In this article we will discuss about:- 1. Introduction to Gantry Girders 2. Gantry Girder Sections 3. Wheel Base 4. Loads 5. Design Details.
Introduction to Gantry Girders:
In workshops and factories a very important and useful requirement is to have the means for lifting and moving heavy loads from one part to the other part of the shop area. Gantry girders supported on columns, carry the moving cranes. The hook-chain system is supported on a pulley.
The load can be lowered or raised from the overhead crane. The crab housing the winches is supported over the crane. The moving cranes are provided with wheels which move on rails which are fixed over the top flange of the gantry girder.
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. Cranes may be of the slow motion type or quick motion type. 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.).
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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. Hand operated cranes have a lifting capacity up to 50 kN. Electrically operated cranes have a lifting capacity in a wide range from 10 kN to 3000 kN.
Gantry Girder Sections:
Small cranes may consist of an integrated double beam unit. Large cranes consist of a double truss unit. Fig. 11.2 shows the types of beam sections used for gantry girder.
These beams are subjected to vertical and horizontal loads due to dead load of the crane, the hook load and dynamic loads. Since they are also subjected to horizontal loads a larger top flange is generally provided.
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For a light gantry girder a universal beam only or a universal beam with a channel attached to the top flange is used. A heavy gantry girder consists of a plate girder along with a surge girder (Fig. 11.2).
Fig. 11.3 shows the types of rails which are fixed to the top flange of the gantry girder.
The rails over the gantry girder are firmly fixed to the girder by bolted clamps or hook at a spacing of 0.50 m to 1 m to prevent the rail from lateral dislocation due to lateral forces. It is not usual to weld the rails to the gantry girder since welding makes it difficult for any readjustment of alignment of rails. Welding also makes it difficult for any replacement of worn out rails. The design loads on the gantry girder depends upon the minimum or the closest approach distance of the hook from the axis of the gantry girder.
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In the usual case, the gantry girder is laterally unsupported. Hence it is a practice to strengthen the top compression flange by making it wider using channels. Sometimes a walk way connected to the gantry girder is provided in which case the girder may be regarded as laterally supported.
Wheel Base:
Wheel base means the distance between the two wheels resting on one gantry girder.
Gantry girders may be supported on brackets attached to columns or stepped columns or on a separate column set on the inner side of the main columns.
Crane and Gantry Girder Data:
Relevant data about the crane can be obtained from the manufacturer’s literature. The data needed for gantry girder design are, Capacity of the crane, Crane span, Weight of the crane, End carriage wheel centres (Wheel base), Minimum hook approach, and Maximum static wheel load.
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End Carriage Wheel Base:
Wheel base means the distance between the centres of the two wheels of the cross head resting on one gantry girder. This depends upon the span of the crane across the shop. The wheel base may be taken as follows.
Allowances for Impact of 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.
The rail provided over the gantry girder is assumed to offer no assistance to the gantry girder in supporting the wheel loads.
Loads on a Gantry Girder:
In most cases, a gantry girder is a laterally unsupported beam subjected to loads accompanied by impact. The girder is subjected to unsymmetrical bending due to various forces transmitted to it.
These forces are the following:
(i) Vertical loads transmitted by the crane.
(ii) Lateral forces transmitted due to sudden stopping or starting of the crab on the crane.
(iii) Longitudinal forces transmitted due to sudden stopping or starting of the crane.
(i) Vertical Loads:
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). The load transmitted is maximum when the crab is closest to the gantry girder.
The girder is also subject to loads due to its own weight and the weight of the rails. The weight of the crab and the lift load is shared by the two gantry girders inversely proportional to the distances of the crab from the girders. For instance, if the span of the crane is L and if the crab is at a distance a from the girder A and W is the weight of crab and lift load the load transmitted to the girder A (Fig. 11.7).
(ii) Lateral Forces on the Gantry Girder:
The lateral or horizontal forces on the gantry girder are caused by the following:
(a) Sudden stopping or starting of the crab and load while moving on the crane.
(b) When the crab drags the load across the floor of the workshop.
(iii) Longitudinal Force:
Sudden stopping or starting of the crane introduces a longitudinal force on the gantry girder. The longitudinal force in fact is first induced in the rail which transmits the same to the gantry girder with a bending moment due to eccentric transmission of the force. The longitudinal force transmitted can be estimated as-
Let the coefficient of friction be µ. Then the total longitudinal force for one gantry girder will be times the actual wheel loads on the girder. Usually the coefficient of friction is assumed to be 0.12. Hence the longitudinal force on the gantry girder will be 12 per cent of the total wheel loads on one girder.
The longitudinal force on the girder may also be determined from another approach.
Suppose the crane moves at a speed of 1.6 m/s. Suppose the crane brakes itself in a distance of 4 m. Let the retardation be a m/s2. The retardation may be assumed to be uniform and can be determined from the equation-
This load will be shared by the two gantry girders if the load lifted by the crane is at the middle of the workshop.
Design Details of Gantry Girders:
The relevant details in the design of a gantry girder are briefly given below:
1. Determination of the Maximum Wheel Load:
For the gantry girder under consideration the crab on the crane should be at the closest permissible distance from the gantry girder in order to produce the most critical shear force and bending moment. This distance is the minimum hook approach distance obtained from the manufacturer’s literature. In this condition maximum load is transmitted to the gantry girder.
With the crab in this position we can determine the part of [Lift load + crab weight] transmitted to the gantry girder. The total vertical load transmitted from the crane is equally distributed to the two wheels on the gantry girder. The gantry girder carries the two wheel loads and its own weight.
2. Maximum vertical shear force and maximum vertical bending moment for the gantry girder are now determined.
The wheel loads must be placed on the gantry girder in such positions:
(i) To produce the maximum shear force, and
(ii) To produce the maximum bending moment.
In the usual situations the span of the gantry girder is considerably large compared with the wheel base distance.
The following discussions are based on this condition:
(i) Maximum Shear Force for the Gantry Gilder:
Let W = Each wheel load
a = Wheel base
l = Span of the gantry girder
The maximum shear force for the girder will occur when one wheel load tends to reach a support (Fig. 11.9). For this condition, maximum shear force-
(ii) Maximum Bending Moment for the Girder:
The maximum bending moment occurs when the centre of gravity of the loads and one wheel load are placed equidistant from the mid section of the span of the girder (Fig. 11.10)
With this condition, Maximum bending moment = W/8l (2l – a)2
The shear force and bending moment due to impact effect are calculated.
Thus, the maximum vertical shear force and the maximum vertical bending moment are calculated.
3. Corresponding to the wheel loads the appropriate lateral forces are determined. Maximum horizontal shear force and maximum horizontal bending moment due to the lateral forces are determined. Position of loads for maximum horizontal shear is the same as that for maximum vertical shear. Position of loads for maximum horizontal bending moment is the same as that for maximum vertical bending moment.
4. Selection of Gantry Girder Section:
Approximate plastic modulus (about z – axis) required is estimated as-
A section consisting of an I-section with a channel section attached to the top flange is proposed. The depth of the I-section may be 1/12 to 1/13 of the span of the girder. The width of the flange (the depth of the channel) may be about 1/30 of the span of the girder.
5. Analysis of the Selected Girder Section:
(i) The properties of the selected section are noted. Their sectional classification (Plastic semi- plastic etc.) is made. The preferred choice for a plastic type, i.e., b/tƒ < 8.4 and d/tw < 8.4 for both I and channel sections.
(ii) Plastic moduli about the z and y axes are determined for the combined section. In case the top flange of the girder is laterally supported then the section is checked for the moment capacity of the entire section. For this condition, the design bending strength-
This design strength should be greater than the maximum vertical bending moment determined.
For the case of the girder whose top flange is laterally unsupported the design bending strength is determined as-
(iii) Check for Local Moment Capacity:
where, Mz = Total vertical factored moment
Myƒ = Maximum horizontal moment (resisted by top flange only)
Mdz = Design bending strength about z-z – axis
Mdyƒ = Design bending strength of compression flange about y-y – axis.
(iv) Check for Buckling Resistance:
We will determine the local torsional buckling moment Mcr given by-
