Civil Engineering Reference
In-Depth Information
21
Reinforced Concrete Building
Systems
21.1
INTRODUCTION
To this point we have been concerned primarily with the design of individual components
of structural systems. Even when we looked at parts of a frame to determine the end ef-
fects for the purpose of designing columns, we were predominantly designing the compo-
nents rather than the whole systems. In this chapter our focus changes from components
to entire structural systems. With this investigation we will try to develop a better under-
standing of the relationship between structural analysis and structural design as well as
the impact of design decisions on the overall behavior of structural systems.
1
Figure 21.1 presents a flowchart of the process of structural system design. In this illus-
tration, you should observe that we must develop an acceptable system for both strength and
serviceability requirements. The strength requirements are fundamentally that the structure
must support the loads to which it is subjected. Using the procedures discussed in this text-
book, design for strength is based on a limit states theory. Serviceability requirements, on the
other hand, are usually requirements based on elastic response and in particular on elastic de-
formation response. These requirements include a consideration of live load deflections and
vibrations as well as cracking of finish surfaces. The illustration also shows that the selection
of loads and the geometry of the structure are very important considerations when designing
a system. Furthermore, unless the structure is statically determinate, changes in design will
probably affect the distribution of force throughout the system. We must evaluate these ef-
fects after designing the system—structural design consists of several design-analyze cycles.
An equally important part of structural system design is the consideration of design
alternatives. As we progress through the design-analyze process, we should always be
looking for ways to improve the design. We should be seeking to better the performance
of the system, reduce the overall cost of the system, or simplify construction. Rarely does
the design that is actually built reflect the first pass at the design.
As we proceed through this chapter, we must always recognize that there is not a single
design solution for a given structure or for a particular situation.
Rather, when designing a
structural system, we should realize that there is likely to be a family of acceptable design
1
Much of the information presented in this chapter has been extracted from Jack C. McCormac and James K.
Nelson,
Structural Steel Design: LRFD Method
, 3rd ed. (Upper Saddle River, NJ: Pearson Education
Incorporated, 2003).
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