NAI TECHNICAL REPORT

Technical Brief: Single Girder vs. Double Girder Bridge Cranes

 

Common Misconception: Double Girder Cranes Have Greater Lifting Capacity

Based on the Crane Manufacturers Association of America (CMAA) specifications, both single and double girder cranes are equally rigid, strong, and durable. This is because single girder cranes use much stronger cross girders than double girder cranes, and single girder cranes have lateral bracing.

 

Ultra Low Headroom (ULH) trolleys

On double girder cranes, the hoist and trolley typically ride on top of the cross girders. As an option, we can provide a trolley that rides between the cross girders. This may be appropriate if:

 Maximum hook height is required

 Your cross girders, as well as your hoist, must be the maximum height above the floor beam

 You must minimize the distance above your runway

 You need the crane to be more compact

Cost Considerations

 

Single girder cranes may cost less for several reasons:

 Only one cross girder is required

 The trolley is simpler

 Fright expenses are reduced

 Installation is faster

 Runway beams are lighter

However, not all cranes should be designed with a single girder. Single girder cranes have been built in spans of over 100 and capacities of over 20 tons, but cost-effective engineering dictates that single girder cranes be considered per the chart to the left.

The Difference is in the Hook Height

The principle difference between single and double girder cranes is hook height (the distance above the floor that the hook will rise). Double girder cranes provide greater hook height, typically 18-36 inches more than single girder cranes.

Double girder cranes can provide more lift because the hoist is placed between or on top of the cross girders rather than under them (the trolley can run on top of the girders). Therefore, the depth of the cross girder is gained in switching from a single to a double girder.

Single Girder Crane Capacity/Span Guidelines

First Quarter 2010

Capacity (tons)

Max. Span (feet)

0-7.5

80

10

75

15

65

20

60