Tuesday, August 7, 2007

Most bridges lack dampers

A quick review of relevant Google pages convinces me that the vast majority of highway and train bridges lack dampers for absorbing vibration induced by earthquakes, vehicles, construction activity or other causes. Also, a question arises as to whether bridge repair crews should be required to bring portable dampers to the job site.

A federally financed study found in that computerized "smart dampers" could extend the lives of the nation's decaying highway bridges by many years. But no action seems to have followed that study.

A Taiwanese study pointed out the peril of resonance effects on truss-style train bridges and suggested that dampers would eliminate the dangerous resonance vibrations. Likewise, there is little or nothing on the net about use of such safeguards on U.S. railway bridges.

However, earthquake damage has prompted authorities in California and other quake-prone regions to order the retrofitting of bridges with dampers.

One type of damper is a counterweight that moves opposite the direction of swing and tends to tame the violence of the swinging. This type is found in skyscrapers and would have been in the twin towers, for example. Another type is a viscous or hydraulic damper which works exactly like an automobile shock absorber and is sometimes attached to cables on a suspension bridge or on a stay cable between the deck and the anchorage. (See Friction factor, Bridge Design and Engineering, July 10, 2003, via Google.)

Virtually no cantilever truss bridge -- the type that collapsed in Minnesota -- appears to come with dampers.

When tuned mass dampers are used on bridges, they are generally meant to subdue vibrations of the deck, rather than the structure as a whole, it appears. Of course, such dampers would still tend to curb vibration of the remaining structure if the vibration originated principally on the deck, as presumably occurred in Minnesota.

(For a good glossary of bridge parts, see http://nireland.com/bridgeman/Dictionary.htm )

A federally financed study published in 1999 by a University of Oklahoma team found that computerized "smart dampers" reduced maximum stress on highway bridges by as much as 65 percent in their computer simulation. The authors of the report, W.N. Patten, J. Sun and Annie Zeng, argued that such dampers could extend the lives of America's highway spans by many years, offering an intermediate alternative to the immediate replacement of the many substandard structures.

A Taiwanese study reports that though "steel truss bridges possess the advantages of light weight, high strength and ease of construction," a design problem is that high-speed trains may set off dangerous "multi-resonant peaks" of vibrations. That is, the bridge shakes in tune with its natural frequency, and may collapse.

Jong-Dar Yau, a construction technologist at Tamkang University, said his computerized model showed that tuned mass dampers effectively suppress these resonances during shaking from high-speed trains.

The tuning of a mass damper means to set it to absorb vibrations at specified frequencies. So one consideration might be whether in future construction crews bring mass dampers with them to offset construction vibration.

Regarding the resonance conjecture
I had forgotten the fact that soldiers are often ordered to break cadence when crossing bridges in order to avoid setting off resonance vibrations. We also know that the Millenium Bridge, a footbridge in London, had to be retrofitted with dampers after people walking on the bridge tended to match strides, triggering a resonance wobble.

Bizarre blackout
A spot check shows no further media interest in the matter of vibration or wobble and no interest at all in the matter of the use of dampers in order to safeguard lives. Either the search engines are being jiggered while I use them -- a distinct possibility -- or the word is out to avoid issues prominently mentioned here -- also a distinct possibility.

Aug. 10 2007.Yesterday's N.Y. Times had a story on a conjecture that a truss connector plate was the culprit. The story also acknowledged that some experts viewed jackhammer effects as a potential trigger.

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