Steel form deck with concentrated load

[red22]

I have a mezzanine floor framed with open web steel bar joists 2'-0" o.c., 9/16" 26 gage steel form deck, 4" total depth slab with 6x6-W2.1xW2.1 WWF. The contractor would like to put a 14,000 pneumatic tire lift on the slab. Actually, they put the lift on the slab without asking and then were told to take the thing off until the EOR approved it. I cannot find a way to make this work by the numbers even though it did work in reality. Any ideas?

 

[csd72]

The reason why it probably worked in reality is due to the tensile strength of the fresh concrete. This strength is highly unpredictable, however and if it is exceeded a sudden collapse could occur without warning if this is the only thing holding it up.

What about putting some thick plywood over it to help spread the load?

 

[red22]

I was considering using steel plate over the slab to span between the joists. However this is still a significant load potentially applied between joist panel points. My other concerns are crushing of the deck at the bearings, and cracking of the slab. I have not been able to find web crippling values or other literature on this type of load on form deck.

 

[JAE]

Well, it may be that the lift was placed on the floor and the joists didn't fail due to some spreading out of the load via the floor stiffness and the safety factor within the joists themselves. Someone underneath was also probably pretty lucky the thing didn't collapse.

Tough call to approve the use of a vehicle on a building floor that wasn't originally designed for it; especially since you don't know where it will be driven, what loading will be on it, etc. Be careful.

 

[csd72]

red22,

Here is that point load reference you were after:

http://www.njb-united.com/usd/ddds/data33.pdf

Hope this helps.

Also the other option is to provide temporary props to the floor below and share the loads (depending on relative deflections)

 

[csd72]

There is also a design example

http://www.njb-united.com/usd/examples/example1.pdf

 

[red22]

Would you consider this example problem appropriate for form deck (example is based on composite deck)? Values used in this example are not provided for form deck.

 

[UcfSE]

Order Vulcraft's publication "Designing with Steel Joists, Joist Girders, and Steel Deck". It's only $15 and contains a lot of good information concerning special design considerations for steel joists and steel deck, including a point load on composite steel deck design example. You can find other examples in the SDI Handbook.

 

[twinnell]

Deck crippling is not a design concern for non-composite deck. The reason it is an issue for composite deck is that the stresses induced by the loads are shared by the concrete and the deck, composite action. If the deck becomes overstressed, the web can cripple. There isn't composite action with non-composite deck.

 

[lkjh345]

Looking at my Vulcraft catalog, I get that the deck is good for 400 psf live load.

14,000 pounds/400 PSF is 35 sqyuare feet. Not a terribly large area.

What is are the plan dimensions of the lift? Probably bigger than 35 square feet if the lift is that heavy.

As for the joist: What live load were the joists designed for? What else is up there at the moment? If not much else is up there, and the load is shared between a few joists, the joists may not be that much overloaded, if at all.

Given the size of the deck, and the spacing of the joists, this does not seem entirely unreasonable to me. I have allowed similiar sized lifts on elevated floors in the past. If the floor is of a fairly heavy design (and the 400 psf LL capacity floor idicates that it might be), then 14,000 pounds is not all that much area occupied.

 

[red22]

The lift is a "lull" type lift (four tires, driveable), so the loads we are dealing with are concentrated, not distributed. The floor is designed for 100 psf for assembly areas. Joists are 20K3 (good for about 330 plf total load). The only loads at this point would be a construction live load, the lift and the slab dead load.