Using a complex array of more than 300 sensors, engineers
say they can now remotely monitor the structural health of the new I-35W bridge
in Minneapolis.
The
concrete bridge, which replaces the steel spans of the old bridge that
collapsed and killed 13 people in 2007, incorporates sensors in the foundation
elements, sub-structure columns, main span, box girders, expansion joints, bridge
deck and bearings.
The
bridge's engineers, Figg Engineering
Group, Inc., say they outfitted the new structure with the sensor system
because they wanted to monitor its "health" and because they wanted to participate
in advanced engineering research on the new structure with the University of Minnesota's Institute of
Technology.
"It's unusual
to do this on a new bridge, even today," says Alan Phipps, design manager for
the I-35W project at Figg Engineering. "Structural health monitoring systems
are typically applied to older structures."
Figg
outfitted the new bridge with at least six different types of sensors. In all,
the company embedded 323 sensors, including: vibrating wire strain gauges in
the concrete; temperature sensors on the top of the bridge deck and on the
underside of the bridge; accelerometers to measure forces near the center of
each box girder span; long-gauge strain gages in the main span; linear
potentiometers to monitor movement of the expansion joints and bearings and
embedded corrosion sensors to monitor corrosion to the reinforcing bars at
various depths of the concrete.
"Minnesota uses a lot of
salt, so the top 2.5 inches of the wearing surface is intended to be
replaceable," Phipps says. "It's like the shingles on your house; eventually
you have to put a new roof on."
Figg worked
with sub-contractors to outfit the bridge with the sensors. Roctest Telemac provided wire strain gauges,
temperature sensors and corrosion sensors. Accelerometers came from Minnesota Measurement Engineering.
Engineers
can monitor the strains in the bridge in real-time over the Internet. All 323
of the sensors are connected by wire to a central computer, which collects data
and stores it. The sensors made their debut recently when the Department of
Transportation placed eight 25-ton trucks in various patterns atop the bridge
deck and then monitored strains on the central computer.
University of Minnesota engineering professors are said
to be interested in examining the effect of temperature differentials between
the top of the bridge – which can be exposed to sunlight – and the structure's
underside, which is in the shade and faces down at the river below.
"This is just a way of
starting off on the right foot with this new bridge," Phipps said. "It provides
information on how to maintain the bridge, starting with Day One. We think this
kind of structural health monitoring is going to become more and more common in
the future."
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