Crane Runway Steel vs Building Steel
It is a frequent situation that we are asked to install new cranes and runway rail in a new building where the contractor has provided the runway beams. Many times, we discover runway beams that are so far out of alignment tolerance that it is impossible to perform a proper installation that meets the required standards. Left uncorrected, the issues will likely result in crane wheel wear and early failure of the runway itself.
Why is Runway Steel different that Building Steel?
One big problem is that runways usually are built with building steel (wide flanges), fabricated by building steel fabricators, and installed by building steel erectors, but runway steel is not building steel. In fact, building steel and runway steel have the same appearance but are very different.
The runway alignment specs—written by the Crane Manufacturers Association of America (CMAA) and adopted by the Metal Building Manufacturers Association (MBMA), the American Institute of Steel Construction (AISC), and the Association of Iron and Steel Engineers (AISE)— fill an entire page and take considerable time to interpret.
A simplistic summary is that the tolerance for runways must be ±1⁄4 inch in a single bay and no more than ±3⁄8 inch over the full length of the runway while the mill tolerance for structural wide-flange beams basically is 1⁄8 inch per 10 feet of length. These tolerances must be maintained in four ways: left/right, up/down, parallel to each other, and level in respect to each other.
Therefore, in a common 30-foot bay, the wide-flange beam can have a sweep (horizontal bow) of 3⁄8 inch, which means that putting up this first piece of steel exceeds the acceptable CMAA/MBMA/AISC runway tolerance already. To compound the problem, the opposing runway can have an equal (but opposite) sweep, doubling the problem.
The Wrong Fix
Many times, we find that the rails were simply adjusted the rail laterally in relation to the girder. Although this solution is the most commonly used, it is bad engineering practice and actually is prohibited by the AISC specifications. The runway beam/girder is the wide-flange structural shape that supports the runway, while the rail (commonly American Society of Civil Engineers (ASCE) rail, similar to railroad rail) is the track upon which the end truck wheels traverse.
It is a common misconception that the runway beams have no particular installation tolerance and that only the rail is at issue. Further, this assumption seems to be confirmed by the lateral adjustment of the rail fasteners (for example, J-bolts/ hook bolts or patented clips). Actually, the tolerance of the beam installation is governed by the tolerance of the rail installation. This is because, according to AISC Design Guide 7, paragraph 19a, the centerline of the rail should be within ±3⁄4 inch of the girder web thickness. This prevents top flange rollover and subsequent fillet cracking and possibly girder failure.
Provide the Contractor with a Detailed Runway Fabrication Specification
If you buy the runway steel from the building supplier and install with the same building installation crew, you very likely will end up with runways with the tolerance of building steel. It is unlikely that the building contractor will buy, fabricate, and install 20+ pieces of apparently identical red primed steel to a tolerance two to four times tighter than the other several thousand pieces of red steel in that same building.
So if you do plan on buying the runway steel from the building contractor, protect yourself by providing a runway tolerance specification and using the crane supplier or third party to verify the adherence to the tolerances.
See required tolerance specifications in this blog: https://www.crane1.com/about-us/blog/cmaa-aisc-crane-runway-rail-tolerance/
Have the Crane Supplier Supply the Runway Steel
Qualified crane and hoist suppliers are used to working to the tighter tolerances and will ensure that the runway beams they purchase come from suppliers that understand the required tolerances. In addition, they will take the additional care to properly shim the beams so that they are correct in elevation as well. The cost might be a little bit higher, but the chances of having runway issues will be dramatically reduced.
Summary
Runway steel is not building steel. Poor runways will result in premature wheel failure, motor and or gearbox failure, and premature runway replacement. With a typical wheel replacement costing $10,000 and new runways costing $50,000 or more the advice in this blog can save you many thousands,
