Friday, August 3, 2007

The physics of jackhammers and bridges

Jackhammer resonance may be a plausible suspect in the Minnesota bridge failure.

We know that the Twin Cities interstate bridge was structurally deficient but not considered highly unsafe.

The National Transportation Safety Board says it will take about a year to publish its analysis of the collapse. In the meantime, here is something reporters might want to ask experts, such as engineers and physicists: Is it plausible that, in a freak confluence of circumstances, a jackhammer set off resonance vibrations in the structure that led to a sudden critical weakening of a truss?

Slow-moving bumper-to-bumper traffic was stressing the bridge to the max. The faster traffic moves, the less the load per square foot.
However, despite substandard ratings, there had been no indication that the bridge could not support such a load.

But the combination of maximum load, substandard condition and vibrations set off by the jackhammer might well have triggered breakage of a joint or truss.

A witness told MSNBC of hearing the pneumatic hammer shortly before collapse. Construction crews were on the span doing deck repaving at the time. The construction company, which was not touching structural elements, would have had no reason not to proceed with work.

When a jackhammer is used on a ground-based roadway, it sets off minor seismic waves. But the earth is so immense and dense that the waves are rapidly dissipated. However, on a bridge, the waves first have to travel through the steel structure to reach earth.
Hence an undulating motion must occur, though in most cases one would expect the waves to dissipate harmlessly. But, in a freak choice of location, the jackhammer could set off waves that set up a near resonance with the bridge span or structural part.

Additionally, the waves travel different paths through the steel support system, meaning that it is possible that most or all might recombine at some critical point, giving a laser-like energy burst. Some waves, rather than fanning out and weakening, would travel in a near linear path, preserving their strength. And there is always the possibility that, after taking different paths, they merge in phase at full strength.

In the early 1940s the new Tacoma Narrows Bridge collapsed during moderate winds. Photos showed that the bridge span twisted at a near resonant frequency. Since then, no engineer has failed to take into account the potential for wind resonance.

But, how many would think of jackhammer resonance? A jackhammer doesn't put out a great deal of energy with respect to the potential energy of the bridge. Yet, the tool is a driver: it keeps adding energy, which could possibly result in the resonance-like focusing of wave energy at a critical stress point.

The jackhammer pumping energy into the bridge from some critical point, and at just the right frequency, is equivalent to someone pushing a child on a swing, giving a shove at regular intervals. It isn't long before the child is swinging at maximum arc and, if the pusher doesn't halt the resonance, an accident will occur.

This just in: "I would be stunned if this didn't have something to do with the construction project," said David Schulz, director of the Infrastructure Technology Institute at Northwestern University. "I think it's a major factor."

Murdoch skirts anti-trust safeguards
The fact that no concerns have been voiced by U.S. antitrust regulators over Murdoch's takeover of Dow Jones and the Wall Street Journal shows either that U.S. trust laws are flawed and need overhaul or that Justice Department enforcement is inadequate.

One consideration should be the globalization of the Murdoch media. The heft of News Corp. overseas is not unrelated to the menace of media cartelism in America, but regulators are not making that connection.

Similarly, one would think that British and Australian regulators would be concerned that the acquisition of Dow Jones poses anti-competition concerns in those countries.

No comments: