Pallet Rack manufacturer’s beam capacity tables are generally based on a UDL (uniformly distributed loads) calculation. Both the capacity and the deflection of a beam will vary if the load is not uniformly distributed on the pallet rack beams. A short list of things that can cause non-uniform loading are:
- Feet or runners on any load that can cause point loading of beams.
- Loads that have non-uniform weight or mass over their projected area.
- Loads that are not symmetrically placed on the shelf (either in depth or width).
- Loads that are undersized for the shelf. (These loads can be placed towards the center of the beam rather than towards the rack frame. This will aggravate the stress and deflection of the beam.)
To illustrate the importance of this, let’s take a look at an example of changing the beam size can affect the capacity rating for a pair of beams.
Specific Case:
2500# pallets are 40” wide but the customer wants a 108” bay for increased operating clearance. How does this effect beam design?
For a 96” wide bay, it is common to assume a uniformly distributed load (UDL) acting on the beam. For this case the beam centerline moment for pinned ends formulas would be: WL/8.
Since L = 96 this would equal 96W/8 = 12W (in-kips).
This is a close approximation for two 40” pallets where each pallet is placed 5” from the upright frame with 6” between the pallets at the center.
The error comes when the warehouse uses a wider, 108” long pallet rack beam to try and add extra clearance around the pallets. The same two 40” wide pallets can be placed 11-1/2” from the upright frames leaving 5” between the pallets at the centerline of the bay. This causes the uniform load assumption to be in error and this error can be significant.
Example:
108” beam – uniform load assumption (pinned ends): Moment = 108W/8 = 13.5W
Actual moment for loads placed with 5″ apart at the center of the beam: Moment = 15.75W
The error for this simple example is 21%. This means that the uniform load assumption upon which manufacturer’s beam tables are based would be 21% in error in the unsafe direction. The deflection of the beam will also be more than the UDL deflection.
For the case of pallets that are undersized for the width of a pallet rack bay, special beam design is needed to make sure the beam is strong enough and stiff enough for the application.