Notice the top of the beam? Looks like it was cut, then the rest looks like a normal fracture. 
No.
The loading on that beam is primarily tension from axial loading, and upwards bending, meaning the beam curves up under load (I mean like a smile, or a "u"). It is a bridge, that's what they do.
So what certainly happened is that a flaw on the bottom-most fibers occurred. This would be a micro-crack. Could even have been an in-spec manufacturer's flaw. Then the cycles on the bridged expand and contract on the flaw, and the crack grows over time.
The bridge is decades old, and every car and truck, every single one, will cause that beam to flex, just like every other beam on the bridge.
And so the crack grows, following the weakest path through the grain of the metal, as can be seen zig-zagging up from the bottom. The remaining intact section has to carry the full load on the beam and the stress rises. Past a certain point, there is no longer enough section to carry the total stress and it just unzippers.
And that's what you see. About 3/4 up from the bottom, the crack stops zigging and just goes straight to the top.
A classic fatigue failure from the looks of it. Can't know for certain until the beam is cut and we can inspect the surfaces.
But I spent four years performing fatigue tests on helicopter rotors and that there is a fatigue crack.
Thing thing that should be not that the bridge was cracked. That is going to happen, it's what metal, even iron, does, and it's a fact of life. This is why bridges are inspected.
But look at the rust. How long has the beam been open to the weather? It's apparently evenly rusted, top and bottom.
When was it last inspected and, since we can determine the age of the crack, if it was missed earlier, why and what needs to be done in the future?
First thing that needs doing is to have Congress define "infrastructure" as "bridges, roads, pipelines, power grids, water distribution, and other MATERIAL articles of public use". "Knee Grows" are not infrastructure.
