Elevated Residential Driveway 1

[jdgengineer]

Wondering if any of you have designed relatively small elevated driveways connecting the street to a residence on a steep slope. I am looking at a project that may involve constructing one of these and I'm not sure I have run across it before so I thought I would get some input. The bridge is located over a steep hillside but I believe we should be able to support it along it's length (likely with drilled piers). In the neighborhood (constructed mostly in the 70s I believe) these appear to be wood framing with a thin slab poured on top. See below for some pictures.

It seems to me an elevated structural slab supported on concrete piers would be a good approach, but figured I would check in to see what else has been done. We do not have snow loads, but do have heavy seismic loads. I'm a bit concerned about a hard link from the street to the house with the bridge. Seems like we may want to get an expansion joint on one side.

 

[STrctPono]

I'll take a crack. I've never designed a driveway bridge before but I do design bridges.

A couple of thoughts. Take them for what they are worth.

1. I don't like the idea of pouring concrete on wood. I've never done it and I probably never will. So many reasons why not. If possible to avoid, wood should not be in contact with concrete. Wood is way too flexible to support concrete. Concrete is gonna crack like hell. People are cheap though and prefer wood cause it's cheap.

2. It's weird to see the driveways in your photos have joints in them. I can't seem to figure out why they have joints. Maybe they have lots of supports underneath and it's not really a bridge.

3. I would go with a CIP structural concrete slab. You could also go with some steel beams and a thinner concrete deck on top.

4. If the ground is sloping from the street to the house, you could probably go with some micropiles and some shallow pile cap. Set your superstructure on that and joint it.

5. See attached sketch for some ideas. Not saying it's correct, but something I would consider. Many times, this stuff isn't governed by what's right but rather what is standard practice of the Contractor's in your area.

 

[Phil1934]

Consider a couple concrete double t's for a driveway bridge. Will span the distance between abutments.

 

[Bill West]

I've seen many of these, mostly wood span to a wood garage. Many go back to 1930 summer cottages. The one lay observation I can offer is to make the design structurally flexible. After a decade or two all four corners are likely settle down or up 3-4" or more and not in synch with each other either. If the maintenance for this is nothing more than redoing a fill strip (asphalt?) the customer would probably consider it to be a smart piece of design. I guess one virtue of the wood approach is its ability to twist. A concrete finish of course is still a good idea for traction.

How about the beam idea from the house but with the deck made of 3 or 4 separate slabs? An abutment at the street could provide a seat for the beams to slide in/out a bit as you noted. An abutment might cooperate with street settlement too. A drain grate could be a way to cover the sliding joint. A grate is also very relevant at the low end of the bridge to keep runoff out of the garage.

Bill

 

[phamENG]

I live in the Southeast/Mid-Atlantic, so I've certainly never done anything like this (think pancake flat), but I have a couple ideas.

I saw a book once that was published in the late 70s or early 80s with a bunch of typical wood construction details and example calculations. One section was for elevated garages. All the details show wood framing with concrete poured on top. The idea is that the concrete is a non-structural, protective surface. They would typically be designed for 40psf and a point load requirement commensurate with typical wheel loadings. Throw on a few inches of concrete, and you have a heavy but certainly feasible load for the wood to carry all by itself. I think that explains the joints and relative lack of visible cracking in the pictures.

To do it today, I agree with staying away from the wood on concrete. Do either or. Cast in place concrete beams and elevated slab would be one option. Though with that I imagine you'd need to add supports adjacent to the house - wouldn't do to bear the concrete beams on or in the wood walls. That would solve the seismic separation with a joint at the house. Or...do treated glulam beams and heavy timber deck. Girders spanning from the house to the street would extend up a foot or so above the deck with beams/joists running perpendicular to the bridge span, and then put in 2" or 3" tongue and grove heavy timber deck. It would look really sharp. That can be connected to the house, and the joint would have to be on some sort of elastomeric bearing pad on an abutment at the street (bridge guys can tell you more about those).

 

[ldeem]

I would be leery of a RC slab if you don't have quality control over the contractor. If they hire a typical residential contractor the contractor may not understand the importance of maintaining rebar locations. I once had a SOG built where I wanted rebar placed on stands. I provided a drawing for the contactor to bid on and told him what I wanted. They ended up putting the rebar in and reaching into the wet concrete and pulling up into position. It worked out (I moved a year or so later) but ever since then I have tried to consider the quality and experience of potential contactors in my designs.

 

[oldestguy]

Hey you fussy concrete guys. I well remember in college structural reinforced concrete prof (from Germany to US) with a lot of experience. He commented on jobs he has seen where the re-bars might even have been in the wrong face, yet they held up. I suppose the story is "don't get too worried about it on this job. Could be a lot worse".

 

[jdgengineer]

Thanks guy. I appreciate all the input. Given the architect is somewhat unknown to us, we've decided to walk away from the project. I appreciate all the discussion and ideas!

 

[XR250]

They ended up putting the rebar in and reaching into the wet concrete and pulling up into position.

Have never seen that with rebar but it is a common practice here with WWF.