Crane gantry girders

The function of the crane girders is to support the rails on which the

traveling cranes move. These are subjected to vertical loads from crane,

horizontal lateral loads due to surge of the crane, that is, the effect of

acceleration and braking of the loaded crab and swinging of the suspended load

in the transverse direction, and longitudinal force due to acceleration and braking

of the crane as a whole. In addition to the weight of the crane, impact and

horizontal surge must be considered.

The crane girder spans from column to column, usually having no lateral

support at intermediate points excepting when a walkway is formed at the top

level of the girder which restrains the girder from lateral bending.

under normal circumstances, the crane girder must be designed as laterally

unsupported beam carrying vertical and horizontal load at the level of the top

flange. Apparently a girder with heavier and wider compression flange is

required.

The fibre stresses in the gantry crane girders should rationally be computed

considering bi-axial bending combined with torsion produced by the lateral force

being applied at the top flange.

To simplify analysis,it is assumed that the lateral moment is resisted only by the

top flange bending horizontally without any assistance from the bottom flange.

The crane girders are supported either on brackets connected to columns

of uniform section or on stepped columns. Brackets are used for lighter crane

loads and the stepped columns for heavy crane loads and taller columns.

The crane girder is supported on a suitably formed seat and it is also connected

to the column near the top flange in each case in order to restrain it from lateral

bending and twisting at the support point.

To permit longitudinal movement, due to temperature and deflection, slotted holes are

used to connect the channel with the column. plates are flexible for horizontal bending

no slotted holes are necessary. Where roof leg and crane leg are provided separately

in a built-up column.The two legs are properly braced together to act as one piece.

The bracing is designed to take 2 ½% of column load plus shear due to

bending under crane loan and wind. The load of roof leg and crane are eccentric

to the combined column axis and should be considered as such.

The crane columns must be properly braced in the longitudinal direction of

the crane girders to be able to take the longitudinal forces due to crane. Such

brancing may be provided every fourth or fifth bay. In other bays, struts must be

used to transmit the longitudinal force to the bracing frame.

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