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Dear Matthew Alice:

I work in Mission Valley, and I spend a lot of time waiting at red lights on top of the bridge that crosses Interstate 8 at Mission Center Road and Auto Circle. When trucks use the bridge there's a whole lot of shaking going on. Was that bridge designed to be extra flexible or something?

-- Mark Brocklehurst, idling on the bridge

According to Caltrans, shaky bridges get a lot of attention from the motoring public. We call them a lot asking what the story is. Since most of us jet through life pursued by secret fears of imminent doom, a big wobbly concrete thing does take on some significance. But I'm glad to say, a shaky bridge is a safe bridge. With no flexibility in the structure, it would eventually snap like a breadstick or crumble like a Lorna Doone shortbread cookie. If you want a true hair-raising experience, try staring out a window on the 60th, 70th floor of a city office building and watch the thing sway in the wind. A Dramamine moment.

The bridge at Mission Center Road is no more or less flexible than any other. You notice it more because (1) it does carry a lot of truck traffic that causes big vibrations and (2) you happen to sit on that particular bridge for a long time trying to occupy your mind with something (like, exactly how many people would show up at my funeral, anyway?). Bridge decks are designed in sections held together with strips in a sort of loose tongue-and-groove pattern to aid flexibility and expansion from heat. Unfortunately, one of the lessons learned from the Loma Prieta earthquake in San Francisco was, if you shake a deck hard enough horizontally, the joints pull apart and the deck falls down. The first stage of the state's earthquake retrofit project was to add restraint cables to the decks to limit the horizontal movement and keep the joints intact.

Does that make you feel better? Yeah, well, maybe not. Sometimes knowing the facts only helps us worry in clear, terrifying, undeniable detail. So forget I said anything.

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