Wheel Loads on Structural Floors

[mgardina]

Does anyone know of a publication that addresses how wheel loads are applied to structural floor slabs? I have a case where forklift is appling load to a structural floor made of 4" concrete slab on 2x10 floor joists. My preliminary calc shows the joist would fail. However, they have been using the slab since the 70's. there are starting to get some failures at the joist hangers not the wood. What this is telling me is my simplified approach to distrbuting the load is too conservative.

any suggestions?

 

[whyun]

On the joists, try applying a regular dead load plus live load for typical condition. Then, second check using dead load plus wheel load applied to the worst location (typically centered about the mid span of the beam/girder).

Justification for this is that chances of full live load being present simultaneously with the forklift load is very slim. How muc"reduced" live load to use for the second check above is engineer's judgment and how much the local building officials are willing to live with.

If anyone is aware of a specific code section that addressed this, please reply.

Cheers.

 

[RockEngineer]

You haven't stated what your joist spacing is but the relative stifffness of the 4" concrete slab to the wooden joists will cause load sharing by the joists. The individual wheel load would determine if the concrete fails but the gross load will determine if multiple joists will fail. Unless you have widely spaced joists a single joist will never see the large wheel load. You can take the time using relative deflections to determine how many joists take the load of the fork truck but it will be at least the number goverened by the overall dimensions of the fork truck. It would be conservative to assume the fork truck gross weight is distributed evenly over the joists defined by the overall dimensions of the fork truck. The slab will do the distribution between joists. If you have a large fork truck with a big wheel base you may want to look at two rows of concentrated loads corresponding to the front and rear. Use AISC Steel Manual load cases for moving loads (Beam Diagram formula 42) to determine your critical load locations.

 

[RG88]

Most 'codes' identify a distributed load and the clause of X lbs as a point load. For example, the IBC lists 50 PSF and 2000 lb concentrated on 20 in^2.

A few good sources on this subject are USACE Army TM5-809-12, which is free to download from their website. This manual covers forklift loads. Another source is the steel decking manual by USD, free as well. The decking manual can provide some insight on the application of the Building Load outlined in the various codes. I don't have it handy, but I think ACI 315 is the slab design manual (if that is not the number, I apologize, I am going from memory, but they do have a 60 page ACI code for concrete slabs under load).

One other point, the floor system you describe may not have been sized 'correctly'. It may not have failed because because it was never loaded to capacity. Oftentimes, structures don't fail under loading, not because they were designed correctly, but because they were never loaded to capacity.