Culvert footing design question - LRFD

[leighcheri]

I'm designing cast in place footings for a precast 3-sided culvert using LRFD and am looking for a design example.  I want to include live load in the vertical stress calculation but I'm not sure if I should distribute it over 1 ft "strips" and apply the Strength I factor of 1.75 or if I should estimate it as a surcharge and apply the EV factor of 1.35 (AASHTO LRFD Figure C11.5.5-1).

I'm using HS-20 loading and the soil engineer gave me the ultimate bearing capacity of the foundation soil with a resistance factor of 0.45.

 

[engineertipsman]

I would distribute over 1' strips, with the 1.75 factor. Is this a buried structure?