Fork truck on mezzanine slab 1

[BSE05]

I'm researching the new construction of a mezzanine slab (100,000 sf) that will carry a fork truck in the range of 5-6K pounds total weight. Pallet weight is about 2000#.

The structure is steel with composite metal deck. There is a real concern to prevent slab cracking since fork truck wheels tend to eat at the cracks causing more spalling etc.

Another concern is that wheels can "crack" a rib so slab thickness is critical also.

My thought is to limit overall span deflection to L/540, use fibermesh concrete, and saw cut joints.

Does anyone have any guidance on this?

 

[wale01]

I've dealt with this on a couple of jobs. Why is cracking such a huge issue? I know you want to control it but its not avoidable(for the most part). Design the slab to carry the point load. Treat it as a simple span between supporting steel members. You definitely don't want to cut joints in your slab. Its a structural slab.

 

[JAE]

I once did a study of a steel joist, metal deck, concrete slab system for an office supply retail store which used the rear portion as a truck dock and they had heavy forklifts traveling all over the store depositing their wares onto racks.

The floor in the vicinity of the truck dock was a mess. Lots of cracks and spalling, very bouncy due to the loss of stiffness in the slab. The owners were fearful because directly below the dock was a movie theater.

What we discovered in our study was that the wheel loads, positioned on a single floor joist (I think they were 22" bar joists) would deflect the joist to a degree that would cause bending in the slab relative to the adjacent joists and the stress was higher than the cracking stress.

Once the cracks started, others propogated out from there and the constant loading and unloading began to totally deteriorate the concrete to the point that it was individual chunks of concrete sitting on the metal deck.

The joists only had horizontal bridging and were fairly low-Ix for the spans.

I would definitely study your floor framing and ensure that cracking is minimzed as much as possible. Look especially at joist-to-joist relative deflections with specific wheel load placements.

We've used fiber (steel plus poly) reinforcing in slabs to help but these won't prohibit stress cracking, just tend to keep them smaller.

 

[jike]

First of all, it is not recommend to use composite metal deck. Research shows that the composite deck will debond from the concrete under repetitive concentrated wheel loads.

Second of all, fibermesh is used for shrinkage but not to carry structural bending moments created by the wheel loads.

You will have to calculate both positive and negative bending moments created by the wheel loads and reinforce the slab with sufficient reinforcing steel. Make sure that you have not only the proper main steel (both positive and negative) but also adequate distribution steel (bottom at right angles to the main steel) to allow the concentrated loads to spread out over an effective slab width. Make sure you calculate the overlapping bending moment created by the adjacent wheel loads. Since you will have two layers of steel, you might want to consider a 6" minimum slab thickness.

Since the size of the mezzanine is sufficient to have two or more trucks operating at the same time, you might want to consider two loaded trucks passing in an aisle.

If you design the reinforcing properly for all the various stress conditions created by the wheel loads, you should minimize any cracking. The size of the mezzanine might require some expansion joints.

Good Luck!

 

[GregLocock]

You might also want to consider the entire weight of the truck plus the full lift capacity exerted on just the front wheels.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mrengineer]

1) There was an article in Modern Steel Construction (not sure which issue, I just made notes for posterity) on San Diego baseball park where they had concourse subject to small trucks and forklifts. They used 2" composite deck, 4 1/2" topping, #4@12" each way in top of slab.

2) In the publication "Designing with Vulcraft Steel Joists, Joist Girders, and Steel Deck" by James Fisher et al., they say "The use of composite deck is not recommended where forklifts are used unless the use is an infrequent occurrence".

 

[jike]

JAE:

Why would you use fibers to limit "stress" cracking instead of whatever rebar size may be required?

GregLocock's suggestion is a good one. I would also use an impact factor.

Often owners will want to increase the capacity of trucks over the years. This might be something that you may want to discuss with the owner.

 

[dik]

Steel fibres and not polypropylene increase the toughness of the slab and will likely hold the concrete together at a crack. I've used this with steel deck (not considered as composite, only as a form) with OWSJ and used top reinforcing (bar) to take care of -ve moment. Also limited joist deflection to L/1000 (arbitrary) and I had lots of headroom so depth of joist not an issue and I wanted the stiffness. Also critical load was when two units were passing and also considered the maximum loading to be concentrated on the two front wheels. Client should also undertake an annual inspection and repair program to make sure it doesn't progressively deteriorate.

 

[BSE05]

Thanks to you all, your comments are all very helpful. I have had varying degrees of success with mezzanines over the years. As was stated they all cracked! Some years later, like a cannon going off.

The problem with this job is that the steel design is being controlled by a pre-engineered building manufacturer, I get the concrete design. The adjacent warehouse mezzanine had joists 3 foot on center ( I didn't design this one) and the slab cracked over virtually every joist with a great deal of spalling. Trucks were only hand trucks and limited to one pallet or 125 psf.

So this thread will help my confrontation with the steel fab. Can you image that they ever limited deflection to L/1000.

 

[BSE05]

Has anyone ever saw cut such a slab?

The theory being to force cracks to the location of a uniform joint you can fill with epoxy.

I have never done this since the reinforcing (and deck) ties everything together.

 

[mrMikee]

GregLocock's suggestion is a good one. I've seen fork trucks overloaded to the point that the rear wheels lift up. It depends on the culture of the facility where the mezzanine is to be located because some manufacturing plants, for example, have an anything goes mentality. After all, everyone knows the engineers put in a 5 to 1 safety factor.

Regards,
-Mike

 

[jike]

I have found that most structural floors that have cracked due to fork truck loading have not been correctly designed for the stresses caused by those wheel loads. This is an area of building design that is not well understood by many engineers.

When you hear on crack that sounds like a cannon going off, that could be a sign that perhaps some rebar was overlooked, either in design or construction.

I don't like the idea of using joists for these floors. I prefer rolled beams.

 

[dik]

I use an ancient copy of PCA program used for analyzing aircraft loadings... works really well...

Dik

 

[mrMikee]

In my comment earlier today I did not mean to suggest that everything actually does have a 5 to 1 factor of safety. In my haste to post information about what I saw frequently during my years in a Plant Engineering department of a metal fab facility, I failed to notice that my post was worded poorly and the sarcasm possibly was not obvious. Part of my job back then was to design hatches in truck aisles and floor plates around machines where fork trucks would go. My observation was that fork truck drivers would lift anything they could, and if it didn't work the first time they would try again. This attitude is probably different in other industries, but fab shops and foundries are pretty bad.

So, as Greg says, assume "the entire weight of the truck plus the full lift capacity exerted on just the front wheels."

Thanks,
-Mike

 

[JAE]

jike - I didn't say to use fibers to reduce stress, I said, "We've used fiber (steel plus poly) reinforcing in slabs to help but these won't prohibit stress cracking, just tend to keep them smaller."

 

[Ron]

jike is onto the issue....

You must design the slab for two actions....the macrostructural to hold the forklift and its load in a concentrated fashion and as GregLocock stated to consider the load distribution, AND, the stresses imparted to the concrete from the wheel loads. They present different problems and you can't pick one or the other....you have to design for both. For instance, the wheel loads might initially dictate the concrete thickness, while the structural loading will dictate the overall deflection and bending resistance.......however, the overall deflection under the macroloading must be considered with the lateral strains and lateral stress at the bottom of the slab caused by the wheel loads. Compound all that by the formed deck interaction and you actually have an engineering problem!

 

[mfrad]

I used 18 gage composite deck and a composite beam system (with Nelson studs) with #5 rebar in the deck flutes to distribute wheel loads with no problems. Need minimum 6.5 to 7.5 inch total slab thickness. Also place filler beams closer together since this distributes forklift loads to more beams. This system was shown in an old Robertson Q deck catalogue for fork lift and wheel loads. Also check AISC design guide on parking decks. Concentrated loads from autos are less than the forklift wheel load since the tire pressure is higher. I used 85% of total forklift plus lifted load which represents maximum tipping on front two wheels.

 

[dik]

and to add to Ron's comments, depending on the slab stiffness, the location/pattern of the wheels, they can either add or subtract from the stress pattern at the wheel under consideration.

Dik

 

[rmw]

Look, just keep the forklift off of the mezzanine deck. Here's how;

http://www.cargolaw.com/2000nightmare_singlesonly4.html#safetylift

No cracking in that slab.

rmw

PS: Sorry, I just couldn't resist. The thread was quite well developed by now.