Quantcast Section 4: Description of Assemblies and Components

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Section 4:
Structural.  The current Navy inventory of cranes contains a mix of older
designs of portal and floating cranes, whose structures are riveted or bolted
assemblies of rolled structural plates and shapes; and newer cranes, which make
extensive use of major weldments bolted at the main connections.  These
differences are most apparent on the booms and portal crane bases.
The ASTM A7 structural steel of the older cranes has been replaced by the
modern ASTM A36 and higher strength steels.  The improved physical properties and
chemistry of the modern steels were brought into tighter control to ensure high
quality and uniformity of welds.  Structural steel used on all outdoor cranes is
now required to have a minimum fracture toughness of 25 foot-pounds at 40 degrees
Fahrenheit (per Charpy V-notch tests).  The older ASTM A307 structural bolts were
supplanted by ASTM A325 and A490 high strength bolts.
Standard commercial cranes use structural tubing round, square,  or
rectangular - in many of their structural components.  Structural tubing and other
steel shapes of higher strength than ASTM A36 require strict compliance with the
welding procedures of American Welding Society or the original equipment
Bridge Girders for OET, Gantry, and Semi-Gantry Cranes.  Bridge girders
of the box section design are ideal for OET cranes because of their high capacity
to withstand lateral and torsional loads.  These girders have parallel top and
bottom flanges, two parallel webs, full depth diaphragms for rigidity,
intermediate short diaphragms for rail support, and in some cases, longitudinal
stiffeners.  The girders are designed with a camber to compensate for deflection
under load.  The flanges and webs are welded together with continuous fillet
welds.  The interior diaphragms may be welded with intermittent welds.  For
intermediate spans and capacities, box section girders are sometimes fabricated
from structural shapes, or a combination of structural shapes and plates.  This
type of girder is usually a manufacturer's standard design utilizing proven design
details.  The use of custom (non-standard) designed girders of this type should be
approached with caution.  (CMAA #70 does not address the design of this type of
girder in sufficient detail.)
For cranes in CMAA #70, Class C service, with spans of 40 feet or less,
and with rated capacities of 20 tons or less, single web girders may be used.
(Navy Crane Center (NCC) policy is to specify only Class C or heavier duty
service.)  The single web girders may be built-up (welded) with the web profiled
for the desired camber or they may be standard rolled structural shapes bent
(bowed) to the proper camber.  External vertical stiffeners are often required on
the sides of the web and the flanges.  The ends of the bridge girders, both box
section and single web type, should be notched (stepped) to fit over the end
truck.  The girders must be reinforced at the notches with vertical diaphragms and
horizontal stiffeners.  The bottom flanges should be tapered towards the notches.
The trolley rails should be centered over the top flange and secured with
rail clips.  The rail clips may be welded directly to the top flange or fastened
with nuts and threaded studs welded to the top flange.  The rails should be free
to shift longitudinally but the amount of movement must be limited by end stops.
Optionally, the rails may be positioned directly over one of the box girder webs,
in which case


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