MIL-HDBK-1038
The portal base is affected by the radius of the boom, and by the
orientation of the boom axis to the portal base axis. For design purposes, the
portal base should be analyzed for three orientations of the boom axis: parallel
to the rails, perpendicular to the rails, and at the orientation which produces
maximum corner load. Each of these three orientations should be analyzed for two
boom radii: the radius which produces maximum overturning moment on the portal
base, and the radius which produces maximum axial load on the portal base (unless
they occur at the same radius). The load case of the boom at minimum radius with
no load should be examined to see if it produces maximum overturning moment.
Also, each of the radius/orientation cases described above should be combined with
spreading forces, and separately with squeezing forces.
Several other load cases should be investigated for various areas of the
crane. For example, the auxiliary hoist and whip hoist govern the design of the
outer section of the boom (beyond the main sheaves), as well as their associated
sheave supports and hoist foundations. The boom foot is usually governed by non-
operating wind forces. The rear portion of the machinery deck may be governed by
the load case of the boom at minimum radius with no load, or with the boom removed
for maintenance.
Structural design criteria for portal cranes are established entirely by
NCC, and they reflect successful traditional practices of Navy crane designs
tailored to the shipyard environment. The following paragraphs prescribe the
design load combinations and maximum allowable stresses for each structural
component of portal cranes. The maximum stress levels listed below represent the
percentage of AISC allowable values for the particular material.
a)
Boom, A-frame, Strut, Pendants, Machinery Deck and Hoist
Foundations.
Three load cases are considered:
1) Dead load, rated hook load with a vertical impact factor, 40 mph
wind, and acceleration forces due to rotate and travel motion. The maximum
stresses are limited to 85 percent of AISC allowable values.
2) Dead load and non-operating wind load from the front, rear, or
side, with the boom at the specified radius. The maximum stresses are limited to
133 percent of AISC allowable values for the particular material.
3) For fatigue analyses: the stress range is defined as the
algebraic difference between the stresses due to: (a) dead load plus 50 percent of
the main hoist rated hook load, and (b) dead load with no hook load. If the crane
is straight-line rated, the main hook will be at its maximum operating radius for
this load case. If the crane is variably rated, the main hook will be at 75
percent of its maximum operating radius for this load case. (And the load will be
50 percent of main hoist rated capacity at that radius.) The maximum allowable
stress range is limited to 100 percent of AISC allowable values for Loading
Condition 2 (that is, 100,000 to 500,000 cycles).
Additionally, this load case is to be applied to the boom section between
the whip hoist sheave and the main hoist sheave nest, using the dead load and 100
percent of the whip hoist rated hook load. On cranes with auxiliary hoists, this
load case is also applied to the boom section between the auxiliary and main hoist
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