Civil Engineering Reference
In-Depth Information
The reader should realize that working-stress design has several disadvantages. When
using the method, the designer has little knowledge about the magnitudes of safety factors
against collapse; no consideration is given to the fact that different safety factors are de-
sirable for dead and live loads; the method does not account for variations in resistances
and loads, nor does it account for the possibility that as loads are increased, some increase
at different rates than others.
In 1956, as an appendix, the ACI Code for the first time included ultimate-strength
design, although the concrete codes of several other countries had been based on such
considerations for several decades. In 1963 the Code gave ultimate-strength design equal
status with working-stress design, the 1977 Code made the method the predominant
method and only briefly mentioned the working-stress method. From 1977 until 2002
each issue of the Code permitted designers to use working-stress design and set out cer-
tain provisions for its application.
In the 2002 Code, however, permission is not included
for using the method.
Today's design method was called
ultimate-strength design
for several decades; but,
as mentioned, the Code now uses the term
strength design
. The strength for a particular
reinforced concrete member is a value given by the Code and is not necessarily the true
ultimate strength of the member. Therefore, the more general term
strength design
is used
whether beam strength, column strength, shear strength, or others are being considered.
3.2
ADVANTAGES OF STRENGTH DESIGN
Among the several advantages of the strength design method as compared to the no
longer permitted working-stress design method are the following:
1.
The derivation of the strength design expressions takes into account the nonlinear
shape of the stress-strain diagram. When the resulting equations are applied, de-
cidedly better estimates of load-carrying ability are obtained.
2.
With strength design, a more consistent theory is used throughout the designs
of reinforced concrete structures. For instance, with working-stress design the
transformed-area or straight-line method was used for beam design, and a
strength design procedure was used for columns.
3.
A more realistic factor of safety is used in strength design. The designer can cer-
tainly estimate the magnitudes of the dead loads that a structure will have to sup-
port more accurately than he or she can estimate the live and environmental loads.
With working-stress design the same safety factor was used for dead, live, and en-
vironmental loads. This is not the case for strength design. For this reason, use of
different load or safety factors in strength design for the different types of loads is
a definite improvement.
4.
A structure designed by the strength method will have a more uniform safety fac-
tor against collapse throughout. The strength method takes considerable advan-
tage of higher-strength steels, whereas working-stress design did only partly so.
The result is better economy for strength design.
5.
The strength method permits more flexible designs than did the working-stress
method. For instance, the percentage of steel may be varied quite a bit. As a result,
