Risk Management Series Designing for Earthquakes
A Manual for Architects
FEMA 454 / December 2006

SEISMIC DESIGN--PAST, PRESENT AND FUTURE BY ERIC ELSESSER
7.1 INTRODUCTION
Design of any building is a challenge for architects and engineers, and the 
challenge is made more complex by providing for earthquake resistance. During 
the past 100 years, seismic design philosophy and details have progressed from 
simply considering earthquakes to be the same as wind loads, to a sophisticated 
understanding of the phenomenon of the earthshaking that induces a building 
response. 
This chapter covers the 100-year history of seismic structural systems as 
developed by engineers and architects, ranging from simple to sophisticated 
solutions. Basic structural behavior is outlined; guidance for selecting a good 
structural system is suggested, and the following issues are discussed:
  Scale and size of buildings and structural components
  The impact of building configuration
  Force verses energy
  Drift or movement
  Structural mechanisms (passive to active)
  Costs and post-earthquake repair costs
7.2 A BRIEF SUMMARY OF 100 YEARS OF   	STRUCTURAL SEISMIC DESIGN
Seismic exposure has extended over many centuries, but systematic seismic design 
has occurred only over the past 100 years, especially in California since the 
1906 San Francisco earthquake.
A group of thoughtful and creative engineers, responding to the observed damage 
in the 1906 earthquake, started to study, conceive, and design a progression of 
structural solutions to solve the earthquake response problem. This creative 
work has extended over a 100-year period, and continues today. A brief 
progression of key milestones in this seismic design history follows:
   Initial seismic designs for buildings were based on wind loads, using static 
force concepts. This approach started in the late 1800s and lasted to the mid-
1900s.
   After the 1906 San Francisco earthquake, concepts of building dynamic 
response gained interest, and in the early 1930s, initial studies of structural 
dynamics with analysis and models were initiated at Stanford University. This 
approach ultimately led to a design approach that acknowledged the importance of 
building periods and dynamic rather than static design concepts.
   Dynamic design concepts were enhanced by the acceleration spectra method used 
for design as developed by Professor Housner at the California Institute of 
Technology (See Chapter 4, Section 4.5.3).
   While analysis methods were being developed, engineers needed additional 
knowledge about nonlinear behavior of structural components. Substantial testing 
of materials and connection assemblies to justify actual behavior were 
undertaken from 1950 through 1990 at numerous universities (University of 
California, Berkeley; University of Illinois; University of Michigan; University 
of Texas; etc.).
   Since 1980 to the present, sophisticated computer analysis programs have been 
and continue to be developed to facilitate design of complex structural systems 
and the study of nonlinear behavior.
In the past 70 years substantial change and progress have taken place, not only 
in California but also over the entire United States, so that concepts and 
systems can now be utilized that previously could only be dreamt about.
7.3 HISTORIC AND CURRENT STRUCTURAL- SEISMIC SYSTEMS
7.3.1 Early Structural Systems-Pre-1906 
         San Francisco Earthquake
San Francisco in 1906 had a varied building stock with a few basic structural 
systems widely represented. Almost all common residential buildings were of 
light-frame wood construction, and most performed well in the earthquake, except 
for those on poor, weak soils or those with unbraced lower story walls. Most 
small- to medium-sized business buildings (about five to six stories in height) 
were constructed with brick masonry-bearing walls, using wood-framed floors and 
roofs. These buildings had a variable performance, with upper stories 
experiencing partial collapse and masonry walls typically showing shear cracks 
to varying degrees. Tall buildings, constructed during the previous 10 to 15 
years (prior to 1906), utilized a steel frame to support gravity loads and 
provided unreinforced brick/stone perimeter walls which served to provide 
lateral load resistance. These buildings generally performed well during the 
earthquake. Most buildings, when subjected to the firestorm after the 
earthquake, did not do well.
The general conclusion following the 1906 earthquake was that a steelframed 
building designed to support gravity loads and surrounded with well-proportioned 
and anchored brick walls to resist earthquake forces was a superior structural 
system, and it was commonly adopted by the design profession.
7.3.2 The Early Years (1906  1940)
Immediately after the 1906 earthquake, when reconstruction and new buildings 
became essential, a variety of new structural concepts were adopted. Brick 
masonry infill walls with some reinforcing were used, and steel frames were 
designed to carry lateral loads using one of the following ideas: knee bracing, 
belt trusses at floors to limit drift, rigid-frame moment connections using 
column wind-gussets, or top and bottom girder flange connections to columns.
As concrete construction became popular after 1910, concrete momentframe 
buildings together with shear walls, emerged for industrial and lower height 
commercial buildings. Concrete slowly replaced brick as a structural cladding 
after 1930, and buildings commonly used a light steel frame for floor support 
with a complete perimeter concrete wall system for lateral loads.
7.3.3 The Middle Years (1945  1960)
Immediately after World War II, construction of large projects started again. 
New ideas were common, and some refinement of framing systems for tall buildings 
was proposed and adopted.
Expressive structural systems were studied and used, but they were usually 
covered from view with conventional exterior finishes.
The transition from riveted connections to high-strength bolted joints occurred 
in the 1950s. By 1960, another steel connection change was starting to occur; 
girder flanges welded directly to columns to create moment frame connections. 
Because engineers initially did not trust the limited use of moment frames, 
structural designs were conservative, with substantial redundancy created by 
utilizing complete moment-frame action on each framing grid, in each direction.
7.3.4 The Mature Years (1960  1985)
The 25-year period from 1960 to 1985 represents the mature years, in that 
substantial projects were completed using the concepts of either ductile moment 
frames or concrete shear walls.
The structural engineering profession accepted the validity of 1) ductile 
concrete-moment frames, 2) ductile shear walls, or 3) ductile welded steel-
moment frames as the primary structural system for resisting lateral loads. The 
primary design activity became optimization of the system or, in other words, 
how few structural elements would satisfy the minimum requirements of the 
building codes. Substantial connection tests were carried out at university 
laboratories to justify this design approach.
7.3.5 The Creative Years (1985  2000)
After the damage caused by the 1989 Loma Prieta earthquake (San Francisco Bay 
Area) and the 1994 Northridge earthquake (Los Angeles), the structural 
engineering profession began to ask itself about actual earthquake performance. 
Would real performance differ from the solution obtained by simple compliance 
with the building code? This investigative process defined many issues, and one 
of the most important was the dissipation of seismic energy by the building 
structure. The pursuit of this issue led engineers to the consideration of dual 
systems and seismic isolation to limit lateral displacement. 
Several significant solutions have been developed using the dual-system concept 
with stable cyclic seismic behavior:
Each of these systems is part of an overall framing concept. These dual and 
stable mechanisms represent the current search for reliable seismic performance.
7.4 BACKGROUND AND PROGRESSION OF STRUCTURAL-SEISMIC CONCEPTS
The progression of seismic systems selected by structural engineers has resulted 
from three factors:
1.   Study of Past Earthquakes
	Learning from past earthquake performance: Successful seismic structural 
systems continue to be used; unsuccessful systems are eventually abandoned. New 
and better ideas frequently flow from observed earthquake damage.
2. 	Research Data
	New ideas for structural concepts are frequently developed jointly by 
design engineers and university research laboratories. These systems are 
physically tested and analytically studied.
3. 	Building Codes
	Finally, structural systems, that are listed in building codes eventually 
are used by many engineers as approved. The problem with code concepts, in 
these times of rapidly changing systems, is that codes are created about 5 to 10 
years after new ideas are developed, so that codes may no longer be current or 
at the cutting edge of new thinking; overly specific codes may tend to stifle 
and delay new ideas.

7.4.1 Development of Seismic Resisting Systems
Over a 100-year period, seismic resisting systems have developed substantially. 
The use of San Francisco buildings as a typical measure of the evolution 
provides a good summary of the past and present, and some indication of the 
future.
A summary of individual buildings gives a clue as to thinking. The plot of 
systems (Figure 7-1) connects the concepts and indicates the progression of 
ideas.
7.4.2 Pictorial History of Seismic Systems
The following pages provide a visual history of key features in the evolution of 
seismic systems developed and utilized in San Francisco and other Western 
regions of high seismicity.
7.5 COMMENTARY ON STRUCTURAL FRAMEWORKS
Both the steel-frame and concrete-frame buildings previously tabulated can be 
summarized on the basis of a primary framing system, considering both frameworks 
and structural cladding.
7.5.1 Steel Building Frameworks
It can be seen from the previous tabulation that steel frameworks have 
progressed from a simple steel frame, braced laterally by unreinforced masonry, 
to complete moment frames with full lateral load resistance. However, the 1994 
Northridge earthquake in southern California created serious doubts as to the 
integrity of welded moment frames. Actually, several years before the 1994 
earthquake, thoughtful structural engineers recognized the advantages of dual 
structural systems for the structural redundancy required to resist large 
earthquakes. The engineers had two separate tasks. The first task was to 
minimize the structure for economy, and the second task was to provide a secure 
load path to protect the structure.
A review of the basic eight steel-frame concepts tabulated in Figure 7-2 
indicates that they are all reasonable, but some have substantially more 
redundancy than others:
1.   The simple 1890-1920 steel framework with unreinforced brick infill: The 
system has vertical support with an infill system that allows brick joint 
movement for energy dissipation. It is a good inexpensive system that allows for 
simple repair of brick after an earthquake.
2.   The 1910-1930 column-to-girder gusset-plate connection with nominally 
reinforced concrete infill walls: A good low-cost steel riveted detail with 
concrete providing stiffness for controlling lateral drift.
3.   The 1910-1940 trussed girder wind brace providing inexpensive drift control 
of the frame: The encasing concrete also provided substantial lateral stiffness, 
and forced the steel column sections to actually be stronger and stiffer and to 
create girder yielding, a good contemporary concept.
4.   The 1920-1940 knee-braced moment frame with concrete encasement provided a 
nominally stiff frame system.
5.   The 1930-1970 riveted (or bolted) top and bottom girder fixity to the 
column created a steel-moment frame. The concrete fire-proofing encasement on 
buildings through 1960 enhanced the moment-frame stiffness.
6.   The 1950-1970 top and bottom bolted haunched girder moment frame provided 
inexpensive girder stiffness and was especially strong if encased in cast-in-
place concrete.
7.   The more recent 1970-2000 all welded girder moment frame relied only on the 
steel system for seismic resistance and produced the most flexible of steel 
frames. These steel systems were not encased in concrete and were clad only with 
precast concrete, metal panels, or glass, not providing added structural 
strength or stiffness.
      After the Northridge earthquake, these conventionally welded frames were 
seen to be vulnerable, providing far less ductility than anticipated. A major 
FEMA-funded study has attempted to find solutions to this very significant 
problem. The current solutions tend to be expensive to fabricate.
8.   The 1995-2000 steel-moment frames with a dual system of dampers, unbonded 
braces, or eccentric-braced frames, all clad with light-weight materials, appear 
to be good solutions.
The engineering profession had progressed fairly slowly until the early 1980s 
from the basic framing concepts that were first evolved in the early 1900s. When 
the concerns about seismic performance and energy dissipation became paramount, 
researchers and design engineers investigated mechanisms and configurations to 
supplement the basic rectangular grid framing in use for over 100 years.
7.5.2 Concrete Building Frameworks  
Reinforced concrete framing became popular in California immediately after the 
1906 earthquake. The significant framing systems that evolved are shown in 
Figure 7-3.
1.   The early years, about 1908 to 1915, featured nonductile frameworks of 
reinforced concrete column-girder moment frames. The earliest utilized wood 
floor infills between concrete floor girders, followed rapidly by all-concrete 
floors. It was not until the early 1960s that nonductile frames were recognized 
as an unsafe collapse mechanism, because of inadequate shear capacity, or 
because of poorly confined concrete.
2.   Early steel frames for gravity load support were initially encased in 
masonry cladding, soon to be changed to reinforced concrete frame infill walls, 
1910 - 1930. This was a simple change which added definable strength around the 
steel.
3.   Soon after the bare concrete frames were developed, they were infilled with 
concrete shear walls for lateral stability, 1910 - 1930. Not all of these 
buildings were well conceived structurally, and because of the ease of adding or 
deleting walls, torsional problems became common for this type of building.
4.   Significant reinforced concrete shear-wall buildings acting as bearing 
walls evolved in the1920s, without moment frames.
5.   It was not until the late 1960s and early 1970s, after substantial research 
at the University of Illinois, that the benefit of confined concrete columns and 
ductile concrete frames was recognized and adopted for dependable seismic 
resistance.
6.   In the 1970s and 1980s, a dual system of ductile concrete moment frames 
coupled with confined concrete shear walls was recognized. This concept works 
best with a perimeter moment frame and an interior shear wall core, or the 
reverseusing a perimeter ductile wall and an interior ductile moment frame. 
Finally, a dependable concrete dual system evolved.
7.   Shear walls coupled together with yielding shear links were developed in 
New Zealand in the 1980s after successful tests carried out by Park and Paulay 
in 1975. This coupled-wall system is another excellent example of creative 
research used to develop a low-cost mechanism for seismic energy dissipation.
7.6 System Characteristics
Selecting a good structure requires engineering common sense. Common sense 
requires understanding the earthquake motion and its demands, and understanding 
the structural behavior of the individual systems available. There are 
differences of scale (small versus large), differences between elastic and 
inelastic behavior; and differences of dynamic responses and seismic energy 
dissipation. Structural and architectural configurations (such as regular versus 
irregular forms) are also significant in the performance. The many variables 
often make it difficult to select an appropriate system. The building code lists 
numerous structural systems, but it does not provide guidance in the selection 
of a system, and the many systems are not equal in performance.
Key performance issues are elastic behavior, inelastic behavior, and the related 
cyclic behavior resulting from pushing a structure back and forth. This behavior 
should be stable, nondegrading, predictable, and capable of dissipating a large 
amount of seismic energy.
 7.6.1 Elastic DesignLinear Systems
The simple building code approach to seismic design requires diminishing an 
acceleration spectra plot by use of an R value. Elastic design is expressed by 
an R value, which is used to modify the acceleration spectral value to a simple 
seismic design force (Figure 7-4). This is a simple but frequently questionable 
method. It does not consider performance, nonlinear cyclic behavior, or most 
importantenergy dissipation.
7.6.2 Post-Elastic DesignNonlinear Drift
Inelastic design is a better indication of realistic lateral drift or deflection 
that results from real earthquake motions (Figure 7-5). Nonlinear drift impacts 
structural and nonstructural behavior. For significant seismic energy 
dissipation, the drift should be large, but for favorable nonstructural or 
content behavior this drift should be small. A building with a large but 
unstable structural drift will collapse. A building with a limited or small 
structural drift generally will not dissipate significant seismic energy without 
significant damage. 
7.6.3 Cyclic Behavior
A good measure of seismic performance is stable cyclic hysteretic behavior. The 
plot of load vs. deformation of an element, for motion in both directions, 
represents cyclic behavior (Figure 7-6). If the load curves are full, 
undiminished, without necking down. they represent a stable system that is 
ductile and has sufficient capacity to deliver a constant level of energy 
dissipation during the shaking imposed by an earthquake. Degrading cyclic 
systems  may be acceptable if they degrade slowly and in a predictable manner.
7.6.4 Performance-Based Seismic Design
Years of physical testing and corresponding analytical studies have provided a 
greatly increased understanding of earthquakes, materials and assemblies. Recent 
seismic performance design has forced engineers to look at the full range of 
structural behavior, from linear to nonlinear to failure.
Good, dependable seismic systems behave with a stable, nondegrading behavior; 
others do not. The ones that are not stable diminish in capacity with continuing 
cycles, until they have little capacity to dissipate seismic energy. If the 
earthquake continues for a long duration (over 10 to 15 seconds), the structure 
can become weak and unstable, and may fail.
The single conventional system with only one means of resisting seismic forces 
is especially vulnerable to long-duration earthquakes, because when that system 
degrades, no alternative exists. This concern has evolved for high-performance 
structures into the concept of utilizing multiple-resisting structural systems 
that act progressively, so that the overall structural capacity is not 
significantly diminished during the earthquake.
7.6.5 Nonlinear Performance Comparisons
Considering only strength and displacement (but not energy dissipation), six 
alternative structural systems are represented by seismic performance pushover 
plots for comparison for a specific four-story building, but  a useful example 
of a common low- to mid-rise building type (Figure 7-7).  For other building 
heights and types, the values will vary. Each system was designed and sized for 
code compliance, all with a lateral drift of less than one inch. However, the 
nonlinear displacement (or drift) for each alternative system is significantly 
different:
  The braced frame (BF) needs to reach the 5% damped spectral curve, but the 
system fails at a drift of about one-and-a-half inches, far below the target of 
5%t damping. A possible solution would be to design the brace for about four 
times the code value; excessively expensive, this solution deals only with 
strength, not with seismic energy. 
    The shear wall (SW) needs to reach the 10% damped or spectral curve, but can 
only reach a drift of about two inches, about half of the required drift. Again, 
significant over-design might solve the strength problem, but may not solve the 
energy issue.
 The eccentric braced frame (EBF) needs to reach the 10% damped curve at a drift 
of about 5.5 inches, which exceeds its ductility and would create excessive 
deformation. The solution is to add more capacity.
  The steel-moment frame (MF) without a supplemental system needs to reach a 5%t 
damped curve at a drift of about nine to ten inches, far in excess of its 
capacity. The only solutions are to over-design the MF or to add a supplemental 
system.
  The steel-moment frame with passive dampers (MF + Dampers) needs to reach the 
20% damped spectra curve which it does at a drift of about four inches. This is 
a reasonable solution.
  The base-isolation solution (BI) satisfies the demand at a drift from 10 to 12 
inches, which it can easily do without damag,e because most of the lateral drift 
occurs in the isolation itself.
7.6.6 Energy Dissipation
During the duration of ground shaking, seismic energy is stored in a 
structure. If that input energy can be stored and dissipated without damage to 
the structure, the behavior is favorable. If, however, the structure cannot 
store the energy, the system may rupture with either local or catastrophic 
behavior (Figure 7-8A). Earthquake energy accumulates during ground shaking so 
that the total energy continually increases. If the structure can safely 
dissipate energy as rapidly as it is input to the structure, no problem occurs. 
In fact, the structure can store some of the energy due to ground shaking, and 
safely dissipate the balance after the earthquake stops. If the structure cannot 
keep up with the energy input, the structure may suffer minor cracking, or 
rupture, or major failure (Figure 7-8B).
7.6.7 The Design ApproachForce vs. Energy
Searching for the perfect system with conventional solutions has been limited to 
date, because seismic forces are used that are based on accelerations, and then 
the resulting lateral drift or movements of the structure are reviewed to check 
the behavior of both structural and nonstructural elements, such as rigid 
cladding. This process has an inherent conflict.
Seismic accelerations are used because a design force, F, from the relation 
F=Ma, can easily be obtained where M is the building mass, and a is the ground 
acceleration.
For the typical stiff-soil site, the larger the acceleration a, the larger the 
seismic force F. The larger the force F, the stronger the structure is; the 
stronger the structure, the stiffer the structure. The stiffer the structure is, 
the higher the seismic acceleration, and so on. A strong, stiff structure 
appears to be good conceptually. The lateral drift (or horizontal displacement) 
is at a minimum, which is also good. However, the contradiction of this approach 
comes from seismic energy dissipation, which is the fundamental need and 
characteristic of good seismic design; energy dissipation comes from large 
displacements, not small displacements.
Large displacements are needed to dissipate seismic energy, and small 
displacements are needed for lateral drift control to protect cladding, glazing, 
and interior systems. This produces a conflict for most of the classic 
conventional structural concepts and normal nonstructural components.
The most useful seismic systems are those that have predictable, stable, 
nondegrading cyclic behavior. Contemporary structures with these characteristics 
are base-isolation systems; moment frames with dampers; shear-link systems, such 
as coupled shear walls and eccentric-braced frames; and other dual-resistance 
systems with built-in redundancy.
The most useful nonstructural components are those that accommodate large 
lateral movements without failure.
7.7 THE SEARCH FOR THE PERFECT SEISMIC SYSTEM
The 100-year review of seismic systems indicates a slow development of 
structural solutions. In fact, most new development occurs after a damaging 
earthquake. Perhaps this slow periodic development is due to the need to discern 
whether previous ideas were successes or failures. The following topics 
represent steps and ideas in the search for the perfect system.
7.7.1 Structural Mechanisms
There have been periodic significant conceptual breakthroughs in structural 
thinking. For example, Dr. Ian Skinner and his team, working at the New Zealand 
National Laboratory, developed in 1976 a set of energy dissipating concepts 
suitable for use in seismic protection. Among the most notable concepts was the 
practical use of elastomeric isolation bearings for global protection of 
complete structures. Some of the other concepts developed in New Zealand have 
also been utilized in building design. Figure 7-9 illustrates the set of 
mechanisms published in 1980.
Many of these relatively simple mechanisms were new to the design professions 
and to constructors; consequently, they represented unknown processes and 
unknown costs. Development and education are the keys to the acceptance and 
adoption of these systems. Perhaps a few well-publicized, prototype projects 
would familiarize the design professions and constructors with the details and 
costs of these good ideas. To date, this has only been done with the flexural 
plate and the lead-rubber isolation bearing.
7.7.2 Semi-Active and Active Dampers
Recent research and design work in Japan and the United States has focused on 
approaches for structural control during actual wind storms and earthquakes. 
These approaches have been summarized by Hanson and Soong, and can be divided 
into three groups: passive systems, such as base isolation and supplementary 
energy dissipation devices; active systems that require the active participation 
of mechanical devices whose characteristics are used to change the building 
response; and hybrid systems that combine passive and active systems in a manner 
such that the safety of the building is not compromised, even if the active 
system fails.The current systems being studied are characterized by devices that 
control properties and behaviors. Some of these systems are in limited use; 
others are still in development.
The goal of these devices is to respond actively to the variable character of 
wind and earthquake displacements, velocity, accelerations, etc., by adding 
damping or altering stiffness. This controlled behavior will provide the needed 
resistance to respond to ever-changing earthquakes. However, the challenge with 
semi-active or active control systems is maintaining their behavior trouble-free 
over an extended period of time - specifically over many years. These concepts, 
although very appealing, will require some time to perfect and bring to market.
7.7.3 Cost-Effective Systems
The most important measure of good earthquake-resistant design is the impact on 
the structure after the earth has stopped shaking. With little building damage, 
repair costs are low. With significant damage or collapse, repair or replacement 
costs are high. The measure of success in seismic design is selection of a 
structure that will suffer minimum damage, and with corresponding low post-
earthquake repair cost. The behavior of each structural system differs with 
earthquake ground motions, soil types, duration of strong shaking, etc. Our past 
observations of damage yields the best measure of future repair costs. Systems 
with stable cyclic behavior, good energy dissipation, and controlled interstory 
drift will yield low repair costs (Figure 7-10).
The above five attributes, which are tabulated in Figures 7-11A and 7-11B for 
the various significant seismic structural systems, show that a range of 
possibilities is indicated.  These tables are presented for discussion, and the 
evaluations stated are the authors opinion, based on observations, analysis, 
and common goals for seismic performance.  The green colored boxes are favorable 
conditions; the red tend to be unfavorable. The favorable structural systems 
will do the following:
   Possess stable cyclic behavior
   Control lateral drift
   Dissipate seismic energy without failure
   Create a low post-earthquake repair cost
The design reduction value R, discussed above in section 7.6.1,  does not 
necessarily correlate with performance. The R value was a consensus value 
developed for conventional elastic design. With the advent of performance design 
based on nonlinear evaluation the R value serves only as a rough estimate of 
system behavior, but not a realistic estimate of performance.
7.7.4 Avoiding the Same Mistakes
Architects and engineers learn from their detailed investigations of past 
earthquake damage and can document the significant issues and lessons that can 
be learned about particular structural problems. Some problems occur because of 
inappropriate building or structural configuration, some because of brittle, 
nonductile structural systems, some because the building or structure could not 
dissipate sufficient seismic energy, and some because of excessive loads caused 
by dynamic resonance between the ground shaking and the building. Figure 7-12 
illustrates some classic problems.
Why, with all our accumulated knowledge, does all this failure continue? 
Buildings tend to be constructed essentially in the same manner, even after an 
earthquake. It takes a significant effort to change habits, styles, techniques 
and construction. Sometimes bad seismic ideas get passed on without too much 
investigation and modification.
The challenge for architects, engineers, and constructors is to ask why so much 
damage occurred, and what can be done to correct the problem. Understanding the 
basic seismic energy demand is a critical first step in the search for 
significant improvement. Not all structural systems, even though they are 
conventional and commonplace, will provide safe and economical solutions.
7.7.5 Configurations Are Critical
Configuration, or the three-dimensional form of a building, frequently is the 
governing factor in the ultimate seismic behavior of a particular structure. 
Chapter 5 covers conventional configuration issues where conventional 
rectangular grids are used in building layout, design and construction. However, 
contemporary architectural design is changing, in large part because the 
computer allows complex graphic forms and analyses to be generated and easily 
integrated into a building design. 
The resulting irregular, random, free-form grids and systems have just begun to 
be explored from the structural engineering viewpoint. They are frequently 
rejected because of various cost issues, and because of unproven real earthquake 
behavior. 
The potential for optimizing seismic resistance with respect to structural 
configuration is an obvious direction for the future. Structural form should 
follow the needs. How can we define seismic needs? 
Buildings must dissipate energy; how does one configure a structure to dissipate 
energy? There are natural forms and design concepts that act as springs, rocking 
mechanisms, flexural stories, yielding links, articulated cable-restrained 
configurations, pyramid forms, cable anchors, etc. Any system that can dissipate 
seismic energy without damage is a candidate. Figure 7-13 illustrates some 
special concepts utilizing building configuration to dissipate energy.
7.7.6 Common-Sense Structural Design-Lessons Learned
The simple way to reduce seismic demand within a structure is to understand the 
actual demand. An earthquake is a dynamic phenomenon with all its classic 
characteristics. If one can reduce the effective damaging character of the 
earthquake, the behavior of a structure or building will be significantly 
improved. The following five issues can significantly reduce earthquake damage 
and related costs.
  Select the Appropriate Scale
The size and scale of the building should determine the appropriate structural 
solution. It is common sense to use a light braced frame for a small structure, 
such as one- or two-story wood or light-gauge steel systems. The light seismic 
system is compatible with the light building mass. As the building mass 
increases with the use of heavy concrete floors, the mass increases by a factor 
of 8 to 12 times. This would imply a much heavier seismic frame or brace (Figure 
7-14), 8 to 12 times stronger per floor, and if the building grows from two to 
ten stories, the total mass increases from two to 80-120 times. 
The same frame or brace which works well at two stories no longer works well at 
ten stories, because structural components when simply scaled up no longer 
behave in the same fashion. For instance, a light steel section with 3/8-inch 
flanges is ductile, but no longer has the same ductility when the flanges are 
two inches thick. The thick sections and welds are now subjected to high shears 
and are prone to failure. 
Some structural systems are very forgiving at small sizes but not at a large 
scale. Alternative structural systems must be used for the larger, more 
demanding buildings. The appropriate systems must not degrade in strength and 
should have ample sustainable damping. It is critical that scale be considered, 
even though it is not considered by the seismic building codes. This scale issue 
requires study, observation, and common sense, but the issue of appropriate 
scale continually confronts designers. Careful, unbiased research is necessary.
  Reduce Dynamic Resonance
It is important to significantly reduce the dynamic resonance between the 
shaking ground and the shaking building, and to design the structure to have a 
period of vibration that is different from that of the ground period. The 
difference is simply illustrated with a classic resonance curve (Figure 7-15).
The relationship between the building period and spectral acceleration of a real 
earthquake varies between a site with firm soil and a site with soft soil 
(Figure 7-16).
If the building has a period of vibration, T1, corresponding to the peak ground 
acceleration, the most severe demand occurs. Shift the period, T2,  by altering 
the structural system to lengthen the period (firm soil) or shorten or lengthen 
the period T2 (soft soil). 
This requires design effort by both the architect and engineer, but the result 
can be a significant reduction in the seismic response.
  Increase Damping
It is valuable to significantly increase the structural system damping. Damping 
reduces vibration amplitude similar to the hydraulic shock absorbers in an 
automobile, and damping reduces the structural demand (Figure 7-17), as 
illustrated in the spectral acceleration plot.
Significant damping can be introduced into a structure by 1) adding a passive 
damping system (hydraulic, friction, etc.); 2) utilizing a fractured concrete 
member such as a shear-link serving to couple two walls; 3) fracturing a 
concrete shear wall; 4) utilizing a seismic isolation system (with 10 to 20% 
damping), or 5) utilizing a tuned-mass damper (a challenging solution for most 
buildings). An increase of damping by using a non-distinctive system is a most 
positive solution for reducing seismic demand.
  Provide Redundancy
It is also important to add redundancy or multiple load paths to the structure 
to improve seismic resistance. After experience in many earthquakes and much 
study and discussion, the engineering profession has generally concluded that 
more than a single system is the ideal solution for successful seismic 
resistance. If carefully selected, multiple systems can each serve a purpose; 
one to add damping and to limit deflection or drift, the other to provide 
strength. Multiple systems also serve to protect the entire structure by 
allowing failure of some elements without endangering the total building.
An informative sketch of a classic redundant-framing concept, with frames on 
each grid line, versus a contemporary multiple system with two types of framing, 
one for strength, the other for damping, is shown in Figure 7-18. The current 
dual systems now being developed and utilized are a significant improvement over 
the historic single seismic resisting systems.
  Energy Dissipation
Solving the seismic energy dissipation problem is the ultimate test of good 
earthquake-resistant design. Since large building displacement is required for 
good energy dissipation, while minimum displacement is required to protect the 
many brittle non-structural components in the building, only one seismic 
resisting system adequately solves both aspects of this problemseismic base 
isolation. 
However, seismic isolation is not suited for all building conditions: 
specifically, tight urban sites adjacent to property lines (a movement problem), 
and tall buildings over 12 stories (a dynamic resonance problem). Cost of 
construction is another consideration (about a 1 to 2% premium). Seismic 
isolation is an ideal solution for irregular buildings and unusual or creative 
building forms that are difficult to solve with conventional structural systems.
Other dual systems can also solve the energy dissipation problem. Six seismic 
systems are outlined in Figure 7-19. Each system has a cyclic load-deformation 
loop which is full and nondegrading. Most importantly, each has a predicted 
minor-to-moderate post-earthquake damage, with a correspondingly low post-
earthquake repair cost. This fact makes these good seismically sustainable 
buildings, expanding the meaning of green building.
7.8 CONCLUSIONS
We must continue to develop and use increasingly realistic analysis methods to 
design new buildings and to modify existing structures. This is not a simple 
task because of the numerous variables: duration of shaking, frequency content 
of the seismic motion, displacement, velocity, acceleration, direction of the 
earthquake pulse, proximity to the active fault, and soil amplification effects. 
In addition to the ground effects, the structure has its own variables: size; 
shape; mass; period of vibration; irregularities in shape, stiffness, and 
strength; and variation in damping. 
After the past 100 years of seismic design, the perfect architectural/structural 
solution is still elusive, but with creative thought, testing, and computers, 
the problem is more transparent, and we can now more rapidly study variables 
than in the recent past. Design work today has become part research, part 
invention, and common sense, and we are on the threshold of creative 
breakthrough.
7.9 REFERENCES
1.  Park, R. and Paulay, T, 1975. Reinforced Concrete Structures, Wiley
Interscience.
2.  Paulay, T. and Priestly, N, 1992. Seismic Design of Reinforced Concrete and 
Masonry Buildings, Wiley. 
3.  Hanson, R. and Soong, T.T, 2001. Seismic Design with Supplementary Energy 
Dissipation Devices, Earthquake Engineering Research Insitute, Oakland CA.

[End of Chapter 7]