Connection of Adjacent Bridge Structures - why?

The details you provide are not that uncommon. And, in fact, many more bridges exist side by side which are exactly identical but rest on seperate yet identical substructures.

As for why, it may be just a simple explanation as the bridge width which is standardized (and there could be several different bridge widths) to accomondate a variety of roadway widths were developed. Hence the slab, design, girder design, and other appertunances are somewhat standard as well. This is my experience with one such state DOT.

Another reason might be construction. With the introduction of paving machines, a contractor is limited in the width of bridge deck he can place without having joints in the bridge decks. Where possible joints are always eliminated.

Just a couple of ideas. [sig][/sig]

I'm not sure of my understanding of your problem, but I guess that you are speaking of structures that were designed and calculated to act as completely separate structures.
This means that there will be relative displacements under load at the joint and you should connect them with a sufficiently soft and/or ductile material, otherwise you could experience cracks (and water infiltrations) in your protective superstructure. The best solution should be to keep the superstructures separate.
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Many structures on the interstate system are like this. One consideration for leaving an open joint was to accomodate concrete shrinkage and temperature forces in the transverse direction. These bridges could get very wide if the deck had no joints and there was a concern that the forces could damage the deck or bearings. Unfortunately the open joints led to probably the more serious problem of deterioration of the beams adjacent to the joint. It is because of this problem that many open joints are now being closed. Perhaps the concern about damaging the deck or bearings wasn't as much of a problem as originally thought.

One consideration would be a break in the load path in the event of impact damage.