Civil Engineering Reference
In-Depth Information
Berkeley.
4
This equation, which is shown in Section R10.3.6 of the ACI Commentary,
follows:
1
P
ni
1
1
1
P
o
P
nx
P
ny
where
P
ni
the nominal axial load capacity of the section when the load is placed at a given
eccentricity along both axes.
P
nx
the nominal axial load capacity of the section when the load is placed at an ec-
centricity
e
x
.
P
ny
the nominal axial load capacity of the section when the load is placed at an ec-
centricity
e
y
.
P
o
the nominal axial load capacity of the section when the load is placed with a
zero eccentricity. It is usually taken as
c
A
g
f
y
A
s
.
0.85
f
The Bresler equation works rather well as long as
P
ni
is at least as large as 0.10
P
o
.
Should
P
ni
be less than 0.10
P
o
, it is satisfactory to neglect the axial force completely and
design the section as a member subject to biaxial bending only. This procedure is a little
on the conservative side. For this lower part of the interaction curve, it will be remem-
bered that a little axial load increases the moment capacity of the section. The Bresler
equation does not apply to axial tension loads. Professor Bresler found that the ultimate
loads predicted by his equation for the conditions described do not vary from test results
by more than 10%.
Example 10.8 illustrates the use of the reciprocal theorem for the analysis of a col-
umn subjected to biaxial bending. The procedure for calculating
P
nx
and
P
ny
is the same as
the one used for the prior examples of this chapter.
EXAMPLE 10.8
Determine the design capacity
P
ni
of the short tied column shown in Figure 10.23, which is sub-
jected to biaxial bending.
c
f
4000 psi,
f
y
60,000 psi,
e
x
16 in., and
e
y
8 in.
SOLUTION
For Bending about
x
Axis
20
25
0.80
8.00
(15)(25)
0.0213
g
e
h
16
25
0.64
4
Bresler, B., 1960, “Design Criteria for Reinforced Concrete Columns under Axial Load and Biaxial Bending.”
Journal ACI
, 57, p. 481.
