Civil Engineering Reference
In-Depth Information
Column forms. (Courtesy of Economy Forms Corporation.)
mally round, but they also can be made into rectangular, octagonal, or other shapes. For
such columns, circular arrangements of the bars are still used. Spirals, though adding to
the resilience of columns, appreciably increase costs. As a result, they are usually used
only for large heavily loaded columns and for columns in seismic areas due to their con-
siderable resistance to earthquake loadings. (In nonseismic zones, probably more than 9
out of 10 existing reinforced concrete columns are tied.) Spirals very effectively increase
the ductility and toughness of columns, but they are much more expensive than ties.
Composite columns
, illustrated in Figures 9.2(c) and 9.2(d), are concrete columns
that are reinforced longitudinally by structural steel shapes, which may or may not be sur-
rounded by structural steel bars, or they may consist of structural steel tubing filled with
concrete (commonly called
lally columns
).
9.3
AXIAL LOAD CAPACITY OF COLUMNS
In actual practice there are no perfect axially loaded columns, but a discussion of such
members provides an excellent starting point for explaining the theory involved in design-
ing real columns with their eccentric loads. Several basic ideas can be explained for
purely axially loaded columns, and the strengths obtained provide upper theoretical limits
that can be clearly verified with actual tests.
It has been known for several decades that the stresses in the concrete and the rein-
forcing bars of a column supporting a long-term load cannot be calculated with any de-
gree of accuracy. You might think that such stresses could be determined by multiplying
the strains by the appropriate moduli of elasticity. But this idea does not work too well
practically because the modulus of elasticity of the concrete is changing during loading
due to creep and shrinkage. So it can be seen that the parts of the load carried by the con-
crete and the steel vary with the magnitude and duration of the loads. For instance, the
