Civil Engineering Reference
In-Depth Information
3
Strength Analysis of Beams
According to ACI Code
3.1
DESIGN METHODS
From the early 1900s until the early 1960s, nearly all reinforced concrete design in the
United States was performed by the working-stress design method (also called
allowable-
stress design
or
straight-line design
). In this method, frequently referred to as WSD, the
dead and live loads to be supported, called
working loads
or
service loads
, were first esti-
mated. Then the members of the structure were proportioned so that stresses calculated by
a transformed area did not exceed certain permissible or allowable values.
After 1963 the ultimate-strength design method rapidly gained popularity because
(1) it makes use of a more rational approach than does WSD; (2) a more realistic consid-
eration of safety is used; and (3) it provides more economical designs. With this method
(now called
strength design
) the working dead and live loads are multiplied by certain
load factors (equivalent to safety factors) and the resulting values are called
factored
loads
. The members are then selected so they will theoretically just fail under the factored
loads.
Even though almost all of the reinforced concrete structures the reader will encounter
will be designed by the strength design method, it is still rather desirable to be familiar
with WSD for several reasons. These include the following:
1.
AASHTO permits the design of reinforced concrete for highway bridges using ei-
ther the working-stress method (WSD) or the strength design method. Such de-
signs are to be made today in accordance with the requirements of the 2002
Standard Specifications for Highway Bridges plus interim specifications.
2.
Some designers use WSD for proportioning fluid-containing structures (such as
water tanks and various sanitary structures). When these structures are designed
by WSD, stresses are kept at fairly low levels, with the result that there is appre-
ciably less cracking and less consequent leakage. (If the designer uses strength de-
sign and makes use of proper crack control methods as described in Chapter 6,
there should be few leakage problems.)
3.
The ACI method for calculating the moments of inertia to be used for deflection
calculations requires some knowledge of the working-stress procedure.
4.
The design of prestressed concrete members is based not only on the strength
method but also on elastic stress calculations at service load conditions.
65
