Static Friction Coefficient Steel on Asphalt?

[ColonelMonk]

Howdy:

Have searched quite a bit trying to find this. Closest I have come is this document:

[URL unfurl="true"]https://apps.dtic.mil/dtic/tr/fulltext/u2/270810.pdf[/url]

The only coefficients in the document are for dynamic friction, based on testing for landing experiemental aircraft on asphalt and dry lake beds. Pretty interesting, but not what I need.

Doing a wind load test on an equipment trailer. Trailer is supported by four steel footed jacks, and asphalt is the most likely surface it would be set up on.

Using a sort of average from these graphs, we're using 0.25 as the coefficient, but of course this is dynamic and lower than the static friction number would be.

Extremely close to meeting our design goal, like by several mph..... A value > 0.31 static would get the job done, so close....

I have found sliding values of steel to concrete of 0.45 and static values steel to concrete around 0.70

If the difference between dynamic and static for asphalt is anywhere near the magnitude of concrete, then we're there. Just can't find a published value.

Anyone? Any idea of any references texts that would contain this information?

Thanks

 

[JStephen]

Here locally, the rural asphalt roads are all chip-seal surface. I observe that the identical material will be nearly 100% rocks on the surface when fresh and about zero% rocks when aged. That being the case, I would expect a wide range of values, especially with steel being the contact surface.

 

[CANPRO]

Fairly easy to test. Some known weight, steel plate, and some sort of spring scale/load cell is all you need. The test can be as accurate as you have money to spend - I’d opt for a relatively cheap spring scale just to get feel for what to expect.

“Asphalt” could describe an endless combination of asphalt mixes, quality of construction, and wear conditions - I would expect a significant range friction.

 

[JAE]

Look through this document - you might have to dig a bit but I bet it's in there.

https://apps.dtic.mil/dtic/tr/fulltext/u2/270810.pdf

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[ColonelMonk]

JStephen,

Thanks and agreed. Not the most homogeneous surface.... That paper observed as much.

Still, seems rather certain a static value (if one exists) will get the job done. Last resort we're looking at contracting a test lab. Can always specify that rubber pads be used as a friction modifier at the upper end of requirements, but not the perfect solution.

Thanks again.

 

[jayrod12]

Obviously a connection would not be ideal, but if it's asphalt, could you not just drive a couple nails through the feet? If you're that close to it working, a couple of nails would ensure you're ready to go.

 

[dhengr]

ColonelMonk:
While you are looking for the coef. of friction for that particular asphalt, you might also want to look for some punching shear strength for the asphalt. Many times, four jacks under a heavy trailer have to be dug out of the pavement at the end of the operation, because they have punched into, or through, the asphalt. Maybe your friction problem is steel jacks on wooden blocking which spreads the load to the asphalt. You could also weld a piece of expanded metal to the bottom of the jack, which would mechanically engage with the base material below.

 

[BridgeSmith]

dhengr's expanded metal idea is a good one. Another option is bonding or attaching a section of tire tread to the feet of the jacks.

 

[ColonelMonk]

Guys Thanks much for the replies, my apologies for being absent it's been a busy week.

All good ideas, we realize that a friction modifier will do the trick easily, but prefer not to rely on it if we don't have to. Nearly anything is better than just steel. Wood blocks are often used... Expanded metal is a great idea also...

Yeah the variability of asphalt is probably the issue and reason why the numbers don't exist, the aggregate used is all local so it can be river stones or crushed rock or about anything in between. Just looking for a number better than the dynamic friction number I have.

Will check out these links soonish.

Thanks for the replies, I appreciate them.

CM

 

[ColonelMonk]

JAE SAID
"Look through this document - you might have to dig a bit but I bet it's in there.

https://apps.dtic.mil/dtic/tr/fulltext/u2/270810.p..."

Haha, yeah that's the same document that I linked to above. It only has dynamic values of friction though....

For now, we are rolling with what we have, and noting that rubber pads or some other friction modifier must be used at 90 mph cert. Not a bad result, just annoying that it's so close.

Agree with all about the variability of asphalt, but that doesn't stop anyone from listing common static frictions for rubber on asphalt and others.....

Thanks

 

[JAE]

Yikes - sorry ColonelMonk. I didn't stop to see that.

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[IRstuff]

What do the feet look like? Are they smooth or toothy?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[BridgeSmith]

All this effort and time to try to use something that may or may not work, when simple and cheap solutions that will work have been proposed...why not just put something on the footpads that will give you the grip you're looking for?

 

[cvg]

since static is always significantly higher than dynamic, your design is almost certainly acceptable. no need for any pads.

to prove it, I think canpro hit it, you could test this in your driveway in about 5 minutes. flat steel plate with a brick on top for extra weight, use a cheap fish scale to measure the force and check it for several types of asphalt.

 

[ColonelMonk]

CVG yeah exactly, with a static coefficient a mere 0.06 higher than the dynamic one given, we meet our goal.... We know the design is acceptable, but we have to do a 3rd party PE approval and the assumptions can't be made to satisfy them. Got to be a published number. I even found an old survey of a previous product before my time where they used 0.35, but there was no source listed and the 3rd party doing the testing won't accept it.

HotRod,

Well, It's not possible to convey all restrictions of my situation over this thread..... Many fine solutions have been given and they would be perfectly acceptable if it were a one off.... It's a production environment, on equipment used by state employees to their requirements. Not to say that a friction modifier can't be used, just that from the perspective of being in business in a competitive environment, it would be so much better to not need that requirement at all - and the only thing holding us back is 0.06 difference from what is probably a conservative sliding coefficient to begin with. I answer to our president, and we all answer to the 3rd party firm doing the survey and our picky customers. So there you have it.

For now, we're going with what we have that satisfies 90% of our customers, the few that have stronger wind requirement will probably accept it with the caveat.

If anyone ever does find a static coefficient for steel on asphalt, please do post it here with the source information. Thanks!!

And thanks for the helpful suggestions.

CM

PS, the jacks are a typical trailer jack, smooth steel contact surface. They come zinc plated which is one the reasons we don't want to have to weld stuff to it. thx

 

[BridgeSmith]

I understand the limitations of working for a state agency; I work for one as well. As a bridge designer for for my state DOT, I get it. While I can appreciate the effort to be responsible with public funds, the safety of my fellow state employees and the public is the priority. I always weigh the consequences of 'failure' when determining whether to 'push the envelope'. I'll ask the simple question, what happens if it slides? If it's not going to get anyone hurt or killed, then I would be ok with being a few percent over. Otherwise, I'm very cautious when dealing with uncertainties.

 

[ColonelMonk]

Hot Rod:

Surely understand how it is in the bridge design world; those designs are "permanent" and as you state, people's lives are on the line. I wish I could say more, but you all probably want to be somewhat anonymous here as well.... The answer to what would happen if it slid, is probably absolutely nothing. I won't tell you more about the equipment but at the target wind velocity folks should not be around it and it should not be in use.... But that won't stop state agencies from wanting it to perform like something which is permanently installed so that is one of the difficult conditions of our business.

Design performed perfectly in tipping due to wind load, and slightly less in staying put, but in the end we're fine with the result as it passes for the large majority of state and federal realms. For the few with the higher wind velocities, the simple addition of a rubber pad to the jack plates put it well above the requirement. That's been noted in the report and it's all good.

Thanks for all your help and perspectives,

CM

 

[Tmoose]

What is the nominal loading ( psi) for the jack feet ?

As a minimum I'd be thinking about making the feet out of this in place of flat steel ( if that indeed is what you will be using )

https://www.industrialmetalsupply.com/steel-tread-plate-products#1

My concern about rubber feet would be retaining them when the trailer is dragged, although maybe the trucking industry has that figured out already.

 

[POPEYES]

Are there published values for how much checkered plating steel increases the COF?

 

[BridgeSmith]

Doubt the diamond plate would make much of a difference. It's still a smooth finish and large enough that it won't really interlock with the aggregate. Anyway, I believe the jacks in question were to be off-the-shelf items, not fabricated, so he's pretty much stuck with what's commercially available.

ColonelMonk, we make the same kind of judgment calls, and generally come to a similar conclusion you did - if it slides at the design wind speed, it's fairly inconsequential. If I'm understanding your situation, there is negligible risk to life and limb, and only minimal risk of even property damage. I wouldn't lose any sleep about recommending just going with the steel feet.