CUREE
To intrude on the floor plan as little as possible, it is efficient to use steel beams to carry gravity loads horizontally
to widely spaced supporting steel columns. With proper seismic design, this efficient gravity-load-resisting
system can also resist earthquake loads. There will always be a need for a framework that frees the interior space
of a building from permanent walls and opens up the exterior to light and views, while using a minimum
amount of a ductile material, and the steel moment-resisting frame is the structural element ideally suited to
that role.
CONSORTIUM of UNIVERSITIES for RESEARCH in EARTHQUAKE ENGINEERING
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Plywood, which became mass produced and inexpensive after World War II, provided a much stronger and stiffer sheathing material
than board siding, which was a finish material used since the early years of the development of the United States. In the past decade,
oriented strand board (OSB) appeared on the scene and competes in popularity with plywood for shear wall sheathing for woodframe
("two-by-four") construction. The nailing around the edges of a sheathing panel is designed to have a closer spacing than over the
intermediate studs, and when a wall is loaded laterally, deformation is absorbed mostly by the nails around the perimeter of each panel.
Traditionally, each full-story-height rectangular panel, e.g., the solidly sheathed area on each side of a window, has been regarded
as a shear wall (segmented shear wall design method). Increased efficiency results, especially for resisting overturning moments,
if the individual panels on both sides of the window opening are connected with metal straps to form larger wall bracing units, which
is called perforated shear wall design. (See Breyer, Fridley, and Cobeen. (1999, p. 10.22.)
Recent full-scale woodframe building shake table experimentation (Fischer et al., 2001 and Mosalam et al., 2002) in the CUREECaltech Woodframe Project has demonstrated the significant influence that finish materials such as drywall and stucco can have on
seismic performance, even though these finishes are now typically considered "nonstructural" materials. As one example, the peak
ground story drift for the two-story and three-story test specimens cited above, comparing the wood-structure-only case with the same
structure with stucco and gypsum wallboard, was reduced by approximately two-thirds when subjected to a Northridge Earthquake
Home under construction in the 1971 San Fernando Earthquake. Note
displacement of relatively rigid roof diaphragm and racking of plywood record with a peak ground acceleration of 1/2 g. Cyclic dynamic testing by Gatto et al. (2002) of wood-sheathed shear walls with
shear panels, which were not yet fully nailed.
stucco showed not only an increase in capacity but a change in failure mode. Adjacent sheathing panels did not rotate individually
photo source: Gregg Brandow, Brandow & Johnston
but instead acted as a unit, rigidly joined by the stucco that extended across them.
Instead of nail yielding around panel edges, which is a relatively benign form of inelastic energy dissipation, wall studs eventually fractured.
The difference in stiffness between plywood- or OSB-sheathed shearwalls and drywall or stucco raises a design issue. If a level of damage
protection far in excess of code minimums is sought, it implies great stiffness to protect brittle finishes, and yet it is difficult to greatly boost
stiffness by "beefing up" the sheathing and nailing of the wood shear walls. Two fundamental design approaches are possible. First, partitions
could be isolated from in-plane motion as is relatively common in high-rise construction, though in most woodframe buildings, many of the
walls are load bearing, which makes this detailing problematic. A related strategy of accommodating drift is to leave the attachments fixed,
but to use more flexible exterior finish materials such as board siding or vinyl siding rather than stucco. A complicating factor is that the exterior
materials are usually selected on the basis of non-seismic criteria, and another challenge is that even if exterior wall surfaces are flexible, interior
walls are very commonly surfaced with gypsum wallboard.
A second approach to the differential stiffness issue does not require radical change to basic construction features. Further research could be
devoted to the goal of obtaining more capacity from finish materials such as stucco, which must consider the reliability issues of quality
assurance in the field and durability over time. Most of the recent innovations in the earthquake aspects of woodframe construction have come
from detailing and steel hardware connection products. A breakthrough in the basic construction materials could be achieved if stucco can
be seismically designed as, in effect, a thin layer of reinforced concrete, with its small-scale reinforcing and connections arranged to provide
for strength, stiffness, and ductility. The ductility criterion is perhaps the most difficult to achieve, because if too much inelastic behavior
occurs, it is by definition damage, and damage to stiff finish materials is the original concern motivating this approach.
Testing of Stepped Woodframe Cripple Walls. Significant
differences in performance are caused by the presence of
stucco on the exterior. The stepped geometry that is common
to hillside houses also influences behavior.
photo source: Chai, Hutchinson and Vukazich, 2002
Wood Shear Walls
CUREE
The “general range of the fraction of wood structures to total structures seems to be between 80% and 90% in
all regions of the US, for example being 89% in Memphis, Tennessee and 87% in Wichita, Kansas,” (Malik, 1995)
and the vast majority rely on wood shear walls, making this the most common of the elements discussed here.
When properly constructed, its performance in past earthquakes has typically been reliable. Ongoing research
aims at improving the efficiency of this element by increasing its strength and drift control capability at a small
additional construction cost.
CONSORTIUM of UNIVERSITIES for RESEARCH in EARTHQUAKE ENGINEERING
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In earthquakes of the first half of the twentieth century in the USA, reinforced concrete shear wall buildings with severe damage were most notable as exceptions to generally good performance,
with isolated instances of collapse from the 1925 Santa Barbara Earthquake (San Marcos Building), 1940 El Centro (Dunlack Hotel), and 1952 Kern County Earthquakes (Cummings Valley
School). Severe damage was usually due to inadequate width of walls and low quality construction. (Steinbrugge, 1970) In the 1964 Alaska Earthquake, core walls failed in the Four Seasons
Building and the six-story building collapsed, probably because of inadequate lap splices. Damage also occurred in the case of the Mt. McKinley and 1200 L Apartment Buildings, though the
more extensive amount of shear wall prevented collapse. (Steinbrugge, Manning, and Degenkolb, 1967) In the 1994 Northridge Earthquake, large concrete parking garages collapsed, but the
main factors had to do with diaphragms, gravity-load-resisting columns with inadequate tolerance for imposed deformations, and pre-cast elements, rather than traditional reinforced concrete
shear walls.
The tilt-up shear walls in a warehouse-type structure or one- or two-story office building, which are poured flat and then lifted up into place, began to display a bad habit of collapsing in the 1964
Alaska Earthquake in the case of the Alaska Sales and Service Building and a large warehouse at Elmendorf Air Force Base (Steinbrugge et al., op. cit.) Tilt-ups usually do not have seismic
deficiencies in the wall panels themselves, but rather in the connections of these elements to the roof diaphragm, which is usually made of wood.
Three factors relevant to reinforced concrete shear walls coincide in Chile: (1) there are large earthquakes, e.g. 1960 (Mw 9.5) and 1985 (Ms 7.8); (2)
there is a prevalence of reinforced concrete shear walls for the vertical elements resisting lateral forces; and (3) the seismic performance of these buildings
has been good. In one survey of approximately 400 buildings in Chile up to 15 stories in height, almost all relied on reinforced concrete shear walls
for resisting both seismic and gravity loads. This survey found that in high seismic regions of the United States, designers used reinforced concrete
shear walls for about half of the buildings up to three stories in height, for only about an eighth of those in the four-to-six story range, and for only one
twentieth of the buildings from seven to fifteen stories tall. (Eberhard and Meigs, 1995) In the Chilean city of Viña del Mar, which was strongly shaken
by the 1985 earthquake, 80% of the reinforced concrete shear wall buildings had no damage. This good performance was attributed to the extensive
amount of shear wall oriented along both axes of the plane, which for each axis added up to between 2 to 4% of the floor area. The Chilean design
philosophy provides for less wall ductility (e.g., less confinement of chord steel at ends of walls) but greater strength and stiffness. “If less wall area
had been used, the displacements during the 1985 earthquake and the resulting damage, would have been larger.” These Chilean buildings were
extremely stiff, with their periods approximating N/20, where N is the number of stories. (Wood, 1991) Where walls with high ratios of height to length
are used, as is common in the US, there may be advantages in lengthening the period of the structure to the point where it escapes the usual short-period
plateau of response, but that longer period is associated with greater story drifts and nonstructural damage.
Large buildings with reinforced concrete shear
walls in the Chilean city of Viña del Mar were
largely undamaged in the 1985 earthquake.
photo source: Sharon Wood
A sample of individuals representing Los Angeles structural engineering firms, testing laboratories, and building departments was surveyed after the
Northridge Earthquake concerning the seismic risk associated with structural systems. Compared to a welded steel moment-resisting frame, 89%
thought the reinforced concrete shear wall option had lower earthquake risk (though the survey was conducted only about a year after the Northridge
Earthquake, when steel frame vulnerabilities were well-publicized but research had not yet produced many answers). When asked what kind of building
they would prefer their own offices to be in, half chose the reinforced concrete shear wall option while the other half of the votes were divided among the steel braced frame, ductile reinforced
concrete frame, and low-rise woodframe candidates. (Gates and Morden, 1995, p. 3-40) The collapse of some very large buildings with discontinuous shear walls in the 1999 Chi-Chi (Taiwan)
and Turkey Earthquakes (Naiem and Lew, 2000) points out the limitations of any material, unless it is used wisely.
Reinforced Concrete Shear Walls
CUREE
The reinforced concrete shear wall is a tried-and-true workhorse that usually performs reliably if its needs are
respected in terms of the layout of the building. In many cases the “failure” mode of adequately reinforced
concrete walls in severe earthquakes has been the formation of horizontal or diagonal cracks which are easily
repaired after the earthquake. As the earthquake shaking continues, the crack keeps working and damage to
this element increases, but within limits, while stability of the overal structure is maintained and drift control
is provided for nonstructural elements.
CONSORTIUM of UNIVERSITIES for RESEARCH in EARTHQUAKE ENGINEERING
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Prior to the 1933 Long Beach Earthquake, reinforced masonry was so rare that the “unreinforced” adjective
wasn’t necessary when referring to masonry walls laid up without benefit of a grid of steel reinforcing embedded
in grout. (See Cowan, 1977, p.225, and Tobriner, 1984 for discussion of very early examples of reinforced
masonry). The statistical picture of unreinforced masonry building performance was as clear as the photographic
pictures of destruction: In the City of Long Beach, out of about 2,000 unreinforced brick buildings, damage that
was severe enough to cause bricks to fall was suffered by 86% of them, and 55% were damaged so heavily that
about half the wall areas collapsed or required replacement. (Wailes and Horner, 1933) The first significant
seismic building code provisions in the United States, the Field and Riley Acts in California, were a direct result.
These laws applied to new construction, and later the risk posed by existing unreinforced masonry buildings
motivated the implementation of the first retroactive seismic upgrade laws in the US. It was also in Long Beach
that this later retrofit phase began when the building official of that city, Ed O’Connor, began implementing
mandatory seismic upgrades of unreinforced masonry buildings in 1959. (Reitherman, 1984) Los Angeles also
enacted an influential ordinance in 1981. As of 2000, three-fourths of the 25,000 unreinforced masonry buildings
in the state inventoried by local jurisdictions were subject to mandatory upgrade ordinances. (CSSC, 2000)
Older mixtures for mortars, especially after decades of deterioration, are sometimes said to “hold the bricks apart,
Reinforced masonry, pseudo-dynamic experimentation, five-story building
not together.” While it might seem that today’s higher strength Portland-cement-based mortars could adhere one
source: University of California, San Diego, NSF-TCCMAR Project
masonry unit to the next, this “glue” is not up to the task of keeping a masonry building intact in a strong
earthquake in the absence of steel reinforcing. Where codes still allow unreinforced masonry buildings in low seismic zones in the USA today,
the design philosophy is to provide enough strength for the low level of loading to keep the structure elastic. Brick construction was modified
after 1933 in California to produce a structural sandwich composed of two wythes of brick separated by a few inches of space (the cavity) in
which was placed a grid of reinforcing steel and, in essence, an especially fluid concrete mixture with small aggregate (grout). In this filledcavity construction, there are no headers, which are bricks extending into the wall and are seen on-end, as compared to the stretchers laid with
their long axis parallel with the wall, which expose their sides.
Unreinforced masonry’s poor performance led to the
development of modern earthquake resistive (reinforced)
masonry. The Long Beach Earthquake also produced the
first significant earthquake accelerogram.
photo source: Security Pacific National Bank Collection
Los Angeles Public Library
Concrete block, (also called concrete masonry unit or CMU, hollow concrete block, or cinder block), also pre-dated the 1933 earthquake. This
construction style evolved to incorporate the vertical seismic reinforcing and grout running up through the hollow cells and reinforcing steel
and grout allowed to run horizontally. Seismic research initiatives have more recently been launched and coordinated to keep pace with the
other structural materials, such as the Technical Coordinating Committee for Masonry Research, or TCCMAR, which was established in 1985.
(Noland, 1987) Experimentation has included a full-scale reaction-wall experiment of a portion of a concrete masonry building five stories in
height. (Seible et al., 1994) Textbooks (e.g., Paulay and Priestley, 1992) and design guides are widely available.
While most seismically designed masonry buildings in the USA in high seismic zones are made of concrete block, other alternatives include
modern hollow brick, which can be reinforced similarly to hollow concrete block and should not be confused with the weaker hollow clay tile
used for fire walls and partitions in the 1800s and early 1900s. Most stonework seen in modern, seismically designed buildings is nonstructural
veneer, though double-wythe, filled cavity reinforced stone masonry construction is possible.
Reinforced Masonry Shear Walls
CUREE
Most of the buildings studied in architectural history are made of masonry: Egyptian and Greek temples, Hindu
siharas, Buddhist stupas, Roman sports arenas, Persian palaces, Mayan pyramids, Byzantine domes, Islamic
minarets, Gothic cathedrals, Renaissance universities, and the list goes on. This is because (1) masonry was
chosen for its high quality; (2) it aesthetically complemented many different styles; and (3) the structures have
endured. Reinforced to carry out its seismic role, masonry can be used in high seismic regions to create beautiful
and durable buildings which perform similarly to concrete shear wall buildings if detailed adequately.
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In discussing the steel braced frame, we can make the analogy between a horizontally spanning truss and a vertically oriented braced frame. Similarly, an analogy can be drawn between the
horizontally spanning steel plate girder and the vertically oriented steel plate shear wall. “Together, the steel plate wall and boundary columns act as a vertical plate girder. The columns act as
flanges of the vertical plate girder and the steel plate wall acts as its web. The horizontal floor beams act, more-or-less, as transverse stiffeners in a plate girder.” (Astaneh, 2000)
Of the elements discussed here, steel plate shear walls, along with the steel eccentrically braced frame, were the last of the elements that were added to the repertoire of the structural designer
who had to contend with earthquake loads. (Energy dissipating devices, seismic isolation, and active control are recent developments also, but are outside the scope discussed here). However,
in the field of ship design, fastening steel plate to structural steel shapes to form wall or diaphragm elements was routinely done for over a century before being accomplished in seismic design.
Ships experience large inertial forces in storms that lead naval architects to design portions of their structures for about 1 g of acceleration. The vertical load carried by bulkheads and hulls in
a large ship can be tremendous, as they bear the weight of what is in effect a multi-story building. Water pressures, even neglecting wave effects, add to the demand. In Twenty Thousand Leagues
Under the Sea, (Verne, 1873) Professor Aronnax gives the harpoon-wielding sailor, Ned Land, a quick lesson in hydrostatic pressure to convince Ned that
thick iron plates would be necessary to keep any undersea craft from being crushed. The professor calculates that going deeper underwater by 10 meters
increases the water pressure produced by the weight of the column of water above that level a kilogram per square centimeter (about 15 psi, or 2,000 psf).
This can be thought of as about 50 times the pressure (live load) for which a residential building floor is designed. In a ship’s hull, at the same time that
large out-of-plane loads are applied, the same steel element acts as part of a beam spanning the other axis, longitudinally, resisting hogging and sagging
bending moments that for a large ship are calculated into the realm of hundreds of millions of Newton-meters (billions of inch-pounds). The configuration
and loading on a ship’s hull is obviously different than on a building’s shear wall. Nonetheless, naval architecture’s use of steel plate walls illustrates the
potential strength and toughness that can be adapted for use in seismic design from such a type of construction. Timoshenko (1953, pp.434 ff.) discusses
early 20th century structural engineering advances in steel ship design.
The twenty-story Nippon Steel Building in Tokyo, built in 1970, was the first seismically designed structure to use steel shear walls. Five “H’s” in plan
were arranged symmetrically in a row down the center of the building. (Thorburn, Kulak, and Montgomery, 1983)
The steel shear wall structure that has been the most dramatically tested by a real earthquake to date is the Olive View Hospital—that is, the new (1988)
medical center, on the site of the original facility of the same name, that was so badly damaged in the 1971 San Fernando Earthquake that it was demolished.
In the 1994 Northridge Earthquake, accelerometers in the new six-story building (two stories of reinforced concrete shear walls surmounted by four of
steel plate shear walls) “registered the highest accelerations ever recorded in a building (0.9g free field and 1.7 g at the roof).” (Holmes and Somers, eds.,
1995, p. 230) The structure managed to resist these tremendous loads without damage, and except for water pipe breakage, most of the facility was
otherwise functional. The cost of damage repair was 11% of its replacement cost, and it was back in full operation within a month. (FEMA, 1997)
Compared to the performance of its predecessor in 1971, the steel shear wall design’s performance in 1994 was excellent.
In addition to its high strength and stiffness, research has been conducted to select the material properties of the flange and web materials to maximize
performance. If one keeps in mind the seismic design strategy of resisting earthquake loads by intentionally designing components that will yield, it is
not surprising that some research has centered on the use of low yield, rather than high yield steel, for the panel material. (Nakashima, 1994)
Experimental research by Kyoto University
DPRI and Nippon Steel.
photo source: Masayoshi Nakashima
Steel Shear Walls
CUREE
If one combines the most material—a solid wall rather than a frame—with the strongest of the structural materials—
steel—it is not surprising that tremendous lateral load capacity can result. While common in ship design, steel plate
walls are only rarely used in buildings, but this type of element offers the designer an option that can
concentrate massive amounts of earthquake resistance into compact spaces where other types of elements
would not be strong enough. Energy dissipation strategies can also be combined with steel shear walls to
provide damping and energy dissipation with this type of element.
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The vertical structural elements that resist lateral forces would be a disorganized crowd of individuals trying to compete in a tug of war if they weren't linked together with horizontal diaphragms,
which allow the walls or frames to work as a group in resisting lateral forces. Because out-of-plane resistance capacities are very small for walls, moment-resisting frames, and braced frames,
it is only through the diaphragms that the vertical elements oriented in line with the earthquake loads at a given instant can convey their stabilizing effect to their partners.
In the 1964 Alaska Earthquake, pre-cast concrete roof or floor beams and planks, (products that were relatively new as of that time), performed very poorly. (Steinbrugge, Manning, and Degenkolb,
1967) Lacking the integral characteristic of poured-in-place concrete, the building that was constructed in pieces tended to fall apart in pieces, as welds connecting adjacent units failed. A basic
solution to the problem is to employ continuous reinforced concrete topping to tie together the precast units, in addition to improving the welds. Although today there are many hotels and other
kinds of construction outside the high seismic West with precast floor or roof units without any topping, it has become a standard solution where earthquakes are more frequent and seismic design
forces higher. Looked at as an individual unit, the pre-cast beam with top flange forming the floor or roof area (e.g., T beams), or flat planks, offer many advantages. For example, the concrete
mixed and poured under factory rather than field conditions can avoid much of the shrinkage and associated cracking and stresses that would occur in a large diaphragm made of monolithic concrete.
As with the other parts of the anatomy of a seismically designed structure, the connections of pre-cast diaphragm units (with each other and with the vertical elements), are often the key concern,
rather than the quality of the members themselves. However reinforcing within the diaphragm element was inadequate in some cases in the Northridge Earthquake where large diameter, poorly
confined chord steel tended to spall off the edge of the concrete diaphragm. “Once the concrete cover spalled, the chord/collector bars were rendered ineffective, and in some cases, catastrophic
building failures ensued.” (SEAOC, 1997, p. 198) Wood, Stanton, and Hawkins (1998) also identified vulnerabilities in the web portion of these large diaphragms in some instances.
In most cases, when the floor or roof will be built with reinforced, poured-in-place concrete, the cost of providing a
seismically resistant diaphragm is very similar to non-seismic construction. Unusually thick concrete slabs, for example
floors in warehouses that may be about 250 mm (10 inches) or more in thickness, are typically designed that way because
of large vertical loads, not lateral loads. From a seismic standpoint, keeping these elements thinner rather than thicker confers
a seismic benefit, since so much of the mass of a typical building is concentrated in its floors. The reinforcement in concrete
diaphragms is specially designed to conform to its role of behaving like a beam on its side, with the edges performing chord
or flange roles. This is precisely where little reinforcing is needed if only vertical loads are considered. The steel decking
often used to erect concrete floors functions primarily as formwork that is left in the floor assembly, rather than reinforcing,
and therefore reinforcing bars or mesh are still needed. (Bare steel decking can be designed as a diaphragm for roofs, but
is not discussed here).
Beam web and flange analogy for a diaphragm
illustration source: Arnold and Reitherman, 1982, p. 38
The choice of concrete allows for high diaphragm strength and stiffness, and this almost always makes for a rigid diaphragm,
rather than a flexible diaphragm that is often assumed for wood diaphragms. Loads are distributed by rigid diaphragms to
the vertical elements in a given story in proportion to the relative rigidities of these elements, not by tributary area, and torsion
can be resisted.
The issues that arise with diaphragms that may pose problems for the structural designer usually have to do with configuration. Like any diaphragm, the reinforced concrete floor or roof can be
challenged to carry out its lateral-force-resisting job when its shape in plan is significantly concave (i.e., there are large re-entrant corners). For purposes of classifying this as an irregularity and
triggering remedial requirements, the NEHRP Provisions (BSSC, 2001, Chapter 5) define the threshold of this condition when both projections or legs beyond the “notch” or corner extend for
more than 15% of the plan dimension in the given direction. If holes (such as the common stairwell, elevator shaft, and atrium) add up to more than 50% of the area of a diaphragm, this is also
an irregularity that triggers special design countermeasures. Analogously, a beam with half its web cut out would have a tough time spanning loads across to supports. If the stiffness of a diaphragm
at one level differs by more than 50% from that of the diaphragm in an adjacent level, another irregularity is identified in the NEHRP Provisions.
Reinforced Concrete Diaphragms
CUREE
A building owner has no particular non-seismic need for diagonal braces or the ductile connections in a
moment-resisting frame. The argument must be made that these are seismically necessary. However, the
engineer does not need to convince the owner or architect that the building should have floors and a roof—i.e.,
diaphragms--which are key to a complete seismic-load-resisting system. A concrete floor system offers some
seismic advantages, but its selection is usually predetermined by non-seismic factors such as fire resistance,
control of footfall vibration, acoustic separation, and vertical load carrying ability.
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A floor or roof, as well as a shear wall, is “a flat structural unit acting like a deep, thin beam. The term ‘diaphragm’ is usually applied to roofs
and floors. A shear wall, however, is a vertical, cantilevered diaphragm.” (APA, 2001, p. 4) As used here, “diaphragm” is limited to floors and
roofs, which act in theory as if they were deep beams on their sides, with the midsection of the diaphragm handling the shear stresses while the
edges act like the flanges of a wide-flange beam and take the tension and compression. The reactions, moments, shears, deflections, and
consideration of end conditions, can be analyzed using the same theory used in the case of beams that resists gravity loads.
Diaphragms framed with nominal two-by lumber (38 mm, 1.5 inch thickness) and sheathed with plywood or oriented strand board are almost
always used when the walls are of woodframe construction. Basic engineering procedures are given in various APA publications (APA, 2001),
and in Breyer, Fridley, and Cobeen. (1999) Prior to about World War II, board siding, either diagonal sheathing (laid out at a 45 degree angle
vis-à-vis the walls) or straight sheathing (boards laid parallel to the walls) were common. (ATC, 1997, Sec. C8.5.1)
The seismic design of wood diaphragms, like all lateral-force-resisting elements, requires care and skill, but the engineer does not usually have
to compete for extra seismic capacity in the layout of the building the way he or she must often struggle to obtain enough shear wall lineal footage.
The vertical-load-resisting function of the sheathing as part of the floor or roof is already a given, which means that there is little extra cost for
the necessary seismic details for the sheathing, nailing, and chords. In all-wood construction, seismic damage to the diaphragm is virtually unheard
of, though this is not to tempt fate and state that this element will always perform as intended, no matter how large the hole in a diaphragm or how
irregular its layout (e.g., complex plan shapes and split levels).
Experimental simulation of earthquake loading on a wood
diaphgram, Virginia Polytechnic Institute.
photo source: Daniel Dolan
Wood diaphragms are often used with large reinforced masonry or tilt-up buildings. While the diaphragm loads increase in these cases, the “beam” (diaphragm) is also deeper, so that even large
wood diaphragms can usually use thicknesses of sheathing and nailing schedules that are not drastically different or more expensive than in ordinary house construction. (The framing, however,
is often more complicated for vertical-load-resistance purposes, with a hierarchy of girders, beams, joists, and purlins.) The most common cause of damage with these large diaphragms in mixed
wood-concrete/masonry construction has been the connection at the wall, though cases of damage within the diaphragm itself were observed in the Northridge Earthquake. Cross-building ties
and collectors are often needed in larger buildings, which pose other connection issues. The problem of inadequate concrete or masonry wall connections to wood diaphragms surfaced in the
1964 Alaska Earthquake in the collapse of the Alaska Sales and Service Building and warehouses at Elmendorf Air Force Base. (Steinbrugge et al., 1967) However, it was the 1971 San Fernando
Earthquake that led engineers and code officials to closely scrutinize this problem. Briasco (1973) provides the standard diagnosis of the weak link provided by the standard ledger connection
of the era. Failure modes included the nails pulling out of or ripping through the plywood, and the ledger splitting in cross-grain tension, both with the same effect: The wall could tilt outward
and fall, simultaneously dropping a portion of the roof. Wood is extremely weak when tensile forces act across the grain, pulling one ring of wood from the adjacent one, the way one can easily
split firewood. With nails connecting the diaphragm's plywood to the top of the ledger, and with the metal joist hangers providing no significant lateral resistance, the connection of wall to roof
exerted a wrenching action at the top of the ledger, inducing a bending or torsional moment on the ledger that causes cross-grain tension. The most common retrofit solution is similar to that
used in new construction: One end of a steel bolt or strap is embedded in the wall (or extended through the wall in the retrofit case) and the other end is connected directly to the sides of the
roof framing. A review of building code provisions for this type of building, in the context of current seismic evaluation standards, is given by SEAONC (2001).
While a common generalization is that “wood diaphragms are flexible” (as compared to the rigid diaphragm analysis alternative), there are in fact ranges of rigidity. Factors that increase diaphragm
rigidity include: gluing the sheathing to the joists; walls or partitions attached to the top of the diaphragm that act somewhat as stiffeners on the web of a plate girder; low aspect ratio of length
to width; and of course the particular sheathing and nailing details (e.g., blocking or concrete soundproofing topping increases stiffness). Wall rigidities are the other key factor: It is the relative
rigidities of diaphragm and walls that determines whether the loads are distributed according to the flexible or rigid analytical assumption. Experimental results are discussed in Fischer et al.,
(2001) and Dolan (2002), design guidance is given in SEAOC (1999, section C805.3), and current code issues discussed by Mochizuki and Fennell (2002).
Wood Diaphragms
CUREE
Just as wood shear walls are the most numerous of the vertically-oriented seismic resistant elements, wood
diaphragms are the most common type of diaphragm in the United States. With careful and usually inexpensive
seismic detailing, wood diaphragms have performed very reliably in earthquakes in all-wood construction.
The vulnerability of inadequate connections of masonry or concrete walls to wood diaphragms in new
buildings has been largely remedied with current joist anchorage and other connection details. Fortunately,
these measures are also relatively inexpensive to implement as retrofits for existing mixed construction.
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H. F. Reid’s elastic rebound theory (Reid, 1908) elegantly explained how the build-up of strain in Earth’s crust released earthquake vibrations as a fault ruptured. Using their knowledge of rocks
and the signs of the past occurrence of faulting, geologists can estimate the likelihood that faults will slip a particular amount, and thus release earthquakes of given magnitudes, over future time
spans. Other earth scientists such as Richter (1935) conceptualized and quantified earthquakes in ways that were increasingly relevant to engineering. A current frontier of seismology and a research
focus of the Southern California Earthquake Center is earthquake source physics—trying to understand in greater detail what happens kilometers under the earth's surface where slip is spread over
a large, irregular rupture surface, which is the true source of an earthquake. Seismologists study wave propagation modeling (for site-specific studies) or general attenuation relationships (to define
regional-scale zones in building codes). In modeling waves from source to site, there lies the hope of being able to predict precisely where motion will be elevated or lowered in severity: “among
the extremely complex wave-motions of the surface of unstable ground, due to reflections, refractions and the superimposing of these motions there are what may be called nodal spots….” (Freeman,
1932, p. 767) See Somerville and Yoshimura, 1990 for a modern view of this phenomenon. By about 1970, the theory of plate tectonics provided earth scientists with the explanation for the global
distribution of crustal strain and earthquakes (Uyeda, 1972, 1978), though a hint of the plate tectonics concept originated with Wegener much earlier (Wegener, 1915). Geodesists have benefited
from the advent of GPS (Global Positioning System) to measure the deformation of Earth’s surface and relate this to strain accumulation.
There are several major geotechnical engineering sub-disciplines extensively involved in seismic design. The geotechnical design contribution
related to foundations is not unique to earthquake engineering, though seismic factors complicate foundation design decisions. For example,
a deep pile foundation may be necessary to handle seismic overturning forces where spread footings would otherwise have sufficed. Soilstructure interaction studies of a very sophisticated nature today grapple with the fact that when structures are relatively stiff and massive, they
are not just a dependent variable being shaken by the ground -- they in turn affect how the localized earth shakes.
Site response is a key seismic design consideration. Borcherdt (2002) found that site response data from the 1994 Northridge Earthquake
confirmed the site coefficient formulation in the NEHRP Provisions, which are explained in Borcherdt (1994) and in the Commentary to the
NEHRP Provisions (BSSC, 2001). Sites are assigned to six (A through F) Site Classes based on their measured or inferred mean shear wave
velocities. The S factor introduced into the UBC in 1976 was a constant independent of the expected severity of ground motion, whereas in
the NEHRP Provisions, these two considerations are related. As shown by site response for very soft soils in the 1985 Mexico City and 1989
Loma Prieta Earthquakes, “earthquake motions attenuate in soft soils as the strength of the soil is exceeded and the soil behaves in a highly
nonlinear manner,” but until that limiting effect on the motion comes into play, soft soil motion can be greatly amplified as compared to the
acceleration at bedrock. (SEAOC, 1999, p. 89)
NEES Mobile Geotech Lab - A localized earthquake can be
generated by this equipment and field data collected.
photo source: University of Texas at Austin
Nathan Newmark, who was well known to the structural engineering discipline for pioneering work in inelastic design spectra (1960), also
did research in the geotechnical field that is still influential today. He was the first to mathematically elucidate the principles involved that
enable a mass of rock or earth to start moving and continue sliding in landslides. (Newmark, 1965) A yield acceleration is necessary to
overcome the friction restraining the soil mass, which is modeled as a block on an inclined plane. By analyzing strong motion records, one
can quantify the amount of time during which the motion acts above this threshold level, and therefore compute how far the slide proceeds.
Liquefaction was a mysterious form of ground failure for many years until its scientific basis was understood, such as in the work of Housner (1958). Seed and Lee (1966), and Seed and Idriss (1967)
produced early work that continues to be influential as a framework for improving the state of the art, as in the case of Youd and Idriss, 1997. The two most often used parameters are the cyclic
stress ratio and a calibration of field data on the local material such as from the Standard Penetrometer Test (SPT) (ASTM, 1999). Youd and Perkins (1978) devised relationships between the way
over 20 categories of cohesionless soil had been deposited (river channel vs. delta for example), combined with four categories of age of deposit. Because the site’s seismicity cannot be controlled,
the two basic liquefaction factors amenable to remediation projects are dewatering and compaction, thus providing the capability of “designing” the soil itself, as well as the structure.
The Earth Beneath A Structure
CUREE
Progress made in reducing to mathematical relationships the seismic behavior of the earth beneath a structure
(or surrounding it in the case of a tunnel) has paralleled the way engineers have rationalized the behavior of
structures over the past century. In many ways, the soil on which the structure is founded is less amenable to
analysis, observation, and experimentation, than the structure, but it is an integral element of the seismic load
path. Great strides have been made in the earth sciences and geotechnical engineering in recent years to apply
sophisticated quantitative analytical and experimental techniques.
CONSORTIUM of UNIVERSITIES for RESEARCH in EARTHQUAKE ENGINEERING
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20
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