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(c) For columns braced against sidesway and not subjected to
transverse loads between supports, i.e., interior columns of shear wall type
structures, the equivalent moment factor, Cm, may be taken as:
M1
EQUATION:
Cm = 0.6 + 0.4
(73)
M2
The value of Cm
may not under any circumstances be taken less than 0.4.
In lieu of a more
accurate analysis, the value of M1/M2 may
conservatively be
taken equal to 1.0. Therefore, in the design of interior
columns, Cm may
be taken as 1.0.
(d) The critical axial load that causes a column to buckle is
given by:
[pi]2EI
EQUATION:
Pc =
(74)
(kLu)2
In order to apply Equation (74), a realistic value of EI must be
obtained for the section at buckling. An approximate expression for EI at
the time of buckling is given by:
Ec Ia
EQUATION:
EI =
(75)
1.5
in which
Ig + Ic
EQUATION:
Ia =
(76)
2
and
EQUATION:
Ic = Fbd3
(77)
where,
Ia = average moment of inertia of section, in4
Ig = moment of inertia of gross concrete section about
centroidal axis, neglecting reinforcement, in4
Ic = moment of inertia of cracked concrete section with equal
reinforcement on opposite faces, in4
F = coefficient given in Figure 37
2.0899


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