Photo by Rick Geary

Some bridge inspector has the fun of periodically crawling up through the trap door

Matt: While navigating through myriad “seismic retrofit” hazards on area freeways, I've noticed several trap doors on overpasses, with ladders leading inside. I've always assumed overpasses to be solid concrete. I've heard that in Switzerland, many critical bridges are filled with explosives so they can be blown up in case of invasion. Is Caltrans really concerned with earthquakes or is a New World Order vanguard covertly preparing for the United Nations takeover? What are these hidden chambers, and why are they there? — Joe Cesare, La Jolla

Quite a scenario, Joe. So, say we detect a convoy of L.A. developers heading south on 805, out to capture all our women and buildable real estate. We can light a fuse that’s sticking out somewhere on the Mission Valley bridge and blow ’em back to Burbank? Nice thought, but it doesn’t work that way, here or in Switzerland. One common method of constructing a prestressed concrete bridge is to make it in sections like long, hollow boxes held together with metal joints. Then some bridge inspector has the fun of periodically crawling up through the trap door and walking around in the mouse droppings and dead pigeons to make sure the concrete isn’t rotting and everything’s okay.