Single Girder vs. Double Girder Top Running Cranes
Part of the application analysis for a crane system is to determine when to use a single girder crane vs. a double girder crane. In most cases, a single girder crane is less expensive than a double girder crane; however, the cost is not the only deciding factor as both have their merits and shortcomings for particular applications.
The basic construction of an industrial overhead bridge crane consists of, and trucks with wheels running along the length of the runway system and the bridge crane girder fixed mounted to the end trucks with the hoist trolley suspending the hoist and traversing across the bridge.
The difference between a single and double girder top running crane is:
· A single girder crane has only one bridge beam, and the hoist trolley operates or travels on the lower flange of the bridge girder.
· A double girder crane will have two bridge beams, and the hoist trolley travels on rails, usually attached to the tops of the crane girder.
Bridge Girder Construction
There are two basic methods of bridge crane girder design for both types of cranes:
· Standard structural steel bridge girder: A standard structural beam is an I-Beam or Wide flange beam that is readily available from structural steel suppliers. These advantage of these beams is in cost, availability, and efficiency when used for shorter bridge crane spans and capacities under 30 Ton. Also, a structural beam needs very limited engineering and only minor fabrication to attach to the proper end truck using connection plates. Structural beams are generally limited to 60’ spans with slightly longer when ordered in a special mill run. As bridge crane spans and capacities increase, the wide flange girders increase in required depth (height of girder) and weight per foot. In many cases, higher capacity cranes and those with longer spans may have very heavy and tall girders.
· Engineered and fabricated box style bridge girder: Box Girder cranes utilize a specially engineered and expertly fabricated box beam section that must be engineered to CMAA specifications. The box girder, therefore, has much higher engineering and fabrication costs than a simple structural girder. The advantage of the box girder design pays off in longe spans and higher capacities. In these applications of over 60’ span and over 20 Ton capacity, the cost of engineering and fabrication can be offset by lower material cost because a box beam will use less steel and be equally strong. When a crane is over 30 Tons or over 60’ span; a box girder section is the only choice available due to the length limitations of standard structural beams. Other advantages to box girders is the ability to design the girder depth to improve hook height, and the lower weight of the box reduces the structural requirements of the runway design by lowering the crane loads on the runways and support columns.
Duty Classification: Many people mistakenly think that a double girder crane is heavier duty than a single girder or that they will last longer. That assumption is incorrect because the components (End Trucks and Hoist with trolleys) can be of exactly the same duty and be configured into a single girder design or a double girder design. A double girder crane is generally used when the duty requirement is D+ (Very Heavy Duty) or E (Severe Duty) because the special hoisting equipment often includes an open winch with a separate split gearcase, heavy-duty motor and a brake that are mounted on a deck frame. Also, heavy-duty or critical applications often include a bridge-mounted footwalk or maintenance platform for the safety and convenience of technician service and repair access to the hoist and crane components. It is not typically practical to mount an effective service platform to a single girder crane.
The crane or hoist must be provided with 3” of minimum clearance between the lowest building obstruction including lights, HVAC and sprinklers and the moving crane or hoist per OSHA. Double girder cranes generally require more clear height over the elevation of the crane runway beams because the hoist trolley travels the top of the crane bridge girders. Since the hoist is traveling on top of the bridge crane girders, the hoist hook height is generally improved substantially over a single girder crane.
In many existing runways, there is insufficient clearance above the runway girders to fit a standard double girder crane, and therefore a single girder design is most practical. In cases where there is excessive clearance, the crane girder can be stooled up to increase available hoist hook height. Single girder cranes have the hoist trolley traversing on the lower flange of the girder. Also, the beam will likely to be deeper than a double girder. So the available hook height of a single girder crane will likely be less than a comparable double girder.
Higher duty class and special applications such as die flipping and double hoisting systems lend themselves better to double girder cranes. The optional service platforms on a double girder crane can easily accommodate large control panels for specialized functions. Die flipping using dual hoists requires that the cranes be fitted with hoisting drums that are parallel to the crane runways. This is best accomplished with a double girder crane.
The precise application of a crane to a particular application is best left to a qualified applications specialist. CRANE 1 is an industry leader in the design and supply of overhead crane systems in both new and existing buildings. Our experts can help you pick the right style of the crane as well as the feature required to make the installation safe, reliable, and efficient.