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 (c) Columns may be considered braced in the weak direction if
the distance between any adjacent braced points is not greater than *lcr
defined as:
EQUATIONS:
*lcr/ry = 1,375/Fdy,
-1.0 < /= M/Mp < /= -0.5
(110a)
*lcr/ry = 1,375/Fdy + 25,
-0.5 < M/Mp < /= 1.0
(110b)
Beam columns should also satisfy the requirements of paragraph 3.a.(7) of
this section.
b. Effective Length Ratios for Beam Columns. The basis for determining
the effective lengths of beam columns for use in the calculation of Pu,
Pex, Pey, Mmx, and Mmy in plastic design is outlined below.
(1) For plastically designed braced and unbraced planar frames which
are supported against displacement normal to their planes, the effective
length ratios in Tables 19 and 20 shall apply.
(a) Table 19 corresponds to bending about the strong axis of a
member, while Table 20 corresponds to bending about the weak axis. In each
case, *l, is the distance between points of lateral support corresponding to
rx or ry, as applicable. The effective length factor, K, in the plane
of bending shall be governed by the provisions of paragraph 5.c. of this
section.
(b) For columns subjected to biaxial bending,
the effective
lengths given in Tables 19 and 20 apply for bending about
the respective
axes, except that Pu for unbraced frames shall be based
on the larger of
the ratios K*l/rx or K*l/ry. In addition, the larger
of the slenderness
ratios, *l/rx or *l/ry, shall not exceed Cc.
TABLE 19
Effective Length Ratios for Beam Columns
(Webs of members in the plane of the frame;
i.e., bending about the strong axis)
One- and Two-Story
Braced Planar Frames[*]
Unbraced Planar Frames[*]
Pu Use larger of *l/ry or l/rx Use larger of *l/ry or K*l/rx
Pex Use *l/rx
Use K*l/rx
Mmx Use *l/ry
Use *l/ry
[*]*l/rxshall not exceed Cc.
2.08-153
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