Load Factor For Mobile Crane. 4

[IJR]

My friends

I have to check a concrete framed building and slab for the case of a heavy crane that will support it temporarily as a steel superstructure is erected. What load factors would be appropriate for use with ACI?

1.4 DL + 1.4 Crane Weight?
1.2 DL + 1.6 Crane Weight?
1.2 DL + 1.2 Crane Weight?

The last one makes sense to me since crane weight is a well determined dead load and 1.4 would be conservative for such a short term loading(about a week at the same location). The ASCE publication on construction loads is not specific on load combinations. Is it?

thanks in advance
ijr

 

[RPMG]

1.2D + 1.6 * (1.33 x Crane) + 1.6 * Construction Load

Dead weight is permanent and static. Per code, it only includes building materials and permanent equipment. I would also add a dynamic factor on the crane and construction area load.

 

[JStephen]

The crane weight is well determined. The load per wheel or outrigger is not so well determined, and will depend on wind load, among other things.
Soil failure example here:

https://www.youtube.com/watch?v=xz71eVreMDc

 

[hokie66]

And crane on floor failure here:

http://www.vertikal.net/en/news/story/6733/

 

[Lomarandil]

You won't find explicit code guidance for this.

For mobile cranes on bridges (note, AASHTO is generally more aggressive than ASCE/ACI) I would tend to use 1.25DL and 1.35LL. Depending on operating conditions, I usually also factor the crane and/or the pick weight with a dynamic load factor.

Jstephen is right -- you also want to be careful about the assumptions (or provided values) for crane reactions to the structure. What I mention above was using a pretty refined breakdown of how cranes transmitted load to the bridge in a variety of circumstances (based on lots of experience).

Remember -- considering the duration of load to reduce load factors only makes sense for probabilistic loads like wind or seismic events (excluding timber structures). A load is a load (and a crane is a crane) no matter how long it sits there -- you don't want to race against a failure mechanism.

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The name is a long story -- just call me Lo.