Effect of Culvert Span on Wheel Load Distribution - AASHTO LRFD

[espi_2000]

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.

w_w = w_t/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 is acting on should affect its distribution through fill.

 

[BridgeSmith]

You'd probably have to look up the reference document the spec provisions are based on (Poulos and Davis (1974), NAVFAC DM-7.1 (1982) ) to know for sure, but I suspect it has to do with how the soil-structure interaction is affected by the greater flexibility of longer-span top slabs. If I understand it correctly, more flexible buried structures allow for more arching action of the soil, which results in a wider distribution of the load.

It's also possible they're including an increase in the width that accounts for distribution of load within the concrete slab.