Civil Engineering Reference
In-Depth Information
Figure 9.1
Secondary or
P
moment.
ratios are described and evaluated in Chapter 11 and are used to classify columns as being
short or slender. When the ratios are larger than certain values (depending on whether the
columns are braced or unbraced laterally), they are classified as slender columns.
In 1970 an ACI Committee estimated that about 40% of all unbraced columns and
about 90% of those braced against sidesway had their strengths reduced by 5% or less by
P
effects and thus should be classified as short columns.
1
These percentages are proba-
bly decreasing year by year, however, due to the increasing use of slenderer columns de-
signed by the strength method using stronger materials and with a better understanding of
column buckling behavior.
9.2
TYPES OF COLUMNS
A plain concrete column can support very little load, but its load-carrying capacity will be
greatly increased if longitudinal bars are added. Further substantial strength increases may
be made by providing lateral restraint for these longitudinal bars. Under compressive
loads, columns tend not only to shorten lengthwise but also to expand laterally due to the
Poisson effect. The capacity of such members can be greatly increased by providing lat-
eral restraint in the form of closely spaced closed ties or helical spirals wrapped around
the longitudinal reinforcing.
Reinforced concrete columns are referred to as
tied
or
spiral
columns, depending on
the method used for laterally bracing or holding the bars in place. If the column has a se-
ries of closed ties, as shown in Figure 9.2(a), it is referred to as a
tied column
. These ties
are effective in increasing the column strength. They prevent the longitudinal bars from
being displaced during construction, and they resist the tendency of the same bars to
1
American Concrete Institute, 1972,
Notes on ACI 318-71 Building Code with Design Applications
(Skokie,
ILL), p. 10-2.
