A Toast to Technology FusionJune 30, 2006 By: Stephanie vL Henkel, Sensors
Back when I lived in a very tall post-and-beam house in the New Hampshire woods, wind from the west would come zipping off the lake a few yards away and that house would hit what felt like hull speed. It vibrated and it hummed. If I'd had some accelerometers and data loggers I could have determined what my home's hull speed was. Now there's a far fancier way to find out what structures are doing.
Tracking Deflections from Space
Today, I'd like to call your attention to our April cover story, "When Bridges Move: GPS-Based Deflection Monitoring." The technology described in that article is particularly slick because the behavior of Scotland's Forth Road Bridge is being evaluated from 20,000 kilometers on high. The principle could be put to work on other structures too.
John Hancock Does the Twist
That bridge story reminded me of the Famous Gyrating Hancock Tower, built in downtown Boston, Massachusetts, in 1976. A skinny parallelogram in design, the outside of this 60-floor edifice was mostly glass, with 10,000 immense panes in all. Before the tower was finished, that glass began to pop out, intact, and hurtle down onto the streets. Big mystery at the time, and even bigger embarrassment for the architect and builders. Turned out that winds through Boston's urban canyons just shook the panes free. The solution was to install on each window a sensor that reports to a central control room to advise of excessive vibrations which might cause a repeat performance.
The Thrashing Bridge
The Tacoma Narrows Bridge out in Washington State was built in 1940 and self-destructed the same year a few months after it was opened to traffic. Its demise was perhaps the most dramatic of all such structures (save for those deliberately taken down with explosives). Is there anyone who hasn't seen the footage that some amateur cinematographer fortuitously shot with a 16-milimeter camera? Without that documentation, who would have believed even eye witnesses to the catastrophe? Man, that bridge twitched in every known direction and in several that haven't yet been named. Wind-induced vibrations were eventually declared the culprit.
So what does all the foregoing have to do with sensors? Plenty. But let's back up a moment. During the initial site planning for any major construction project, call on the CAD experts who will, at least in theory, avail themselves of data from local weather-watchers. As the structure goes up, incorporate the appropriate sensors that will monitor vibration, shock (especially in earthquake zones), tilt, subsidence, deflections, and the like, and relay their findings wirelessly to an intelligent monitoring station. Because tall buildings and bridges are intended to sway and twist to some degree, some of those sensors could feed themselves off that kinetic energy.
Finally, instrument the finished structure so that it can be interrogated by those friendly satellites that watch over the Forth Road Bridge.
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