Civil Engineering Reference
In-Depth Information
Reinforced concrete columns. (Courtesy of
RSM Advertising, representing Molded Fiber
Glass Products Company.)
As previously mentioned in Section 11.3 of this chapter the ACI Code in Section 10.12.1
states that
k
should be taken to be 1.0 for compression members in frames braced against
sidesway unless a theoretical analysis shows that a lesser value can be used.
In the last para-
graph of Section R10.12.1 of the Commentary, use of the alignment charts or the equations
just presented is said to be satisfactory for justifying k values less than 1.0 for braced frames.
11.6
FIRST-ORDER ANALYSES USING
SPECIAL MEMBER PROPERTIES
After this section, the remainder of this chapter is devoted to an approximate design pro-
cedure wherein the effect of slenderness is accounted for by computing moment magni-
fiers which are multiplied by the column moments. A magnifier for a particular column is
a function of its factored axial load
P
u
and its critical buckling load
P
c
.
Before moment magnifiers can be computed for a particular structure, it is necessary
to make a first-order analysis of the structure. The member section properties used for such
an analysis should take into account the influence of axial loads, the presence of cracked
regions in the members, and the effect of the duration of the loads. Instead of making such
an analysis, ACI Code 10.11.1 permits use of the following properties for the members of
the structure. These properties may be used for both nonsway and sway frames.
(a)
Modulus of elasticity
determined from the following expression given in Section
8.5.1 of the C
od
e:
E
c
w
1.
c
33
c
for values of
w
c
from 90 to 155 lb/ft
3
or 57,000
c
f
f
for normal-
weight concrete.
