Proofroll Loading

[bagger]

We need to proofroll a heavy haul road using a minimum simulated wheel loading of 90 psi. The tandem axle dump truck we are planning on using will have a tire pressure of 110 psi. The issue is, what do you base the loading on; the 110 psi tire pressure or the weight of the load divided over the contact area of each tire? The measured contact area is very subjective based on the squatting of the tires under load. The text books I have studied provide formulas for defining the tires contact area, however, it all comes back to the tire pressure (110 psi). If this is the case, what is the rational for proofrolling with a fully loaded tandem axle dump truck, if the tire pressure is what governs.

Sorry for the long winded scenario. Looking for some opinions.

Thanks
Bagger

 

[ATSE]

Forget tire pressure.
If the tire pressure at the end of a loaded axle is 110 psi, and you raise up the truck body and axle with a floor lift, the tire pressure is still going to be close to 110 psi, with no ground load.
Discussions about tire pressure and ground pressure sometimes do not address the radial tension in the tire correctly.
Contact area estimates are not perfect, but this is what you should use, as this is what the ground "feels."

 

[Compositepro]

Well, you aren't going to get 90 psi loading with 30 psi of air in the tire. Nor will you get any loading with 110 psi in the tire if its not mounted on a heavy truck.

Tire pressure (actual pressure - not cold, unloaded, inflation pressure) roughly limits the peak pressure that a tire can apply to a flat surface. As load on the tire increases, the contact area increases, and the tire pressure will rise slightly.

 

[cvg]

Pressure = force/area = load/contact_area

however a total load of 110 lbs applied over one square inch will not be sufficient to test your road...

Total load is more important, thus the requirement for a large, fully loaded trailer

you should proof roll with the heaviest vehicle that you can reasonably expect to use the road. Let the mechanics worry about how much air to put in the tires. most proof-roll specs do not specifiy tire pressure but they all seem to specify the vehicle - minimum 30 tons up to 50 tons.

 

[ATSE]

I respectfully disagree with Composite.
You can get 110 psi ground pressure with a flat tire (0 psi internal pressure) or with a solid rubber or steel wheel, given adequate truck load.
A pressurized tire under asymmetric loading with non-linear constitution is a complicated thing, and using tire pressure to find ground pressure is unnecessarily complicated. Worse, if you ignore hoop tension in the body of the tire, conclusions relating tire pressure to ground pressure will be incorrect.
cvg is correct that you want a very heavy load to increase your area of influence.

 

[Ron]

Contact pressure has little to do with wheel load for pavement structural purposes. Tires on construction vehicles are commonly radial, so then the tire pressure is maintained under a heavier load, the contact area just increases.

ATSE said it nicely...forget the pressure. Concentrate on the total load. For proofrolling, a loaded dump truck works just fine. Almost any equipment with heavy loads and rubber tires will work.

 

[bagger]

Thanks for the input everyone. Heres what we ended up doing.

1. Determined the tire pressure with the truck empty. (110 psi)
2. Loaded the truck with the maximum load it could carry.(68,880#)
3. Measured the tire pressure. (remained at 110 psi)
4. Measured the contact area of the squatted tire (12.5"x8")
5. Calculated the individual tire load (68,880/8=8610 lbs).
6. Calculated the contact area (110 psi/8610 lbs.=78.27 sq in.
7. Contact area assummed to be an ellipse per Boeing.
8. Calculated the major and minor axis of an ellipse.
9. Major axis=12.66", Minor axis=7.91".
10. Compared the calculated values with the physical measurements. 12.5" vs. 12.66" and 7.91" vs. 8.00" OK
11. Based on this procedure we should be >90 psi, closer to 110 psi.

The proofroll went well. The road failed where you would anticipate it to fail (at the edges) and was ok near the centerline. Thanks again for your input.

 

[FixedEarth]

You say "the proofroll went well", but you have not mentioned anything about deflections, cracks, pumping? The subgrade, in particular the upper 2.5 ft, carries the pavement. Can you check with a senior engineer prior to placing the road base?

http://www.FoundEng.com

 

[bagger]

The road rutted excessively at the edges and showed no signs of instability near the centerline (as stated). The rutted areas will be repaired. The reasoning for all the measurements and calculations was for back-up documentation to show how we estimated our loading conditions. Just saying a fully loaded tandem axle dump truck is not adequate in this situation, ie. nuclear.