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 TABLE 20
Effective Length Ratios for Beam Columns
(Flanges of members in the plane of the frame;
i.e., bending about the weak axis)
One- and Two-Story
Braced Planar Frames[*]
Unbraced Planar Frames[*]
Pu Use larger of *l/ry or *l/rx Use larger of *l/rx or K*l/ry
Pey Use *l/ry
Use K*l/ry
Mmy Use *l/rx
Use *l/rx
[*]*l/ry shall not exceed Cc.
c. Effective Length Factor, K. In plastic design, it is usually
sufficiently accurate to use the K factors from Table C.1.8.1 of the AISC
Manual (reproduced here as Table 21) for the condition closest to that in
question rather than to refer to the alignment chart (Fig. C.1.8.2 of AISC
Manual). It is permissible to interpolate between different conditions in
Table 21 using engineering judgment. In general, a design K value of 1.5 is
conservative for the columns of unbraced frames when the base of the columns
is assumed pinned, since conventional column base details will usually
provide partial rotational restraint at the column base. For girders of
unbraced frames, a design K value of 0.75 is recommended.
6.
FRAME DESIGN.
a.
Introduction.
(1) The dynamic plastic design of frames for blast resistant
structures is oriented toward industrial building applications common to
ammunition manufacturing and storage facilities, i.e., relatively low,
single story, multi-bay structures. This treatment applies principally to
acceptor structures subjected to relatively low blast overpressures.
(2) The design of blast resistant frames is characterized by: (a)
simultaneous application of vertical and horizontal pressure-time loadings
with peak pressures considerably in excess of conventional loads; (b) design
criteria permitting inelastic local and overall dynamic structural
deformations (deflections and rotations); and (c) design requirements
dictated by the operational needs of the facility and, often, the need for
reusability, with minor repair work, following an accidental explosion.
(3) Rigid frame construction is recommended in the design of blast
resistant structures since this system provides open interior space combined
with substantial resistance to lateral forces. In addition, this type of
construction possesses inherent energy absorption capability due to the
successive development of plastic hinges up to the ultimate capacity of the
structure.
2.08-154
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