I am in California. I base most of my asphalt concrete pavement designs on the Caltrans Highway Design Manual. I have done a few AASHTO designs, but it's been a while. For portland cement concrete pavements, I have used Caltrans and ACI 360R-14.
Caltrans uses a "Traffic Index" to determine the long-term traffic loading on a pavement for design purposes. Each type of design vehicle (all trucks, from 2 axles to 5+ axles) has a long-term "ESAL Constant" that is based on the equivalent single axle load (ESAL) of the vehicle and the design life of the pavement (typically 10 or 20 years, but can be longer). Thus, the ESAL is used as part of calculating a long-term cyclical loading, rather than an instantaneous pressure. To give some perspective, here are the 20-year ESAL constants for 2-, 3-, 4-, and 5+-axle trucks: 1,380, 3,680, 5,880, and 13,780. The 10-year ESAL constants are one-half of these values.
A detailed explanation of the use of ESALs within the calculation for the Traffic Index is found in the Caltrans Highway Design Manual, Chapter 610. Flexible pavement design is covered in Chapter 630. You can download these two documents (and other chapters) from the Caltrans website:
If you want to see how all this is done, I have attached my AC pavement design spreadsheet template for your and anyone else's use. I built the first version of this spreadsheet in 1986 using SuperCalc 4.
Now back to your original question. It is possible to take an axle load and convert it to a pressure, just not multiples of the axle load. For example, an AASHTO HL-93 design vehicle has the following loads: 4,000 lbf for a single front wheel, 16,000 lbf for a dual rear wheel set (=8,000 lbf per wheel), and 25,000 lbf for dual/tandem wheel set (=6,250 lbf per wheel). The AASHTO tire contact areas for these wheels are 10 in x 10 in, 20 in x 10 in, and 20 in x 48 in, respectively (the last two include the spaces between the tires). This generates average pressures of 40 psi, 80 psi, and 26 psi.
However, if you're looking for maximum pressure, a good approximation is the actual tire pressure. Last year, I designed some concrete pavements for a city's corporation yard. The City has two non-standard 3-axle, 10-wheel vehicles that I was to use for design, so I asked my counterpart at the City to give me actual loaded vehicle weights and tire pressures. Both vehicles had the same tire pressures: 110 psi for the two front tires and 95 psi for the eight rear tires.
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
Be careful with trying to equate tire pressures to load pressures....they can be vastly different. The more appropriate method is to determine the tire contact area and the load on the wheel. As an example, you can have a tire pressure of 100 psi on a tire that your are rolling down the road on its rim. The load pressure would be very small; whereas you could have a tire pressure of 100 psi and a wheel load of 10,000 lb. The pavement section feels the wheel load. The wearing surface feels the tire pressure.
I completely muddled the piece about tire pressure. I should have dived into my library for the exact thing I thought I was referencing, instead of relying on my 61-year-old memory.
ACI 360R-10 §5.2, Paragraph 3, states "The contact area of a single tire can be approximated by dividing the tire load by the tire pressure (Packard 1976)." My statement implied something different. This method works well for slabs on grade, where traffic is typically slow, often infrequent, and impact loads are small to non-existent.
When it comes to street and highway pavements, we (at least in California) switch methods and use long-term ESAL loadings, traffic indexes, etc. and don't worry about tire pressures and load pressures.
Fred
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
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