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 (b) It is assumed that the plastic bending capacity of the roof
girder, Mp, is constant for all bays. The capacity of the exterior and
interior columns are taken as CMp and C1Mp, respectively. Since the
exterior column is generally subjected to reflected pressures, it is
recommended that a value of C greater than 1.0 be selected.
(c) In analyzing a given frame with certain member properties,
the controlling mechanism is the one with the lowest resistance. In design,
however, the load is fixed and the required design plastic moment is the
largest Mp value obtained from all possible mechanisms. For that purpose,
C and C1 should be selected so as to minimize the value of the maximum
required Mp from among all possible mechanisms. After a few trials it
will become obvious which choice of C and C1 tends to minimize the largest
value of Mp.
(2) Dynamic Deflections and Rotations. It will normally be more
economical to proportion the members so that the controlling failure
mechanism is a combined mechanism rather than a beam mechanism. The
mechanism having the least resistance constitutes an acceptable mode of
failure provided that the magnitudes of the maximum deflections and
rotations do not exceed the maximum values presented in paragraph 1.b of
this section.
(3) Dynamic Load Factors. To obtain initial estimates of the
required mechanism resistance, the dynamic load factors of Table 23 may be
used to obtain equivalent static loads for the indicated mechanisms. These
load factors are necessarily approximate and make no distinction for
different end conditions. However, they are expected to result in
reasonable estimates of the required resistance for a trial design. Once
the trial member sizes are established, then the natural period and
deflection of the frame can be calculated. The dynamic load factors of
Table 23 are presented for both reusable and non-reusable frames. In each
case, the factors for a panel or combined sidesway mechanism are lower than
those for a beam mechanism, since the natural period of a sidesway mode will
normally be greater than the natural periods of the individual elements.
TABLE 23
Dynamic Load Factors (DLF) and Equivalent Static Loads for Trial Design
Collapse
Structure
mechanism
Reusable
Non-reusable
Beam
1.0
0.8
Panel or combined
0.5
0.35
Equivalent static vertical load = qv x DLF = w
Equivalent static horizontal load = qh x DLF = w
2.08-158
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