Civil Engineering Reference
In-Depth Information
Massive reinforced concrete columns. (Courtesy of Bethlehem Steel Corporation.)
Refer to point
D
on the curve. The horizontal and vertical dashed lines to this point
indicate a particular combination of axial load and moment at which the column will fail.
Should a radial line be drawn from point 0 to the interaction curve at any point (as to
D
in
this case), it will represent a constant eccentricity of load, that is, a constant ratio of mo-
ment to axial load.
You may be somewhat puzzled by the shape of the lower part of the curve from
B
to
C
, where bending predominates. From
A
to
B
on the curve the moment capacity of a sec-
tion increases as the axial load decreases, but just the opposite occurs from
B
to
C
. A little
thought on this point, however, shows that the result is quite logical after all. The part of
the curve from
B
to
C
represents the range of tensile failures. Any axial compressive load
in that range tends to reduce the stresses in the tensile bars, with the result that a larger
moment can be resisted.
In Figure 10.11 an interaction curve is drawn for the 14
column with six #9
bars considered in Section 10.3. If eight #9 bars had been used in the same dimension col-
umn, another curve could be generated as shown in the figure; if ten #9 bars were used,
still another curve would result. The shape of the new diagrams would be the same as for
the six #9 curve, but the values of
P
n
and
M
n
would be larger.
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