Civil Engineering Reference
In-Depth Information
Royal Towers, Baltimore, Maryland. (Courtesy of Simpson Timber Company.)
Thus, as shown in Section 10.3.6 of the Code, the axial load capacity of columns may not
be greater than the following values:
For spiral columns (
0.70)
c
(
A
g
A
st
)
f
y
A
st
]
P
n
(max)
0.85
[0.85
f
(ACI Equation 10-1)
For tied columns (
0.65)
c
(
A
g
A
st
)
f
y
A
st
]
P
n
(max)
0.80
[0.85
f
(ACI Equation 10-2)
It is to be clearly understood that the preceding expressions are to be used only when
the moment is quite small or when there is no calculated moment.
The equations presented here are applicable only for situations where the moment is
sufficiently small so that
e
is less than 0.10
h
for tied columns or less than 0.05
h
for spiral
columns. Short columns can be completely designed with these expressions as long as the
e
values are under the limits described. Should the
e
values be greater than the limiting
values and/or should the columns be classified as long ones, it will be necessary to use the
procedures described in the next two chapters.
9.8
COMMENTS ON ECONOMICAL COLUMN DESIGN
Reinforcing bars are quite expensive, and thus the percentage of longitudinal reinforcing
used in reinforced concrete columns is a major factor in their total costs. This means that
under normal circumstances a small percentage of steel should be used (perhaps in the range
of 1.5 to 3%). This can be accomplished by using larger column sizes and/or higher strength
concretes. Furthermore, if the percentage of bars is kept in approximately this range, it will
be found that there will be sufficient room for conveniently placing them in the columns.
