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 (4) The particular objective in this section is to provide rational
procedures for efficiently performing the preliminary design of blast
resistant frames. Rigid frames as well as frames with supplementary bracing
and with rigid or non-rigid connections are considered. In both cases,
preliminary dynamic load factors are provided for establishing equivalent
static loads for both the local and overall frame mechanism. Based upon the
mechanism method, as employed in static plastic design, estimates are made
for the required plastic bending capacities as well as approximate values
for the axial loads and shears in the frame members. The dynamic
deflections and rotations in the sidesway and local beam mechanism modes
are estimated based upon single degree-of-freedom analyses. The design
criteria and the procedures for the design of individual members previously
discussed apply for this preliminary design procedure.
(5) In order to confirm that a trial design meets the recommended
deformation criteria and to verify the adequacy of the member sizes
established on the basis of estimated dynamic forces and moments, a rigorous
frame analysis should be performed. Several computer programs are available
through various Department of Defense agencies which perform a
multi-degree-of-freedom, non-linear, dynamic analysis of braced and
unbraced, rigid and non-rigid frames of one or more stories.
b.
Single-Story Rigid Frames.
(1) Collapse Mechanism. General expressions for the collapse
mechanism of single-story rigid frames are presented in Table 22 for pinned
and fixed base frames subjected to combined vertical and horizontal loading.
(a) The design objective is to proportion the frame members such
that the governing mechanism represents an economical solution. For a
particular frame within a framing system, the ratio of total horizontal to
vertical peak loading, denoted by [alpha] is influenced by the horizontal
framing plan of the structure and is determined as follows:
EQUATION:
[alpha] = qh/qv
(111)
where,
qv
=
pvbv = peak vertical load on rigid frame
qh
=
phbh = peak horizontal load on rigid frame
pv
=
blast overpressure on roof
ph
=
reflected blast pressure on front wall
bv
=
tributary width for vertical loading
bh
=
tributary width for horizontal loading
The value of [alpha] will usually lie in the range from about 1.8 to 2.5
when the shock front is parallel to the roof purlins. In this case the
roof purlins are supported by the frame and the tributary width is the same
for the horizontal and vertical loading. The value of [alpha] is much
higher when the shock front is perpendicular to the roof purlins. In
this case, the roof purlins are not supported by the girder of the frame and
the tributary width of the vertical loading (bv = purlin spacing) is much
smaller than the tributary width of the horizontal loading (bh = frame
spacing).
2.05-156
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