Don’t Repeat Same Bridge Mistakes

* Some 40 years ago when I was a brand new MIT graduate, I began working at Lockheed in the structures division. For beginning engineers there was one rule that was strongly impressed on our collective consciousness by the senior engineers. This was so-called Rule No. 2, and it said: “Thou shalt not fail twice!”

And yet Caltrans appears to have allowed exactly that situation to occur with the recent earthquake failures of the bridges at the Golden State and Antelope Valley freeways. These bridges failed the earthquake test in 1971 and now again in 1994. What’s more, these failures look just like those in the 1989 earthquake in San Francisco.

It doesn’t take much of an engineer to appreciate that these failures are all very similar to what happens when a singer sings the right notes to break glasses sitting on a shelf. These bridges all resonate (i.e., bounce up and down) at about two cycles per second. And that is about the same frequency of the earthquake shocks. The man who witnessed failure of the Santa Monica overpass said it bounced up and down.

What it amounts to is that these high-flying long spans are simply dynamically unstable in earthquakes. Put them down low to the ground where you can put in more vertical supports.

THOMAS C. POLLACK

Sun Valley