huangyhg

influence of footing
there is a box culvert over which i am designing a one-way slab to take pedestrian loading (85 psf). i am also designing strip footings to support the slab. however, i'd like to take the footings to the bottom of the box culvert so that the footings do not impart any loads to the box culvert. am i doing this right?
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that would be preferred, and the simplest route to take as you then don't have to worry about impact on the culvert walls.
if the footings are shallower such that they influence the culvert walls, it may still work, you just need to check the additional load on the walls.

is it feasible bear the slab or short wall on the wall of the box culvert? how does this affect the soil pressures under the culvert?
the other route i am thinking of is to just place a slab on grade and not worry about the footings, as i imagine under-ground box culverts have to be designed for any earth fill or even traffic. i do not know if i can count on that. probably, what i need is to someone tell me what these box culverts are typically designed for.
you haven't indicated who owns this box culvert (federal, state, county, private). this would most likely be built to their design standards although they may be the same or similar. each state dot has there own design requirements so you may need to get a copy of their design manual.
in 10 years time they may need to repair this box culvert (or whatever), you need to make sure that you building will remain stable if it is removed.
if it is not too deep then take a 45 degree line of influence either side and design the footing to span between these lines of influence. check the bearing at each end.
other option is to put a pile/pier/caisson either side of the box culvert and design the footing to span between these and cantilever out to the line of influence.
as jike has mentioned, there may be other criteria (such as clearances) that are required by the local authority and these should be available in a design guide.
you may also need to design the floor to span over as well.
you didn't say if it was actually a building or just a sidewalk type of pedestrian slab. if there is a building involved, i'd want the footings low enough so any work on the box would not undermine the footings. actually i always coordinate footing elevations with buried piping elevations for just that reason. it's unsettling to see what plumbers do to the soil around footings during construction, so my footings are always below piping.
if you are just talking about a slab, the box must have been designed for some fill and live loads above it, so perhaps slab on grade is not unreasonable, but a bit of inquiring into what the box design was based on might be helpful.

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