Did the math, this seems surprisingly spot on, at least from the energy perspective. 100k tons going 7.5 kts is about 160kg tnt equivalent which is a little less than the explosive power of a Mk83/GBU-32, probably not enough to take down an entire bridge, just one span if you're lucky. That said, the issue is the bomb expends most of its energy moving air around, the freighter expended nearly 100% of its energy on the bridge.
