According to AASHTO 9th 3.6.1.2 determining the live load patch width over a culvert, additional width is given based on the clear span of the culvert.
ww = wt/12 +LLDF(H)+0.06Di/12 (3.6.1.2.6b-2)
Why is this .06Di included in this formula? It doesn't seem like the structure the wheel load...