Civil Engineering Reference
In-Depth Information
9
Introduction to Columns
9.1
GENERAL
This chapter presents an introductory discussion of reinforced concrete columns, with
particular emphasis on short, stocky columns subjected to small bending moments. Such
columns are often said to be “axially loaded.” Short, stocky columns with large bending
moments are discussed in Chapter 10, while long or slender columns are considered in
Chapter 11.
Concrete columns can be roughly divided into the following three categories:
Short compression blocks or pedestals.
If the height of an upright compression mem-
ber is less than three times its least lateral dimensions, it may be considered to be a
pedestal. The ACI (2.1 and 10.17) states that a pedestal may be designed with un-
reinforced or plain concrete with a maximum design compressive stress equal to
where
c
,
is 0.65. Should the total load applied to the member be larger
than it will be necessary either to enlarge the cross-sectional area of the
pedestal or to design it as a reinforced concrete column, as described in Section 9.9
of this chapter.
Short reinforced concrete columns.
Should a reinforced concrete column fail due to
initial material failure, it is classified as a short column. The load that it can sup-
port is controlled by the dimensions of the cross section and the strength of the ma-
terials of which it is constructed. We think of a short column as being a rather
stocky member with little flexibility.
Long or slender reinforced concrete columns.
As slenderness ratios are increased,
bending deformations will increase, as will the resulting secondary moments. If
these moments are of such magnitude as to significantly reduce the axial load ca-
pacities of columns, those columns will be referred to as being
long
or
slender
.
0.85
f
c
A
g
,
0.85
f
When a column is subjected to
primary moments
(those moments caused by applied
loads, joint rotations, etc.), the axis of the member will deflect laterally, with the result
that additional moments equal to the column load times the lateral deflection will be ap-
plied to the column. These latter moments are called
secondary moments
or
P
moments
and are illustrated in Figure 9.1.
A column that has large secondary moments is said to be a slender column, and it is
necessary to size its cross section for the sum of both the primary and secondary mo-
ments. The ACI's intent is to permit columns to be designed as short columns if the sec-
ondary or
P
effect does not reduce their strength by more than 5%. Effective slenderness
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