Yeah, even at relatively slow speeds that sort of mass is a HUGE amount of momentum.
Bear in mind that structures like bridges are, to a point, built to be as light as possible for a given purpose. Yeah ok, there are safety factors involved that increase the bulk somewhat, but generally speaking they're not going to be using more material than legislation requires them to. As such, you have a structure built to do the job for normal use, but external factors like A MASSIVE LADEN SHIP DRIVING STRAIGHT INTO IT tend to be outside of the design parameters.
It's hard to imagine quite how much force there was from what is a seemingly slow crash, but that mass is the critical part. I've seen a figure of approx 8kts impact speed, which is around 15kph. Using the 120,000,000kg you mention, Mike, that's like a fully laden artic lorry crashing into the support at something like 38,000kph. That's faster than the escape velocity of the Space Shuttle, and about twice the orbit velocity of the ISS for some context.