Equivilent strip width and AASHTO

[TehMightyEngineer]

So, I've come up a discrepancy in the wording of the AASHTO 5th edition Bridge Design Specification. I'm designing a drainage box culvert with 125% HL-93 loading and have typically used the AASHTO equivalent strip width as the equivalent strip under each wheel (based on the similarities between AASHTO and the original Westergaard equation for a point load on a slab).

However, I noticed today that in Article 4.6.2.1.3 (and especially the commentary) all talk about the strip in terms of "axle load" rather than "wheel load". Now, this would make sense for a deck running transverse to the direction of travel (2 wheels in line on one strip) but for a deck spanning parallel to the direction of travel I would expect there to be two strips (assuming sufficient width between wheels so that there is no overlap) with one wheel load per each. Thus, by that logic am I wrong in taking 2 times the strip width from Table 4.6.2.1.3-1?

I suspect that I'm not wrong as 4.6.2.1.5 - Distribution of Wheel Loads talks about strip widths in terms of "wheel loads" as I would expect it to.

EIT with BS in Civil/Structural engineering.

 

[crossframe]

I would halve the axle load before I would double the strip width.

If there's sufficient cover, the pressure areas may overlap anyway.

If there's no cover (or less than 2 feet) I would use a wheel load (not an axle load) for the unit strip.

Make sure you check the tandem.

 

[TehMightyEngineer]

Yeah, halve the axle load (to get a wheel load) and using 2 strips is what I meant I would be doing.

I shall check the tandem as well as a unique local loading for overloaded logging trucks.

EIT with BS in Civil/Structural engineering.