the short diaphragms may be omitted, but the girder box section must be designed
for the additional torsional loads due to this eccentric loading.
Walkways. OETs should be provided with walkways where space permits.
The drive girder walkway, where the bridge drive machinery and electrical control
cabinets are mounted, must extend the full length of the girder. The idler girder
walkway, as a minimum, should extend from one end truck at least twice the length
of the trolley. It is desirable, where space permits, to have a crossover walk
between the two girder walkways. The width
of the walkways, handrails, toe plates, and gates or safety chains, must comply
with OSHA requirements.
End Trucks and End Ties for OET Cranes. On four-wheel cranes, the end
truck frames provide strength and rigidity to the entire bridge structure to
resist horizontal loads. On large four-wheel cranes, the end trucks are built-up
weldments of box section design, with internal diaphragms located under the bridge
girder webs. The bottom flanges are notched at the ends and the webs are
reinforced to form recessed seats for wheel axle bearings.
On smaller cranes, the end trucks may utilize structural channel sections
for webs with external vertical stiffeners and doubler plates around the wheel
axle seats. The lower flanges are notched for the travel wheels and the channel
webs are slotted from the ends to permit installation and removal of flanged wheel
axle bearing housings. Standard commercial end trucks are normally of this type.
On eight-wheel cranes, the end trucks may be pinned (wheel load
equalizing) type, or fixed (wheel load compensating) type. The bridge girders are
joined by end ties, which provide the strength and rigidity required to resist
horizontal loads. End ties may be rigid (for wheel load equalizing) or flexible
(for wheel load compensating). If equalizing end trucks are used, the bridge
girder and rigid end tie must form a frame that is rigid about its vertical and
horizontal (perpendicular to the rail) axes. If compensating end trucks are used,
the end tie must be rigid about the vertical axis but relatively flexible about
the horizontal axis to permit partial rocking motion for wheel load compensation.
Both end tie designs provide for satisfactory load sharing (equalizing or
compensating) between pairs of wheels in each corner.
Wheel Base. For four-wheel cranes, the wheel base is defined as the
center-to-center distance between the wheels' axles. For eight-wheel cranes, the
wheel base is defined as the center-to-center distance between the outermost wheel
axles. To minimize the undesirable effects of skewing, the wheel base must be at
least 1/7 of the runway span (center-to-center distance between the rails).
Bridge Girder-End Tie/End Truck Connections for OET Cranes. On older
cranes, the alignment of the bridge assembly was often maintained by the use of
close tolerance bolt-to-hole fits. On newer cranes, these connections are slip-
critical (or friction type), made with high strength structural bolts. Tapered
alignment pins (two per corner) are used to maintain original shop alignment
between the bridge girders and the end ties/end trucks when reassembling the
joint. The bolt holes and alignment holes should be located close to the webs of
the structural member in widened flanges or welded-on flange extensions.
Additionally, if required to maintain rigidity in the horizontal plane, gusset
plates may be required between the bridge girder and end tie/end truck flanges.