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Read An Article From Civil Engineering About LUDs

 

 

 

email: adeng@colebrand.com

CIVIL ENGINEERING,
October 1997
ENGLISH LOCK-UP DEVICES MAY SOLVE RETROFIT RIDDLE

Lock-up devices conform to the gradual movements of a bridge but remain rigid during seismic activity.

A series of severe earthquakes in the past few years has prompted transportation agencies throughout the U.S. to bring their bridges up to current seismic standards. Just as many of these cash-strapped departments are realizing the staggering cost of wide-scale retrofits, an overseas manufacturer is pushing to get its new breed of lockup devices (LUDS), popular in Europe and Canada, noticed by American engineers.

The maintenance-free LUDS, manufactured by CI, Ltd, London, provide a temporary yet rigid link between a bridge's deck and its supporting abutments or piers to offer sound seismic protection. The devices work by means of a piston-and-rod configuration housed inside a compound-filled noncorrosive cylinder. During slow movements such as thermal expansion or contraction, the liquefied compound gradually migrates from one side of the piston to the other, allowing the piston and cylinder to expand or contract with the structure. But during sudden applied loads, when the compound doesn't have time to migrate, the piston and cylinder remain in a locked position.

Although U.S. engineers and transportation agencies have yet to adopt LUD technology into their seismic standards, several recent cases in the U.K and Canada have demonstrated the devices' value for retrofit projects. One such example was the recent seismic retrofit of Vancouver Airport Authority's Arthur Laing Bridge in both the transverse and longitudinal directions.

To upgrade the 22-span concrete box girder bridge, engineers installed a series of shear pins and LUDS to transmit the inertia forces of the superstructure to the piers. While the pins were able to transmit the bridge's shear forces, they could not adequately accommodate the structure's longitudinal movements resulting from temperature and creep. To correct the deficiency, project engineer Sandwell, Inc., Vancouver, included 11 CI LUD devices, ranging in size from 700 to 2,100 kN depending on their location, into the design. Crews installed the units at the bridge's critical expansion joints by anchoring them to the girder and the pier caps.

"We found that the LUDS were ideal for installation at piers with sliding bearings," says Richard Reynolds, Sandwell's project manager. "With the LUDS installed, the piers made a bigger contribution toward the resistance of the superstructure's longitudinal seismic shears. This allowed us to avoid an expensive retrofit of pier columns yet still maintain the existing behavior of the bridge."