huangyhg

residential driveway bridge
i am designing a residence to fit a very steep hillside lot. the garage requires an elevated floor, and the driveway is a bridge from the street to the garage. bridges are outside my design experience so i will appreciate comments/criticism of this approach. i cannot find requirements in any code. the driveway dimensions are 25' wide x 36' long from street to garage. i propose a load of a medium sized truck gwv 12,500 lbs. in parallel with a suv (hummer) gwv 10,000 lbs. i have arrived at a composite bridge deck of 5" concrete depth over 2"x6" 18 gage corrugated steel, with wire screen and no.4 rebar for crack prevention. corrugations will run the width direction, all supported by timber substructure. does all that sound reasonable? thanks for any response.
my philosophy is to design for a heavy truck load unless it is physically impossible (due to low headroom, etc.) for a big truck to get there. can you be sure that a dump truck or fire truck will never back into the driveway?
i would recommend designing for at least the ibc live load (category 32: sidewalks, vehicular driveways and yards, subject to trucking) of 250 psf or 8,000 lb point load. depending on the project, i might bump up the point load to 50,000 lb if dump trucks or fire trucks are a concern.
someday, somebody will want to place a prefabricated shed (which weighs almost nothing) in the backyard, and to put it there, they will park a truck crane (which weighs a lot) in the driveway.
mike halloran
pembroke pines, fl, usa
i believe you will need to talk with your insurance provider before making a decision on the required design load.
there are more design problems to be solved than the reinforcement area and slab thickness. handrails will be one of them.
good luck
someone will also want to pour some concrete somewhere along the way and want to drive the concrete truck over the driveway.
there are a number of companies that sell prefabbed bridges that can meet all codes. simple web search will find them.
as always - when out of your area of expertise -- you might want to hire a pro....
yikes - i don't know about using a composite metal deck in the outdoors where moisture will continuously hammer at the bond between deck and concrete. i can see using the deck as a permanent form, and then reinforcing the slab as a traditional reinf. concrete system. you didn't say the spacing of the longitudinal timber beams below (i assume they are also pressure treated). depending on the span, it wouldn't take an enormous amount of rebar in a 5" slab thickness.
some states allow metal deck under slabs, some do not. it depands on the area. any salt used in the area? we never (almost never) support concrete on wood or timber. creep in the timber will cause cracking in the concrete. aashto has bridge design codes. they have one for bike path and pedestrian walkway bridges that uses an hs10 loading, one half of the more standard hs20 loading. the load on pedestrian bridges depends on the width.
my first thought would be to use hollow core precast slabs such as flexicore to span from the street to the garage. they are fairly light and require no forming. many precasters make hollow core deck beams for bridges that will carry much heavier loads if required. is that an option?
i agree that designing the driveway to carry highway loads is a good idea. if you want to get an idea of the magniture and how loads are applied for bridge design see chapter 6 of this document
i would tend to agree with those who would design this as a highway bridge, even if the modern highway design loading is not used. h20 sounds reasonable. if h20 is out of an owner's budget, i always recommend going with at least "h12", that is to say a 12 ton design vehicle in the h 20 configuration. that at least keeps you safe for a big chunk of the kinds of non-construction vehicle deliveries that a residence is likely to see.
i also express my reservation about using the pans as permanent support, if that's what you were doing. one thought would be to use heavier corrugated metal (like 3 or 5 gage) for the deck. decking of this kind is used a lot by local governments and private owners on low volume applications, and is usually filled with bituminous instead of concrete.
in any case, i would strongly recommend that the bridge weight limit be posted, even if the insurance company doesn't require it, although i generally find that they do.
perhaps you can use an overhead canopy that limits the height of a vehicle, like those at parking garages. it would be cheaper than a bridge with hs20 loads. unless you're an experienced bridge designer, i would go with the prefabricated bridge.

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