Steel Plate Overlay 4

[Briansch]

We have a structural floor system consisting of 26H9 floor joists, 40' long, 2' O.C., supporting a concrete slab. The slab is 4" thick normal weight 3500 psi concrete with 1 layer of 6x6x4 gauge WWF over 1-1/2" x 22 gauge composite metal deck (2-1/2" concrete above flute). This floor is a well traveled aisleway approximately 40' wide x 480' long.

Our problem is this floor has severely cracked where material handling vehicles travel.

A structural engineer we retained offered two options: 1. Total replacement of floor system with 5-1/2" thick slab or;
2. Overlay the entire area with 1/2" thick A572 Gr 50 plate

Total replacement is too expensive so we are looking at the steel plate overlay.

Does anyone have experience with this type of overlay? We would bevel the plate edges to accommodate welding the sheets together. Should the plate be fastened in any way to the slab?

I would appreciate any insight you could offer.

Brian Schaible, PE
Marvin Windows and Doors
Warroad, MN
(218)386-4142 Ext 1432
(218)386-4203 Fax
briansch@marvin.com

 

[LPPE]

I think by welding your plate together to form one single playe 40'x480' you may experience some thermal expansion and contraction problems. And why A572 not A36 plate? I know theres a premium on A36 shapes, but is there a preimum on A36 plate also? Thats my two cents.

 

[bengineer]

Just like the US will roll over the competition at the upcoming olympics, equipment rolling over a thin slab will eventually degrade it.

Another solution, which is lighter and quite effective, is to use tounge & groove wood deck. This will absord a lot of the dynamic effects as well as in assist in distributing the moving loads. As you may or may not know, welding against or adjacent to concrete can cause small explosions and further damage the concrete; with the above solution, no welding would be required against or near the concrete

 

[ishvaaag]

The plate proposed by the engineer may well work sinc span is 2' and even if broken the aggregate will providing support. However the rupture of your concrete indicates that maybe due to the not enough thick concrete at the present, or to dynamic effecs of the loads or repeated cycles of solicitation your floor has exceeded its design strength or fatigue life...

The problem is that normally the concrete there is part of the resisting system (or it should be, your present cracking would also be showing). What means that...

If you don't restore the integrity of your slab prior to other work you may in the end lack the necessary strength in the supporting system, by lack of composite action of the slab. Even if it is unlikely a local failure to develop, permanent deformations in the added steel plate will develop. Furthermore, what is completely glued lasts more, behaving as a unit.

Hence if you decide to go for these alternative without further consideration of renewal of some kind, my advice is

first suppress and then restore the integrity of the to be underlaying concrete slab

then prepare a as close as feasible perfectly flat surface

then adhere the entire steel sheet to such prepared surface.

Special mortars and epoxy adhesives should be used in the work.

This will make the structure safe enough, more flat and less noisy.

Consideration to add studs going even unto the supporting joists may be worth as well. You could plug-weld them and then grind.

 

[JAE]

Briansch,

Amazing how similar issues crop up around the country. About 5 years ago I was asked to investigate a deteriorating floor for a retail office supply store (a name brand you'd recognize if you are in the U.S.). It seems they had a steel joist floor supported by steel beams with a metal formdeck and concrete slab (3 1/2" thick I seem to remember). The floor was designed for 100 psf and they were running a full weight (3500 lbs) forklift over it. Needless to say, around the dock area and other high traffic zones the slab was fully cracked and spalled.

The cause, of course, is due to the differential deflection of the supporting steel joists which have fairly low stiffnesses. As a wheel load travelled over a joist, it deflected downward (especially at midspan) and the adjacent unloaded joist did not. This initiated very high flexural stresses in the concrete (which only had WWF in the bottom.)

I first suggested that new steel X-bridging be installed (heavier than standard spacing) to get the joists working together but the owner couldn't get access from below without tearing out a whole movie theatre (Yes....I know....lots of people below in harms way).

The fear was that eventually a wheel would pop through the slab and potentially distort a joist chord, initiating a complete collapse. The owner's engineer (this was a pre-lawsuit) suggested placing two layers of 1" thick high-grade plywood over the floor area where the forklift would travel. This really didn't work as the plywood wouldn't have the capacity to span from joist to joist with a full forklift wheel load on it. I had to assume that the concrete was mush and the plywood would take everything. I convinced the engineer (I think) that plywood was not the answer.

What they ended up doing was repairing the concrete and then buying a very light forklift that I determined would work with the spans.

If you use a steel plate....I would design the plate to span from joist to joist WITHOUT counting on the concrete. Put a wheel load at midspan and see what thickness you need. It may be a very thick plate.

Keep in mind that your MAIN PROBLEM is the flexibility of the joists and possibly the lack of bending and shear capacity in your slab. Slapping a steel plate on it is really just a bandaid.

 

[dlew]

Brian,

The floor system you described seems to be limited by the capacity of the steel joists. The 26H9 is an old denomination for high strength steel joist. I do not have a Joist Load Table for those joists, but a 26K9 (that's the new denomination) joist would support a live load of 115 psf (100 psf if the deflection is limited to 1/360 of the span).

The slab system seems to be a composite steel deck with reinforced concrete. This system would support a live load of at least 400 psf. Disregarding the composite action, a 1-1/2", 22 ga. deck, by itself, could support at least 250 psf.

You did not mention the beams that support the joists. They should also be checked.

Now the question is what kind of loading is being applied to the floor. Obvious the vehicle is producing concentrated loads on the floor system, thus the uniform loads I have mentioned above just give an idea of the adequacy of the floor system.

Some pertinent questions about the vehicle loading are:
What is the full loaded weight of the vehicle? What is the maximum wheel load and wheel spacing? Are the wheel tires solid rubber or air-pressurized? Is the vehicle producing excessive impact or vibration on the floor? Does the vehicle move perpendicular or parallel to the direction of the joists?

Have the cause for the cracking in the concrete been determined?
If the problem is high concentrated wheel loads on the joists producing excessive deflections, providing additional bridging between joists to distribute the load, as discussed by JAE in his response, would help.

Good Luck!
AEF

 

[Imagineer]

Brian,
A couple of thoughts. IF the problem were just forktruck dynamics, I know of several heavy industrial factories that use 2" thick pecan tongue-and-groove decking in all of their aisleways. This works very well and definitely solves that problem. However, I'm not sure that this will solve your problem. I suspect your problem is lack of concrete strength due to underreinforcing. What, I have rebar on the bottom and WWF on top?!? you say... Well, it has been our experience that WWF usually sinks to the bottom during construction before/during the concrete placement. You THINK that you have top reinforcing when in fact you don't. So, we don't design with WWF anymore. (Even the contractors who said that they would put it up on chairs seemed to conveniently forget them.)
Sorry I couldn't necessarily help with a direct solution but I hope you can use this info to come up with a good solution.

Imagineer

 

[Briansch]

Thanks very much for the responses to date. This forum has been very insightful and helpful.

I will offer some additional background on this project. The worst case wheel load (front) is 3,775 lb (Yale GLC030 forklift w/full payload of 2,500 lb). This same vehicle with no load has a wheel load of 1,700 lb (rear). Total vehicle only weight is 6,450 lb. The axle spacing is 48" with a wheel spacing of 31".

A more common vehicle (Pettibone 460-D tow bug) has a max wheel load of 1245 lb (rear) with axle spacing of 49" and wheel spacing of 30". Total vehicle weight is about 4150 lb.

All tires for the most part are solid rubber.

The reinforcing in the slab was assumed to be midway between top of deck and top of slab or 1.25" below top of slab. The engineer we retained modeled the entire floor system (beams, joists, slab) and determined the maximum bending stress in the slab for the loaded forklift was 4672 psi! This considered no contribution from the reinforcing steel. Fr for the concrete is 7.5 x 3500^0.5 or 444 psi! There were some very conservative assumptions made in the analysis but no matter how we slice it, the slab is considerably overstressed. Needless to say we are limiting traffic.

The joists are perpendicular to travel. The aisleway is positioned within 11 feet from the end of the joists. The beams supporting the joists were checked and were OK. The joists, subjected to max wheel loads and total uniform load (305 plf) outside of the aisleway, developed their maximum allowable moment but were considered to be OK. Maximum deflection was about L/200 at midspan.

The proposed 1/2" plate was designed by our retained engineer as 3 span continuous beam (2' spacing) without any support from the concrete (BM=1528 lb-ft, eff. width=16", Fb=30 ksi). I did not mention this before but we did plan to install cross bracing along the joists spaced approximately 4' to 6' along the length of joists. I know we will be losing about 20 psf in live load capacity by adding the plate but we can live with that. Our material handling and inventory control people want this route available to minimize travel distance and reduce traffic on more traveled routes.

Briansch