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Overhead travelling cranes are used in factories and workshops to lift heavy materials, equipments, etc and to carry them from one place to the other. These cranes are either hand operated or electrically operated. The crane consists of a bridge spanning the bay of the shop. A trolley or a crab is mounted on the bridge. The trolley moves along the bridge. The bridge as a whole moves longitudinally on rails provided at the ends. The rails on either side of the bridge rest on crane gantry girders. The gantry girders are the girders which support the loads transmitted through the travelling (moving) wheels of the cranes as shown in figure below. Figure: Gantry girderDesign Loads
The effect of cranes to be considered under the imposed loads should include the following: Vertical loads from the crane,
 The eccentricity effects induced by the vertical loads, and the impact factors,
 Internal (surge) thrust across the crane rail and
 Longitudinal horizontal thrust along the crane rail.
DESIGN BASIS OF GANTRY GIDER
The gantry girder is designed on the assumption that either of the horizontal forces, transverse to the rails or along the rails, act the same time as the vertical loads including the impact load. The horizontal forces act at the rail level. The gantry girder is subjected to bending in vertical plane as well as in horizontal plane along with twisting., the design calculations are simplified by providing a channel at the top flange of the girder, and neglecting the bottom flange for transverse load computations. The transverse loads are comparatively small and this simplification in design calculations does not result in serious error. The channel section provides flange areas to resist bending in horizontal plane due to horizontal forces acting in transverse direction. It increases moment of inertia about yy axis. The flange of channel section resists the bending in the horizontal plane. The bending of the crane gantry girder occurs about the vertical axis as well as about the horizontal axis of the member. The actual bending stresses for bending of the girder in the vertical and horizontal planes are computed. The combined bending stresses are taken as the sum of the two calculated fibre stresses. The combined bending compressive stress is should be less than or equal to the allowable bending compressive stress. Where, = actual bending compressive stress in vertical plane. = actual bending compressive stress in horizontal plane, and = allowable bending compressive stress.
No. 
Type of load 
Additional Load 
1 
Vertical loads 

(a) for electric overhead cranes 
25 per cent of maximum static wheel loads 

(b) for hand operated cranes 
10 per cent of maximum static wheel loads 

2 
Horizontal forces transverse to rails 

(a) for electric overhead cranes 
10 percent of weight of trolley and weight lifted on the crane 

(b) for hand operatedÂ crane 
5 percent of weight of trolley and weight lifted on the crane. 

3 
Horizontal force along the rails 
5 percent of the static wheel loads 