This would be difficult to say in most cases due to the way bridges are designed.
AASHTO has basic guidelines based on the traffic level of service and site conditions. Depending on who has jurisdiction over that bridge can further increase the amount of loads on the bridge.
The engineer(s) then determine the weight of the bridge itself along with environmental factors (soil conditions, wind loads, earthquakes,…), and the design the bridge to withstand all of these plus an additional factor of safety.
The result of all of this is that any given bridge should be able to take an 80,000lb trailer (about 8′x45′) and load the bridge with these; you should have a bridge that can take several times that under “bad” conditions.
Just as a parting thought. It is rarely the weight bearing capacity of a bridge that causes it to fail, and thusly tends to be one of the strongest capacities of a bridge.
MKDesign 