May 23, 2008

New cables help bridges ride out earthquakes

Videos of the damage caused by natural disasters — especially earthquakes — are among the most riveting images seen on television.

The immense damage shown in the pictures of the recent China quake make them the defining images of earthquakes all over the world.

Although the China quake resulted in more damage to buildings than to other structures, there were photos of collapsed bridges that looked much like the bridges that fell in places like Kobe, Japan, and Oakland, Calif., in the last decade or two.

In each case, we saw that whole sections of bridge deck had slipped from their supports, crashing down like a deck of playing cards tossed aside by a child. Wouldn’t it be nice if they could have been secured by cables of some “smart” material that stretches during a quake, then snaps the sections back into place when the shaking stops?

The problem is that concrete-and-steel decks can become unseated from the supports they rest on during a quake. Engineers have tried to solve the problem using steel cables to tie the deck sections to the bridge supports, but that can transmit damaging forces to the supports. Such ties can also stretch during a quake, and need to be replaced afterwards.

But now, in an era when materials science is growing by leaps and bounds, we have something called shape memory alloys, or SMAs, that appear to hold out the promise of bridge decks that snap back into position after an earthquake.

The special properties of SMAs were first observed in 1938, but it wasn’t until the 1960s that serious research resulted in a number of applications — including use in the aerospace industry, and in robotics.

It turns out that there could be a construction application for them. A research team led by Reginald DesRoches of Georgia Institute of Technology, has demonstrated (using a huge shake table) a system using SMAs as an alternative to conventional steel cable ties.

“They can be displaced by a large amount and spring back like a rubber band, without any damage,” DesRoches says. That allows bridges to “give” at their deck joints during a quake, then snap the decks back onto their supports when the quake ends.

Testing showed that the SMA restrainer cables had “minimal residual strain after repeated loading” and were able “to undergo many cycles with little strength and stiffness degradation.”

Half a dozen alloys have been developed to “remember” shapes. The cables used by DesRoches and his associates are made of nitinol, the generic name given to an alloy of nickel and titanium.

In a paper on their research, DesRoches and his team noted that tests showed a deck displacement of 2.5 centimetres, compared with 10 cm for conventional steel equivalents, enough to keep deck sections seated without either stressing the bridge supports or damaging the cables.

Sounds good, but there are hitches. First of all, fitting nitinol cables to existing bridges would be expensive, although it might make economic sense if used on particularly strategic bridges. And there is still no full understanding of the detailed properties of SMAs and how they might respond to the random force patterns of a particular quake.

So don’t expect SMA cables to be installed in a bridge near you anytime soon. Still, it’s an emerging technology that bridge engineers might like to know about.

The research was funded by the California Department of Transportation’s Office of Earthquake Engineering. The paper, Large-scale testing of nitinol shape memory alloy devices for retrofitting of bridges, will be published next month in the journal Smart Materials and Structures (vol. 17, p.35018).

Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@dailycommercialnews.com

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