Civil Engineering Reference
In-Depth Information
Water Tower Place, Chicago, Illinois, tallest reinforced concrete building in the United States (74
stories, 859 ft). (Courtesy of Symons Corporation.)
large sections may be used with small percentages of steel, or small sections may
be used with large percentages of steel. Such variations were not the case in the
relatively fixed working-stress method. If the same amount of steel is used in
strength design for a particular beam as would have been used with WSD, a
smaller section will result. If the same size section is used as required by WSD, a
smaller amount of steel will be required.
3.3
STRUCTURAL SAFETY
The structural safety of a reinforced concrete structure can be calculated with two meth-
ods. The first method involves calculations of the stresses caused by the working or ser-
vice loads and their comparison with certain allowable stresses. Usually the safety factor
against collapse when the working-stress method was used was said to equal the smaller
of or
f
y
/
f
s
.
The second approach to structural safety is the one used in strength design in which
uncertainty is considered. The working loads are multiplied by certain load factors that are
larger than one. The resulting larger or factored loads are used for designing the structure.
The values of the load factors vary depending on the type and combination of the loads.
To accurately estimate the ultimate strength of a structure, it is necessary to take into
account the uncertainties in material strengths, dimensions, and workmanship. This is
done by multiplying the theoretical ultimate strength (called the
nominal strength
herein)
c
/
f
c
f
