steel plate on concrete slab on grade 1

[cmrdata]

Heavy wheel loads on concrete slab on grade when moving equipment into position during the construction phase.
The local stress at the tire contact area is too high and we need to add steel plate on the floor to distribute the loads.
Does anyone know of any design guides/procedures in determining how thick the plate needs to be in order to achieve the stiffness that makes the assumption of "the load been dispersed/distributed" valid?
Thanks in advance.

 

[EngineeringEric]

Should be same as the design of a steel column baseplate.

 

[TehMightyEngineer]

Seconded. Don't forget to check punching shear in the slab.

Maine EIT, Civil/Structural.

 

[cmrdata]

Thank you for your input and you both have a valid point on the steel plate thickness issue.
Let me elaborate a little bit further. Typically, without the added steel plate, we evaluate the slab capacity, when subjected to wheel loads, per PCA design chart that I attach here.
As you can see, when the wheel load is high, the resulting required slab thickness will be off the chart here.
By adding a steel plate, we can increase the tire effective contact area. However, the stress per 1000 lbs of axle would remain high, which would still kick the slab thickness off the chart.
My theory is that if the steel plate is thick/stiff enough, the two tires will be considered acting as one load, then this chart does not apply, and then all we have to worry about is the concrete bearing strength and the punching shear strength.
So, the question is, how thick is thick enough?

 

[dhengr]

Cmrdata:
Why not use 2 - 3' wide wooden crane mats, instead of the stl. pl.? You could lay these 20' long in the direction of travel and space them to be centered under the wheels. You might need a small piece of stl. pl. (bridge) where two mats butt end to end. You can rent these wooden mats. The weight distributing ability and stiffness of these mats might actually be better than a stl. pl. Even though the stl. pl. can act at a higher stress, it also has a fairly high local deflection tendency and thus slab stress , which is the killer in your case.

 

[cmrdata]

That is an excellent idea, dhengr. That is new to me.
As a design engineer, I have never dealt with such crane mats before.
I will look into it in more detail. Meanwhile, do you know, typically, if they are able to tell the contractor what size of mats to use, based on the wheel load and my slab information? Otherwise, the contractor might come back to us and I'm not aware of any design criteria for such application.
Thanks.

 

[JStephen]

You might try contacting the mat rental companies and see if they have recommendations.
Crane mats are very common in coastal areas with soft soils, perhaps less so in other areas.

 

[dhengr]

Cmrdata:
Come on, you’re the Structural Engineer here, and every design problem doesn’t come pre-packaged with design tables and worked example problems, that you just plug your numbers into. Put you thinkin-n-searchin hat on and do a little research. Look in a few Civil and Structural Engineering Handbooks and the like; look at a good Wood Design text or two and the NDS; talk with your GeoTech guy. Google some of this, particularly Wooden Crane Mats. Obviously, you have to know the axle loads of the trucks you are dealing with, the tire foot print, spacing btwn. the tires on the axles and various axle spacings. To spread the load over a greater slab area, you could put a foot of good compactable soil on top of the slab and then the wooden mat on top of the soil. Then the mat might be a beam on an elastic foundation, spanning at least a couple times the spacing of the tandem axles, and corbeling/spreading down through the various layers at about 45̊ . If the mats are directly on the slab, you may want newer mats, not all chewed-up so the thickness of the mat members are all the same, for good bearing on the conc. and a flat top for the tires.

 

[Ron]

Agree with dhengr...crane/dragline mats are made to allow high loads to be distributed to materials that cannot generally take those loads. Use them. They work great for these applications.

 

[JLNJ]

If the slab is significantly stiffer than the plate, then the plate may not end up doing a whole lot of distribution of the forces.


Check the EI for a 6" slab compared to the EI for a 1" plate. You may be surprised. Just because it's steel plate doesn't mean it's magic.