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 girder
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.
The crane loads to be considered are generally indicated by the customer. In the absence of any specific indications, the load combinations should be as follows:
a) The vertical loads with full impact from one loaded crane, or two cranes in case of tandem operation, together with the vertical loads, without impact, from as may loaded cranes as may be positioned for maximum effect, along with maximum horizontal thrust (surge) from one crane only or two cranes of tandem operations.
b) For the multi-bay gantries, the loads specified above, subject to consideration of cranes of cranes positioned for maximum effect on any two adjoining bays of the building, and
c) The longitudinal thrust on a crane track rail to be considered for the maximum of two loaded cranes on the track.
The lateral thrust (surge) and the longitudinal thrust, acting respectively across and along the crane rail, need not be assumed to act simultaneously. The effect of both the forces should however be investigated separately.
While investigating the effect of earthquake forces, the resulting effect from dead loads of all the cranes parked in each bay positioned for the maximum effect should be considered.
The cranes runway girders supporting bumpers should be checked for bumper impact loads.
While considering the simultaneous effects of vertical and horizontal surge loads of cranes, the permissible stresses for the design of crane gantry girders and their supporting structures may be increased by 10 percent.
Where the wind load is the main load acting on the structure, no increase in the permissible stresses is allowed.
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 y-y 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.
Type of load
(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
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 operated crane
5 percent of weight of trolley and weight lifted on the crane.
Horizontal force along the rails
5 percent of the static wheel loads
The additional loads to be considered according to Indian Standard, IS:875 – 1964, for the overhead travelling cranes is given in table above as a percentage of the maximum static wheel loads. The maximum wheel load is the reaction on a wheel due to the total load given by weight of crane plus crab and the lifted load.
The term, maximum static wheel load needs explanation. The extreme position of the crab with respect to the span of crane gives the maximum reaction on one of the gantry girders. This reaction is distributed equally among the crane wheels. The reaction on each wheel is termed as the maximum static wheel load.
The allowable bending compressive stress for bending in horizontal plane is equal to the allowable bending stresses in tension.
The allowable bending compressive stress for bending in vertical plane is reduced in proportion of critical stress in bending.
Indian standard, IS:800 – 1984 recommends that the allowable stress in axial tension, axial compression and bending stresses and allowable stresses for rivets are increased by 10 percent for the design of gantry girder for the combination of vertical and horizontal loads as discussed above. This increase in allowable stress is not in addition to that allowed for erection loads or without wind or seismic forces.