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wheel load distribution on road plate
a collegue of mine asked me about the proper wheel load distribution for h-20 loading on a temporary "road plate" used to cover a trench.
looking through the aashto code, i didn't find anything applicable to spanning with plate steel. did i miss it somewhere?
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use the simple span distribution of the wheel load if you are concerned with the reactions at each end of the trench , but conservatively in this situation i would assume the load is 1/2 the axle load, say 16 kips.road plate is not typically designed. the distribution of the wheel load is more for stringer and girder design. there is a distribution for flat slabs that are loaded perpendicular to traffic, but this is more for concrete slab bridges.
there are formulas in aashto. one for point loads and one for uniform loads, that will give you the percentage of load that is carried by each direction.
with a: being the short span
b: being the long span
point load short span is b^3/(a^3+b^3)
point load long span is a^3/(a^3+b^3)
some judgement will be required to establish the effective width of plate that carries the wheel. but with the wheel contact area of 20", i would not hesitate to double that width for the effective width of plate.
i would assume that the plate would fail by bending at a line in the middle, and solve for average moment at that line. treating the wheel load as a point at the center of the plate would be conservative. you could assume pressure approximately equal to tire pressure to spread it out over a larger area (75-100 psi for large trucks).
is that aashto distribution above based on plate supported on two sides or is that for support on four sides?
the distribution formulas are for two way action.
seems like the load carried in the transverse direction would be zero if the the sides of the plate are unsupported. there's no place to carry that load to.
thanks for all of the comments. it's a continuous trench for practical purposes, so the sides are unsupported. i would have thought double the wheel contact area reasonable, but was looking for something to back it up with.

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