Ask the Architect

A pedestrian bridge collapses

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Q. What could cause the bridge to collapse in Miami last week? Aren’t there all kinds of precautions to avoid this kind of thing?

A. There was a highway safety campaign slogan I remember hearing all the time when I was a kid: “Speed kills.” It was on billboards and in TV commercials. It may be an appropriate campaign to bring back, since it applies to so many things in life. Building systems made of concrete and steel have many issues to be dealt with: the strength of materials, the size, temperature, order of application and following exact planning.

My first observation upon viewing the bridge collapse at Florida International University was the horror of seeing crushed cars, knowing there were people in there, people who kissed loved ones goodbye to head off to work or school and would never come home again. Then I looked at the configuration of the diagonals that form the huge truss designed to span the road. I saw, in later reports, that the reason they weren’t symmetric was because they were to be attached asymmetrically with cables to an off-center tower above, which was not yet in place. Then I noticed that the concrete of the lower bridge level wasn’t cracked like an eggshell, but was pulverized into many smaller pieces.

In bits and pieces, just like the concrete, I began hearing more, along with other viewers. The bridge was being “post-tensioned,” a process whereby cables running internally through tubes embedded in the concrete, parallel to the length of the concrete bridge, were being pulled tighter and tighter, which would cause the bridge to straighten up from any sag developing under its own weight. Many parking garages are built this way. It isn’t unusual, but probably not well understood by the media or the general public.

My observations, after 42 years of analyzing building disasters, are that several factors will need to be studied. The bridge components will literally be looked at under a microscope. The concrete will be “cylinder tested” by cutting round core samples and comparing the result with the cylinders, sampled during the forming of the concrete, by placing them in a crushing machine. The concrete may not have cured completely.

Steel tension cables will be examined and stretched. Steel can distort and/or tear apart. X-rays will be used. The procedure for temporarily supporting or not supporting the bridge before the tower and cables were connected above it will be examined. The engineer’s calculation sheets, the weather, temperature and humidity, moisture content, air speed and direction … it will be an intense scientific study. My general observation is that people assume things, and time plays a key role in construction. The quicker the project, the faster people in the field make snap decisions without question, in the name of money, and the more problems that can develop. Most of the time speed only causes damage. This time speed killed.

© 2018 Monte Leeper. Readers are encouraged to send questions to yourhousedr@aol.com, with “Herald question” in the subject line, or to Herald Homes, 2 Endo Blvd., Garden City, NY 11530, Attn: Monte Leeper, architect.