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."
