Yes, thank you M.Harris for the excellent explanation. It is important to note that the truck involved in the Skagit River bridge collapse was not overweight, but only oversize.
Regarding my post explaining maximum truck gross weights, the 4 and 5 axle container chassis my employer uses can be extended (" stretched " ) to a maximum of 53 feet overall length from kingpin to rear bumper. This stretch puts the container in the middle of the chassis and distributes the load over a greater portion of roadway or bridge. Just like spreading the bricks out along the 2 x 4 as opposed to stacking them all in the middle.
A question for M.Harris : What safety factor ( over-gross weight allowed ) would have been designed into a highway bridge in the mid 1950's ??? And how about today's safety standards ???