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INTRODUCTION

. . . the shoe that fits one person pinches another.

Carl Gustav Jung

This book is primarily about design, which, as I use the term, is the creative process

that seeks the proper blend of essential ingredientsspecifically function, aesthetics,

economy, and, in the context of this book, seismic behavior. There exists no singleformula for creating a good design, for the design process involves making a set of

decisions on issues for which no absolutely right answer exists. Thus the designer is

continually seeking a comfortable rationally based design solution, and two identical

solutions are not likely to be produced even successively by the same constructive

designer.

Tools are essential to the completion of almost every task. I have tried to assemble,

in as concise a form as possible, the tools necessary to the pursuit of a good design.

From the extensive library of experimental efforts, I have selected representativeworks and demonstrated how both strength and deformation limit states might be

predicted. Next, I review alternative design approaches and, in the process, simplify

and adapt them to specific types of bracing systems. Finally I describe how designs

might be comprehensively reviewed.

The focus of the book is concrete and the emphasis is on precast concrete. I

have limited the scope to the satisfaction of seismic behavior objectives because the

topic is complex and, though extensively studied and codified, not necessarily well

understood by the structural design profession. The fact that seismic design can bereduced to an understandable level that can be creatively introduced into a building

program makes it an ideal vehicle to study the design process.

Concrete as a composite material provides a medium that encourages freedom.

The design of structures constructed using composite materials is not peculiar to the

materials selected for any combination of dissimilar materials must satisfy the same

basic fundamental laws and this is because equilibrium, compatibility, and adherence

to the appropriate stress-strain relationship must always be attained. Accordingly,

the choice of concrete as a vehicle should not be viewed as a constraint on the

applicability of the material contained herein.

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Copyright 2003 John Wiley & Sons Retrieved from: www.knovel.com

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2 INTRODUCTION

Precast concrete is but one creative extension of the use of concrete. It is an

especially important extension because the prefabrication of structures can and will

be required to meet the needs of society. The use of precast concrete has traditionally

been viewed with skepticism in regions considered to have a potential for seismicactivity. This is largely the result of a lack of understanding of the basic nature

of seismic behavior and how the attributes of precast concrete can be exploited to

improve behavior. The designer of a precast concrete structure, armed with the proper

tools, can create a structure that will not only survive an earthquake, but do so with

very little, if any, damage. To accomplish this lofty objective requires only that the

designer take advantage of the jointed nature of the assemblage of precast elements.

To present the seismic design of precast concrete as a stand-alone topic would

limit the usefulness of the treatment because a consistent base is critical to bothexplaining and understanding the behavior of precast concrete members and systems.

Accordingly, the basic elements of both seismic behavior and the behavior and design

of concrete must precede any treatment of precast concrete. The precast concrete

seismic systems whose design is described in some detail herein are only intended to

be examples of what can be accomplished with creative thinking. The objective then

is to inspire creative applications of a versatile product.

The design process must be free and dynamic to be effective. Accordingly, a design

must move aggressively to make the many decisions required in an orderly fashionwith a minimum amount of distraction. The process usually starts by tackling the

most difficult decision(s) first and, when necessary, looking quickly downstream in

the decision-making process to confirm that potential problems do, in fact, have a

solution.

I endeavor to place emphasis on the primary objectives of the design process and

relieve, or at least loosen wherever possible, the ever-increasing number of prescrip-

tive constraints being imposed on designs. This is especially important because the

concept development (creative) part of a design must focus on the broader objectivesand leave the details to the development of the concept. The importance of detail is

not discounted by this apparent deferral, for the completed design package must be

very clear on how the broader objectives are accomplished. It is this almost subliminal

awareness of detail that will allow the focus essential to the creation of an excellent

design.

Creative design clearly does not allow regimentation, and this makes it almost

impossible to present design as a subject. My effort toward regimentation is limited

to subdividing the presented material into four broad categories, but even this is notadhered to strictly. Chapter 1 discusses selected basic concepts. The objective is to

provide the designer with the basic insight necessary to the effective development

of a design. A comprehensive treatment of each topic would, in most cases, take

volumes and tend to obscure the basic concepts and objectives. I have tried to identify

references for the reader who is not satisfied with the brevity of treatment contained

herein. Fortunately and unfortunately, the expanded treatment of many of the basic

concepts presented herein has reached a level of development far beyond the technical

capability of most of us. The fortunate aspect is that most of the theory is finding

its way into computer applications that, if properly applied and understood, should


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INTRODUCTION 3

help the designer make difficult decisions. The unfortunate aspect is that most of the

material has been presented, at least to date, in a way that does not allow the rapid

assimilation of concepts by the reader whose primary preoccupation is in an ancillary

area. Perhaps a treatment along the lines of Shakespeare for Dummies would find alarger audience.

I considered including several additional topics in Chapter 1, but elected instead to

scatter them throughout the book. They are covered in the discussions of the design

processes where they may effectively be used. To compensate for the resulting scatter,

I have tried to use the index as an effective locator.

The most important of these topics relates to understanding statics and indetermi-

nate structures and how this understanding might quickly and reliably be reduced to

design methodologies. It seems as though each passing year and each new softwarepackage causes us to become less facile in reducing complex structures to a level that

allows us to make the appropriate design decisions. Design by iteration is becoming

increasingly popular, but it will never be effective as a tool to create the desired bal-

anced design. A learned mathematician once assured me that enough monkeys armed

with typewriters would ultimately produce all of the works of Shakespeare. The prob-

lem from the structural design perspective is that we are given neither the time nor

the money to follow this path. The question usually proposed to the designer is Can

I do this? and the time frame allowed for coming up with an answer is measured indays. Such a time frame does not allow for extensive research or for time-consuming

analytical procedures. The analytical reductions used in various example designs not

only allow design insight and a quick means of evaluating the efficacy of a concept,

but also a quick check of computer solutions.

Chapter 2 deals with the behavior and design of components of bracing programs.

The approach to component design in Chapter 2 starts by reviewing selected experi-

mental efforts and attempting to use the results of the experimental effort to support

or propose design procedures for the component. One need only look at the tablesof contents of the many technical journals to appreciate how much experimentation

is being documented annually in universities around the world. Accordingly, it is

impossible to reduce all available experimental data to a digestible form; thus I have

been very selective.

Components can also be systems as, for example, in the case of shear walls.

I have tried to draw the following distinction in the adopted approach. If a body

of experimental work treats the subject, I have included the subject in Chapter 2.

In the case of shear walls, Chapter 2 explores the experimental evidence and howthese data might be effectively used to create a design approach. The element is

then reintroduced as a part of a system in Chapter 3, expanding on the previously

developed design procedures.

The seismic performance of a component is not exclusively concerned with its

strength, for we know that seismically induced displacement demands will force

members to deform well beyond their elastic limit states. The success with which

a component responds to these postyield deformation demands can only be evalu-

ated by understanding strain limit states and the damage that is likely to occur as a

function of large ductility demands. Critical strains are those that define the inception


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4 INTRODUCTION

of damageshell spalling and strength degradation. The experimental efforts used

to identify probable strain states is of necessity limited, and the reader is encour-

aged to continually review new or other pertinent experimental efforts in order to

establish what he or she believes to be the appropriate limit state. Perhaps focusedresearch will be undertaken to establish and confirm some of the more speculative

limit states.

Having proposed a set of strength- and strain-based limit states, Chapter 2 next

develops a design methodology for each component, and shows how limit states can

be tested by example. Detailing considerations are discussed and details developed.

Where appropriate, codification concepts are discussed and reduced to a level of

analytical simplicity appropriate for design. Occasionally, the limited applicability of

commonly held dogma is reviewed, as are procedures or behavior characteristics notcommonly used by U.S. designers. The goals of this chapter are to reduce component

design to as simple a process as possible and to provide insight into objectives often

well disguised in the codification process.

Chapter 3 is the heart of the book. The focus of Chapter 3 is the design of bracing

systems. The objective is to conceptually create a bracing program that is effective

from both a cost and a behavior perspective. Building behavior must be controlled in

the design process. The building must behave as you, the designer, intend it to, and

only you can make this happen. I, as a grandfather, explain to my students that thebehavior of a building is probably the only thing in your life that you have a chance

of controlling.

One of the lessons I learned early in my career was that a design or a design concept

must be less expensive and better than its alternative if it is to be accepted or adopted,

and that the better part was a distant second consideration. Thus it is incumbent

on the designer to create a cost-effective design in order for it to be realized and to

almost subliminally include the better aspect into every design.

The appropriateness of a well-conceived design may defy codified dogma, and thiswill require courage on the part of the designer. An ethical issue is clearly raised, one

that must be resolved by the responsible designer after careful study and consultation

with peers.

Chapter 3 also presents a variety of design approaches. These include classical

strength procedures as well as displacement-based approaches. I have advocated and

used displacement-based procedures for more than thirty years, and I am convinced

that they offer by far the best chance for producing a successful design. I support the

development and acceptance of a performance-based approach wholeheartedly. Pro-cedures currently proposed are not, in my opinion, easily applied to the conceptual de-

sign process. Further, they do not take advantage of a designers understanding of the

behavior characteristics inherent to structural systems. I, therefore, propose simpler

procedures that include the basic philosophies of each displacement-based approach,

specifically those that follow the equal-displacement tenet and those that treat struc-

tural damping as a system-dependent variable. When I first explored displacement-

based procedures, design earthquakes were nowhere near as strong as they now are

and, as a consequence, strain limit states that I established after analyzing experimen-

tal effort never were approached. Now with design earthquake intensities five or so


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INTRODUCTION 5

times greater, these limit states are being approached, but who knows how strong the

next decades earthquakes might be? Therefore, I treat ductility much as a rich person

treats moneythere simply is no such thing as too much.

The precision suggested by most design procedures is illusory. Response modifi-cation factors now identify more than fifty categories with variations of 1.2% between

vastly different types of structural systems. This suggests a solid technical basis that

does not exist. When I work in three significant figures in examples, it is not because

I believe it to be analytically appropriate to do so, but rather only to allow the reader

a better chance to track the example. Typically, when I prepare a design, I work to

two significant figures and try to constantly review or crosscheck my conclusions to

make sure I get into the ballpark; once Im in the ballpark, it is easy enough in the

analysis phase to find the right seat.Finding the ballpark is an essential part of the conceptual design process. As a

designer you will soon learn that once a program is set it cannot be changed and

the only real option is to mitigate mistakes in concept. On the other hand, if the first

step is in the right direction and allows the latitude to properly consider potential

contingencies, the design will flow smoothly. Early on I found that if I located my

bracing systems in areas that would otherwise not be used, they could maintain

their integrity. Whenever bracing systems can come into conflict with other building

systems, rest assured a conflict will eventually occur. For years I have held the beliefthat mechanical/plumbing engineers, in spite of whatever lip service they may give

during the conceptual design phase, do not start their designs in earnest until the

concrete structure has progressed far enough to make it necessary to core or cut into

it. So make sure your design, if it can be in harms way, has some breathing room.

Chapter 4 introduces the reader to the elastic and inelastic time history procedures

used to confirm or evaluate the efficacy of a design. The objective is not to support

any particular design approach, but rather to better understand the messages time

history analyses can convey. Presumably the designer will know the final answerbefore this type of analysis is undertaken, for it will be very painful if a major

change has to be made at this stage. If the design procedures and checks presented

in Chapter 3 are followed, it is unlikely that a significant change will be required. So

why bother with a confirmation of the design? A retrospective review of any decision-

making process extends the all-important experience base. It should, at the very least,

add confidence to future designs or provide the courage necessary to take on more

challenging designs. The architecture of structural expressionism is a thing of the

past. The free form of todays architectural styles requires boldness on the part ofthe structural engineer, and this must be supported by knowledge, experience, and

confidence.

Chapter 4 also explores the sensitivity of designs to parameters like strength and

hysteretic damping. If parametric studies are used extensively in the retrospective re-

view process, they should allow the designer to more effectively control the behavior

of buildings in subsequent designs. Sufficient strength, for example, has traditionally

been viewed as the key to a successful design. Some professional societies believe

that buildings should be designed so as to respond almost exclusively in the elastic

range. The parametric studies included herein, and those performed by others, suggest


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6 INTRODUCTION

that the benefits associated with an increase in strength are small. This is certainly

not intended to discount strength as an important design consideration, but rather to

point out that associated negative impacts can be greater than the potential reductions

in displacement. Remember, the strength of the yielding element will impose moredemand on the brittle components along the lateral load path. Increases in system

strength will also increase accelerations, and this too will tend to cause more damage

to building contents.

The fullness of a hysteresis loop has always been considered a positive attribute.

How full must it be to produce the desired control over building response? Parametric

studies suggest that, like strength, there is only a vague link, provided that reasonable

levels of both strength and energy dissipation are provided. It is possible that carefully

designed shaking table tests will shed more light on these issues. Until such time,designers will have to use available tools and their intuition to produce the best

possible building. I am convinced that, given todays knowledge base, successful

designs can reliably be produced.

My hope is that the material contained in this book will make it possible for both

the student and practitioner to effectively utilize the vast amount of material that has

been developed over the past quarter century to develop designs with which they can

be comfortable, designs that will serve society well.

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