The girder is also subject to loads due to its own weight and the weight of the rails. The load transmitted is maximum when the crab is closest to the gantry girder. The vertical load transmitted to the gantry girder is numerically equal to the reaction on the crane at its end and is due to the weight of the crane, the weight of the crab and the lift load (crane capacity). (iii) Longitudinal forces transmitted due to sudden stopping or starting of the crane. (ii) Lateral forces transmitted due to sudden stopping or starting of the crab on the crane. (i) Vertical loads transmitted by the crane. The girder is subjected to unsymmetrical bending due to various forces transmitted to it. In most cases, a gantry girder is a laterally unsupported beam subjected to loads accompanied by impact. The rail provided over the gantry girder is assumed to offer no assistance to the gantry girder in supporting the wheel loads. The following allowances are made to cover all forces caused by vibration, shock from slipping of slings, kinetic action of acceleration and retardation and impact of wheel loads. This depends upon the span of the crane across the shop. Wheel base means the distance between the centres of the two wheels of the cross head resting on one gantry girder. Since they are also subjected to horizontal loads a larger top flange is generally provided. These beams are subjected to vertical and horizontal loads due to dead load of the crane, the hook load and dynamic loads. 11.2 shows the types of beam sections used for gantry girder. Large cranes consist of a double truss unit. Small cranes may consist of an integrated double beam unit. Electrically operated cranes have a lifting capacity in a wide range from 10 kN to 3000 kN. Hand operated cranes have a lifting capacity up to 50 kN. In the case of quick acting cranes stresses in the gantry girder are introduced almost instantaneously while in hand operated cranes, which are slow moving cranes stresses are introduced gradually. The crane may be a slow moving manually or hand operated crane (M.O.T.) or may be an electrically operated quick acting crane (E.O.T.). Cranes may be of the slow motion type or quick motion type. The rails are firmly attached to the top flange of the gantry girder by strong clips, preventing the rails from lateral dislocation due to lateral forces.
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