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Existing Bridge--Steel girders cast directly into concrete piers and abutments

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We have been asked to certify that an existing 50-or-so-year-old bridge can support a "water-tender" for our rural fire department.  The 12-foot wide bridge consists of two steel S-shape beams running longitudinally, set 6'-0" apart and spanning 15 feet max, with an 8-inch thick cast-in-place reinforced concrete deck slab on top of them.  The beams are embedded in the concrete bridge abutments and piers.  The deck is poured around the top flanges of the beams, and also on top of the piers and abutments. 
 
The bridge has held up admirably (I wish all of our infrastructure was this durable);  the steel beams have only slight rust, the concrete deck has only a few BARELY noticeable cracks, very minor scour, etc.   The owner also points out that this bridge served a gold mine for many years and has supported concrete trucks carrying 15 yards of concrete--maybe with the extra 5 yards of concrete in the cab with the driver?--and possibly train locomotives (OK, kidding here...).  Initial calculations show that the bridge is adequate in most respects.
 
I see two problems:
 
    1.  Shear in the deck.    This can be mitigated (with the Fire Department's approval) by installing curbs to keep heavy vehicle tires within "d" of the center of the beams, which thankfully are set at the same spacing as the tire tracking width.   Or:   pour another layer of concrete on top of the existing deck, which would worsen problem 2.
 
    2.  The steel beams bear directly on the concrete at the piers and abutments; indeed they are embedded into them an unknown distance.  My "old" reference (1924 Structural Engineers Handbook, by Ketchum, McGraw Hill) gives an allowable bearing of 600 psi on concrete.  This seems reasonable to me for the existing materials.   If I assume (and then verify) that the beams are embedded fully through the piers, and the full width of the bottom flange is effective in spreading out the beam load (which it is not because of longitudinal flange bending) I still do not have enough bearing area for dead plus live loads. 
 
This would not be such a problem if the beams could simply be jacked up and bearing plates and pads slipped under them.  However, in this case the beams are embedded in the piers AND the deck on top of them is cast with no sign that the deck can be raised up off  the piers and abutments.  Casting new corbels under the beams would not help unless the beam load that is already on the existing piers could be transfered onto the new corbels (maybe by filling between the corbels and the beam with expanding grout?)   I would love to get an elastomeric pad in there, but with the beams "locked" into the piers I don't see how to do so economically.  
 
Another option, if we verify sufficient embedment of the beams into the piers, is to consider that the bearing load is carried by both the top and bottom flanges of the beams, since the concrete piers are poured around the entire beam.
 
Suggestions, thoughts, comments?
 
TIA,
 
Thor Matteson