API RP 1102: Origin of Equivalent Contact Area for Wheel Load 1

[George Logans]

Hi all,

I am a chemical engineer working in the natural gas pipeline industry. It often comes up that we will use API RP 1102 as a guideline to check cyclic stress for potential fatigue in welds underneath a highway crossing.

In calculating the cyclic stresses aka the longitudinal stress due to live load, API 1102 recommends maximum applied design surface pressures of 69.4 lbs/in2 and 83.3 lbs/in2 for single and tandem axle loadings, respectively.

To calculate these two surface pressures, API 1102 takes the contact area over which the wheel load is applied to be 0.093 m2.

Would anyone be able to advise where this 0.09 m2 comes from or how it is derived? I am designing a pipeline for a haul road crossing where it will be used primarily by heavy vehicles such as dump trucks. I would suspect that the equivalent contact areas for such vehicles would not be as small as 0.09 m2.

Thank you,

George

 

[LittleInch]

Looking at API 1102 the area is 144 in2, which looks suspisiosly to me like 12" x 12".


Truck tyres do seem to be about 12" wide and their area footprint about the same.

A search shows things like AASHTO-93 quoting 500mm x 250mm for road design so 0.125m2 so 0.093 looks like a conservative number.

You can get a ground bearing force if you look at the right data sheets for the trucks or the tyres, but because it is now spread over a much larger area, you get different load patterns than you do with a normal truck

If you're doing this for things other than tandem axle trucks using 1102 might give you very conservative numbers. There are many other ways to calculate it for large agricultural vehicles and similar on non pavement roads.

Most people just say they can't be bothered and lay a large concrete slab over the crossing point... Overall compaction of the ground and subsidence at the crossing point resulting in shear loads are far more destructive so long as your pipe isn't super thin, very big or less than about 1m cover

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[George Logans]

Thank you for the quick and informative reply LittleInch! Your rationale for the contact area given by API 1102 as a 12'' by 12'' tire area makes perfect sense. Again, my situation is for heavy dump trucks, so the tires they use are heavy-duty and would be more than this area.

When you mention other ways to calculate the longitudinal stress due to live loading, would you be able to specify what some of these might be? Our guideline has almost been entirely 1102 (with some input from AASHTO as you mentioned), so I'd be interested to see what other approaches exist.

Regarding slabs... that is also an option for us which we have gone to in the past. However, with consideration for the fact that this will be a haul road versus a highway, we're rationalizing that there would be less frequency of crossing in comparison. Hence, we'd like to do this fatigue check first before resorting to slabbing the crossing.

Regards,

George

 

[LittleInch]

Try looking at the ALA guidelines attached.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

And this one.

There's enough formulae and graphs to keep you busty for a long time.....

My issue with dump trucks is the rutting that occurs when it gets a bit wet and the potential for holes and impact loads.

You could just stick a few bog mats down instead....



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.