Waiting on my flight so I can get home from the steel bridge symposium and I was thinking about some of the topics that were covered.  Has anyone in the US ever designed one or know any that exist in the states? I thought it was an interesting concept however I don't think aashto addresses them. The speaker made it sound like it was more prevalent in Europe with their code. I plan on doing some research on them once I get to a computer, I just thought it was a very interesting idea. I know it won't ever happen in my state but just wanted to know if anyone else has had any experience with them, good or bad.

By double composite I'm referring to box girders that have a composite topping and a composite slab poured inside the tub on the bottom flange in negative moment regions.

The concept is interesting and definitely doable.
In the US I do not think that such structure was designed or erected.
Although this concept will definitely reduce the steel usage by 10-15%, it will be also more labor intensive, which will reduce the savings.

The concept also have some disadvantages, as the concrete slabs casted directly on steel plates will need dedicated drainage to prevent water from ponding on the plates below. But with some details worked out it is an challenging design.

The idea is indeed quite simple, but very effective. Basically one mainly uses the materials at their best: steel for traction, concrete for compression. It is also very useful to reduce local buckling on the compressed steel plate.

I've designed several bridges with this technique (outside US, mainly Europe). Regarding the extra labor, it is true, it requires some extra work: placement of a simple reinforcement and the pouring itself. As the steel box plays as formwork, the additional work usually is not an issue, since it is only a few hours.

Regarding the drainage, all the bridges I've designed with this technique were sealed (and tested) so, this was not an issue, either. If it is not sealed, I think it is possible to avoid the problem with an appropiate design of the steel box.

I hope it was useful.

regards