Suspended Slab Damage 1

[SteelPE]

I am in the process of designing an addition for a client (a large industrial manufacturer). The project is the 5th project I have done for the client in the past 4 years. The existing structure is a series of 9 metal buildings that have been constructed over the years. The last 5 additions we have done have all utilized conventional framing (a total of 14 building placed on a site if you are trying to keep track).

Somewhere along the addition process (around the 7th structure) the client decided to make his building 2 stories. The client uses forklifts inside of his building to move material around inside the building and the existing suspended floor (4” slab on form deck with open web steel joists at 2’-0” oc.) has deteriorated badly. Buildings #8 on have used a tighter joists spacing with a thicker slab which has fared quite well (no observed damage). Attached you will find pictures of the existing slab.

The latest project will empty the building with the damaged slab and allow for repair. The area is to be repurposed with everything on the table (prohibiting access to forklift trucks, lowering the capacity of the floor for strengthening etc). I have suggested the following:

-placing steel plates on the floor to distribute the loads to the joists
-taking up the slab and casting a thicker slab in the area
-eliminating forklift trucks from the building, remove damaged slab areas and recast a new 4” thick slab

I am wondering if anyone might have any other options to help resolve this problem.

 

[dik]

Deck should not be used with forklift traffic...

Dik

 

[RickyTickyTavi]

Presuming the elevated slab has the capacity to carry the load, the cyclic loading and repeated deflections will eventually tear it apart. This sounds like a serviceability fix. IE, increase the stiffness of the system. Since your existing slab appears toast, I would spec them demo the existing slab, drop in bar joists in between the existing, re-pour maybe 1-2'' thicker (if its even necessary), and be sure any construction joints sit over a joist. This is kind of confirmed with your observations on building 8, which has a tighter spacing and a thicker slab. I would compare deflection between building 8 and the deteriorated building and see what numbers you're getting. The key is, dont tell them they have increased capacity. If it was designed for 100 psf, then your modifications are for stiffness.

I have a project right now where were turning an elevated slab into a concert venue, so "vibration" and floor movement were serious concerns. We elected to demo slab and put in more joists.

Your Steel plate idea isnt bad, but is, in my opinion more of a bandaid. You'll continue to get slab deterioration.

If you can remove forklift traffic from the 2nd floor, then do that. That gives me indigestion for a metal building anyway, those things are normally designed to like 99 percent utilization

 

[structSU10]

SDI recommends that if you use composite metal deck that will be subjected to forklift loads that rebar is used to reinforce the slab, basically treating it as form deck. It references ACI 215R and AASHTO-LRFD for guidance. the supporting beams also need to be sufficiently stiff.

 

[SteelPE]

I should say that the existing slab utilizes 9/16" form deck and not composite decking.

 

[BridgeSmith]

Not sure how the attachment would be accomplished with open web joists, but the joists could be connected with stiff diaphragms or cross frames, so that several joints would share the load and deflect as a group. This would minimize differential deflection and also the total deflection due to the forklifts.

If you do end up removing the deck, and a thicker slab is undesirable, you could look at ways to make the joists composite with the deck. The strength gain would likely be small, but stiffness increase is substantial.

 

[JAE]

Had this happen at an old Home Depot building where the receiving dock was bar joists, form deck, with about a 5 inch concrete slab. Heavy forklift traffic and the whole floor was directly above a movie theater of all places.

Same issue as yours - heavy concentrated wheel load causes bending in one joist but not the adjacent joist - this causes severe bending in the slab-deck system and progressive cracking and spalling.

The fix we did was remove portions of concrete slab and deck, add multitude layers of steel X-bridging, put back more heavily reinforced concrete on form deck, and limit the size of the forklift to a walk-behind lighter unit.



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[SteelPE]

A Home Depot above a movie theater, I can't say that I have ever seen that before.

I don't think the client will accept putting new joists in-between the existing joists. I think I have a better shot of getting them to limit loading.

 

[TehMightyEngineer]

Neat trick I've seen at a paper mill might work for your situation. Demolish the concrete slab down 2 inches or so (hydro demolition or other means, or just pour a new topping over the existing) and place fiberglass bar grating on the remaining slab in the travel areas for the lifts, shimmed to be flush with the top of finished floor, spanning perp. to the floor joists. Pour a suitable topping concrete that can flow into the grating and get underneath the grating and not leave voids. Probably some kind of self-leveling floor topping material. Finish and call it a day.

This embedded grating helps to restrain cracks and reduce damage but most importantly it provides incredible protection against forklift traffic. I wonder if I can dig up some pictures showing how well it works. The difference between the areas that had this done and the unarmored areas was striking.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

Found 'em! I take it back, they used metal bar grating. My caveat to this is the mill floor slab was very stiff. The damage was related to the heavy forklift loads and nearly constant fork truck traffic often making sharp turns.

Armored floor shows no damage or appreciable wear at grating locations; still flush with adjacent floor slab. Unprotected areas adjacent show heavy wear and have required multiple patches.

Unprotected floor adjacent to armored floor, wearing and cracking:

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[SteelPE]

Funny you should mention installing grating in concrete. I was thinking of installing a grating that would be capable of supporting the required weight of the trucks in cocnrete. However, I was considering using a steel or aluminum grating. Although I had never heard of it being done in real life.

 

[TehMightyEngineer]

Although I had never heard of it being done in real life.

Yeah, they used steel bar grating and I retract my idea to use fiberglass; steel bar grating would be better.

I've never seen anyone use that trick other than that one mill, and I've done a number of evaluations of floor slab damage by forklift traffic. It's such a great idea that I've been wanting to find an application for it. I hope you can sell them on it as I suspect it will solve a lot of their problems and be relatively inexpensive.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/