Civil Engineering Reference
In-Depth Information
Figure 9.2
Types of
columns.
buckle outward under load, which would cause the outer concrete cover to break or spall
off. Tied columns are ordinarily square or rectangular, but they can be octagonal, round,
L-shaped, and so forth.
The square and rectangular shapes are commonly used because of the simplicity of con-
structing the forms. Sometimes, however, when they are used in open spaces, circular shapes
are very attractive. The forms for round columns are often made from cardboard or plastic
tubes, which are peeled off and discarded once the concrete has sufficiently hardened.
If a continuous helical spiral made from bars or heavy wire is wrapped around the
longitudinal bars, as shown in Figure 9.2(b), the column is referred to as a
spiral column
.
Spirals are even more effective than ties in increasing a column's strength. The closely
spaced spirals do a better job of holding the longitudinal bars in place, and they also con-
fine the concrete inside and greatly increase its resistance to axial compression. As the
concrete inside the spiral tends to spread out laterally under the compressive load, the spi-
ral that restrains it is put into hoop tension, and the column will not fail until the spiral
yields or breaks, permitting the bursting of the concrete inside. Spiral columns are nor-
