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Solution:
(1) Establish the ratio [alpha] between the design values of the
horizontal and vertical blast loads.
(2) Using the recommended dynamic load factors presented in
Table 23, establish the magnitude of the equivalent static
loads for:
(a) Local mechanism of the roof and blastward column, and
(b) Panel or combined mechanisms for the frame as a whole.
(3) Using the general expressions for the possible collapse
mechanisms from Table 22 and the loads from Step 2, assume
values of the moment capacity ratios C and C1, and
proceed to establish the required design plastic moment,
Mp, considering all possible mechanisms. In order to
obtain a reasonably economical design, it is desirable to
select C and C1, so that the least resistance (or the
required value of Mp) corresponds to a combined
mechanism. This will normally require several tries with
assumed values of C and C1.
(4) Calculate the axial loads and shears in all members using
the approximate methods.
(5) Design each member as a beamcolumn, using ultimate
strength design criteria.
(6) Using the moments of inertia from Step 5, calculate the
sidesway natural period using Table 10 and Equation (112).
Enter Figure 67 of NAVFAC P397 with the ratios of T/TN
and B/Ru, and establish the ductility ratio, [mu]. In this
case, B/Ru is the reciprocal of the panel or the sidesway
mechanism dynamic load factor used in the trial design.
Multiply the ductility ratio by the elastic deflection
given by Equation (115) and establish the peak deflection
Xm from Equation (116). Compare the Xm/H value with
the criteria of paragraph 1.b.
(7) Repeat the procedure of Step 6 for the local mechanism of
the roof and blastward column. The stiffness and
transformation factors may be obtained from Tables 7
and 10, respectively. The natural period is obtained from
Equation (91). The resistance of the roof girder and the
blastward column may be obtained from Table 22 using the
values of Mp and CMp determined in Step 3. Compare the
ductility ratio and rotation with the criteria of
paragraph 1.b.
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