“Fracture critical bridges,” b.s...., those talking and writing heads don’t have the vaguest idea what they are talking about, but using big words and phrases impresses the reader/listener. They are parroting something they have heard with little to no knowledge of the facts of the matter. I must confess, I’ve not seen either of these bridges, and didn’t witness the accidents either. But, I’ve inspected a few bridges after the fact, and been involved in HW&H shipments by truck and rail over the years. I am not trying to downplay the importance of redundancy in structures or the fracture critical nature of some structures. But, we have never designed highway bridges to have primary structural members hit/impacted/ripped out by a 30-40 ton tractor-trailer with a high load and not be seriously, maybe critically damaged. And, we don’t design multi-track railroad underpass bridges to withstand the impact of a fast moving train, or the derailment and collision of two trains, right under the bridge, and not be seriously damaged, maybe moving a pier top a few inches. If we don’t have enough money now to manage, maintain and renew our bridge infrastructure, just wait until we start designing and building for these slim possibilities; we won’t have enough material or money for one bridge.
The likelihood of the collision/derailment right under a highway bridge, as happened in MO, is really pretty slim when you consider ton/miles or track miles of railroad transport activity. This isn’t something we can practically or economically design against, we can’t afford to do that. We haul HW&H (high, wide and heavy) loads over the highways every day. It takes some clearance, structure load cap’y, and actual movement planning and study to do it right. Not just a car with blinking lights in front of the truck. Railroads and highway depts. have people who do this on a regular basis. If the state gave the trucking company a permit for 15.75', they would normally give some advice about critical clearance locations below that elev., but with no guarantees. That trucker should have had a route/trip plan which said stay in the middle lane at the Skagit River bridge, 17' clr. for a 15.75' load, is no biggy, and he should not have been going 40-60mph crossing the bridge either. That wasn’t even a particularly high or heavy load, was it? Someplace, I thought I saw a video, maybe only a recreation, showing the truck hitting the bridge, and continuing on, and the bridge falling seconds later. It seems that we would be wise to let these things develop a bit, before we pass final judgement. As knowledgeable engineers, we might do what we can to prevent (or correct) the talking heads from pushing the negative and spectacular for the sake of air time. And, we should use a bit of common sense about how we approach and analyze these situations, to help minimize the hysteria. And, I certainly don’t mean to downplay our need for infrastructure spending. ASCE has been harping on this for years to essentially no avail. But, we may need more spending on bridges, sewers and water mains, etc., not very glamorous, but essential to our economy and well being, and less on new stadiums and the like.
