The gantry girder spans between the brackets connected to columns, which may either be of steel or reinforced concrete. Thus the span of the gantry girder is equal to the center-to-center spacing of columns. The rails are mounted on gantry girders.
The crane girder spans the bay of the shop. The trolley or crab, mounted on the crane girder can travel transversely along the crane girder. The trolley has four wheels. The crane girder has two wheels at each end and is capable of moving longitudinally on rails.
What Is Gantry Girder?
Gantry girders or crane girders carry hand-operated or electrically operated overhead cranes to industrial buildings such as factories, workshops, steelworks, etc. to lift heavy materials, equipment, etc., and to take them from one location to the other, within the building.
The gantry girders are girders that support the loads that are conveyed through the traveling wheels of the crane. The crane girder spans from column to column, this generally does not possess any lateral support at the intermediate points except when a walkway is created at the top of the girder.
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Uses Of Gantry Girder
The following are Gantry Girder uses,
- To understand the behavior of gantry girders.
- To calculate the different loads acting on the gantry girder.
- To compute the maximum bending moment and maximum shear force in the girders.
- Choose the appropriate cross-sections for gantry girders.
- Design suitable connections.
Characteristics Of Gantry Girders
- The design of the gantry girder is a specific example of a laterally unsupported beam.
- It is subjected to in extension to vertical loads and horizontal loads along and perpendicular to its axis.
- Loads are dynamic and generate vibration.
- The compression flange needs critical attention.
Gantry Girder Design
The design of the gantry girder is a trial and error procedure. In the design, it is assumed that
- Lateral load is resisted entirely by the compression flange with plates, channels, etc.
- The vertical load is resisted by the entire beam.
Design Steps
1. Maximum Wheel Load
The first step is to determine the maximum wheel load of the crane girder transferred to the gantry girder.
U.D.L. on crane girder = Total weight of crane girder/ Span of crane girder
Concentrated load = weight lifted by hook + weight of the trolley
This concentrated load is placed at the point of the minimum hook approach and the maximum reaction of the crane girder is computed. At each end of the crane girder, there are two wheels.
Load on each wheel = Maximum reaction / 2. This wheel load is increased by 25% for impact and then factored.
2. Maximum B.M. In The Gantry Girder
U.D.L. on gantry girder =Self-weight of gantry girder + weight of rail section
For maximum B.M., the wheel loads should be so placed that the C.G. of the two-wheel loads (R) and one of the wheel loads lie equidistant from the center of the span (C). The maximum B.M. will then occur under that wheel load which is nearer to the center of the girder.
3. Maximum Shear Force
U.D.L. on gantry girder = self-weight of gantry girder + weight of rail section. The S.F. due to the wheel load is ultimate when one of the wheels is at the support.
- Horizontal Force (Drag Force) Braking Force
It is taken equal to 5% of the static wheel load on each gantry girder. This load is factored in.
- Horizontal Transverse Force (Surge load)
Surge load = 10% of (weight lifted on hook + weight of crab) For one crane this load is shared by four wheels.
- Surge load on each wheel = Total surge load / 4. This load is also factored in.
4. Section Selection For Gantry Girder
Generally, an I-section with a channel is chosen, though an I-section with a plate at the top flange may be used for light cranes.
Generally, the following guidelines are followed.
Economic depth of girder = L/12
Width of flange = L/40 to L/30
Zp required = k. Mu/Fy
= 1.40 Mu/Fy
The moment capacity for vertical loads should be about 40 to 50% higher (k = 1.4 to 1.5) than the moment due to vertical loads so that the section can resist combined horizontal and vertical moments safely.
5. Calculate the Izz, Iyy and zpof trial section selected
6. Check for moment capacity.
Md should be greater than Mu. The top flange should be examined for bending in both axes utilizing the interaction equation.
7. Check For Shear Capacity
Maxi. S.F. =Vz
Shear capacity,
0.6 Vd should be greater than Vz.
8. Check For Buckling Resistance
When the gantry girder is laterally not supported, the design bending strength of the beam is given by,
Md should be more than Mu.
9. Check For Local Buckling
At points of concentrated load (wheel load or reactions) the web of the girder must be checked for local buckling and, if necessary, load-carrying stiffeners must be provided to prevent local buckling of the web.
Bulking Resistance =
Buckling resistance must be more than maximum factored wheel load.
10. Weld Design
Required shear capacity of weld, Weld is required to connect the channel to the top flange of the T-beam.
The capacity of 1 mm long weld.
P=
= 0.7s
For shop welding,
= 1mm
If P>…. Safe.
11. Check For Deflection
Maximum deflection at center for two-wheel loads,
Permissible Deflection
For EOT crane up to 500 KN Capacity,
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Loads On Gantry Girder
- The gantry girders are unique in themselves
- First, it is unique from the usual beams in buildings. It is laterally unsupported except at the columns.
- Second, it is one of the very limited girders in the buildings that are subjected to impact.
- Third, it must be evaluated for unsymmetrical bending because of lateral thrust from the starting and stopping of the crab.
- Fourth, it is subjected to the longitudinal load through the starting and the stopping of the crane bridge itself.
- Fifth, these are often simply supported. These are subjected to the forces as follows:
- The reaction from the crane girder, working vertically downwards.
- Longitudinal thrust through the starting or the stopping of the crane, acting in the longitudinal direction.
- The lateral thrust, due to the starting or the stopping of the crab acting horizontally, usual to the gantry girder.
Different Types of Gantry Girder Cranes
The different types of gantry girder cranes and their specific applications.
- Full Gantry Crane Systems
- Semi-Gantry Crane Systems
- Portable Gantry Crane Systems
- Adjustable Gantry Cranes
Types Of Load Acting On Gantry Girder
A gantry girder, having no lateral support in its length (laterally supported) has to resist the following loads,
- Vertical Loads From Crane
- Impact Load From Crane
- Longitudinal Horizontal Force (Drag force)
- Lateral Load (Surge Load)
1. Vertical Loads From Crane
A vertical load working over the gantry girder is the outcome from the crane girder and includes the self-weight of the crane, self-weight of the crab, and the crane capacity (the ultimate load that can be hoisted).
To compute the reaction the maximum wheel load is calculated. It happens when the crab is nearest to the gantry girder. In addition, to the reaction from a crane girder, the self-weight of the rail should similarly be evaluated.
2. Impact Load From Crane
The stresses generated in gantry girders due to the above loads are more than those affected by gradually applied loads.
This is due to the forces established by the sudden application of brakes to the quickly moving loaded cranes acceleration, retardation, vibration, feasible slip of slings, etc.
The steelwork which takes these quick-acting cranes must be heavier than the steelwork which supports the slow-moving cranes.
With quick-acting electric overhead traveling (E.O.T) cranes, the stresses in the gantry girders are generated almost instantaneously, whereas, with slow-moving hand-operated cranes, the bending stresses in the girder are generated gradually from zero up to their ultimate values.
- Additionally, impact loads on cranes (IS : 875)
| Sr. No. | Types Of Load | Impact Allowance (%) |
| (A). | Vertical loads Transferred | |
| (i) | For electric operated cranes | Allowance of 25 % from the maximum static wheel load. |
| (ii) | For the hand-operated cranes | Impact Allowance of 10% of the tatal static wheel load. |
| (B). | Horizontal Forces Are Transverse (Surge load) To The Rails | |
| (i) | For electric operated cranes | Allowance around 10 % of the weight of the crab plus the weight lifted on the crane. |
| (ii) | For the hand-operated cranes | Allowed 5 % of the weight of the crab and the weight lifted on the crane. |
| (C). | Horizontal Forces Along The Rails (Braking Load) | |
| (i) | For electric operated cranes | 5 % of the calculated static wheel loads. |
| (ii) | For the hand-operated cranes | 2.5 % of the static wheel loads. |
3. Longitudinal Horizontal Force (Drag Force)
This is caused due to the starting and stopping of the crane girder moving over the crane rails, as the crane girder moves longitudinally, for example, in the direction of the gantry girder.
This force is also known as Braking Force or Drag Force. This force is taken equal to 5% of the static wheel loads for the electrically operated cranes or hand-operated cranes.
4. Lateral Load (Surge Load)
Lateral forces on crane girders may be induced by the,
A) Lateral forces are caused due to the sudden starting or stopping of the crab when moving over the crane girder.
- As with the longitudinal gantry girders, the frictional resistance of the rail is transmitted into the crab girders and from them into the crosshead girders, thence, as point loads through the major wheels, into the top or compression flanges of the gantry girders.
- The positions of the major wheels when maximum lateral bending and shear take place on the gantry girder will be similar to those when a maximum vertical bending moment and shear occurs.
B) Lateral forces are similarly caused when the crane is dragging weights across the floor of the shop.
- The crane is always requisitioned to drag weights across the shop floor. If the load is greatly massive, it is generally mounted on roughly fashioned rollers, probably moving on a timber plank path.
- The lateral thrust and pull on the compressive flanges of the gantry girders, therefore, become a matter of conjecture.
- The resisting forces are, firstly, the friction of the major wheel walks upon the gantry rails and, secondly, the forces given by the flanges of the major wheels bearing against the gantry rails.
- The lateral thrust is supposed to act in the plane of the center of gravity of the upper flange. Working as it does at the rail level, it has a lever arm generating torque.
- This small lever arm and, therefore, the torque are omitted. No support is assumed to be afforded by the lower or tensile flange in resisting lateral thrust.
- Nonetheless, should this help be examined, then the torque due to the thrust multiplied by the distance from the line of action of the thrust to the N.A.
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Limiting Deflection Of Gantry Girder
The maximum vertical deflection of the gantry girder shall not exceed the following values.
| Sr. No. | Category | Maximum Deflection |
| 1. | Vertical Deflection | |
| i) | Manually operated cranes | L / 500 |
| ii) | EOT cranes up to 50 t (500 KN) | L / 700 |
| iii) | EOT cranes over 50 t (500 KN) | L / 1000 |
| 2. | Lateral Deflection | |
| i) | Relative between rails | 10 mm or L / 400 |
Gantry Girder Sections
Small cranes may contain an integrated double beam unit. Large cranes consist of a double truss unit. The figure indicates the types of beam sections utilized for the gantry girder.
These beams are subjected to vertical and horizontal loads due to the dead load of the crane, the hook load, and dynamic loads. Since they are similarly subjected to horizontal loads a larger top flange is mainly provided.
For a light gantry girder, a universal beam simply or a universal beam with a channel connected to the top flange is utilized. A heavy gantry girder contains a plate girder along with a surge girder.
The figure below indicates 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 meter to 1 meter to prevent the rail from lateral dislocation due to lateral forces.
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