Viaduct sensors help CTA bridge budget gap

A deteriorated CTA bridge on the Red Line has an important story to tell about the health of the rail system's viaducts, the Chicago Tribune reports. Each day, thousands of CTA elevated trains operate across 564 bridges, many 80-to-100 years old.

The crumbling bridges, some
inspected monthly and others every two years, pose a potential danger to CTA
passengers as well as to motorists and pedestrians who pass under the viaducts.
But the CTA, saddled with a backlog totaling $7 billion in unfunded
capital-improvement needs, can’t afford to replace bridges.

The antiquated bridges have
exceeded their useful life, experts say. They remain open due, in part, to
luck. The bridges were built to carry steam locomotives, which generate approximately
four times the load of a moving CTA rail car, said CTA chief engineer James
Harper.

"We are kind of
benefiting from that a century later," Harper said. "One hundred
years is beyond anyone’s expectation for a bridge structure."

The bridge at Devon Avenue
and Sheridan Road on the Red Line is one of the most deteriorated crossings in
the CTA system. Its arch design columns have lost significant amounts of
concrete, exposing – and in some spots shedding – the reinforced steel below,
CTA engineers observed. Braces providing additional shoring were installed near
the bridge piers late last year to help the structure support the weight of
trains and to withstand a possible hit by errant vehicles on the street.

But CTA officials had
little idea how much strain, if any, the braces were taking off the bridge.

Then CTA president Richard
Rodriguez picked up a newsletter published by the Infrastructure Technology
Institute at Northwestern University. The newsletter carried a story about
Northwestern researchers monitoring concrete highway bridges in Wisconsin using
an automated system that collects and analyzes data. Rodriguez then surprised
Northwestern officials with a phone call.

"It was unusual for
someone to respond to the newsletter," said Joseph Schofer, associate dean
of Northwestern’s Robert R. McCormick School of Engineering and Applied
Science. "But Rodriguez was particularly concerned about the reinforced
concrete structures on the Red Line and he said, ‘Come on down and meet with
us.’"

In July, the Northwestern
team embedded sensors in the Devon-Sheridan bridge to measure how much the
structure bends when trains pass over. That information is automatically
stored, analyzed and transmitted to a Web site that Northwestern and CTA
officials monitor.

The data collected from the
gauges will tell more of a story over time, but officials have learned the
yellow steel supports that were added are not doing a lot of work – a good sign
that means the original structure is tolerating the daily strain of train
traffic.

But given the CTA’s budget
constraints, the "temporary shoring" at Devon-Sheridan and other
locations likely will be in place for many years, officials said. Replacement
dates for the bridges remain indefinite until funding is identified, officials
said.

The Infrastructure
Technology Institute’s around-the-clock monitoring of the Devon-Sheridan bridge
will provide an early warning to changes that could compromise the bridge’s
integrity, Harper said. The monitoring will also provide good clues about the
condition of other similarly constructed bridges and any retrofits installed
there as shoring.

"Given that we don’t
have a lot of money to install sensors everywhere, the Devon project is a great
predictive tool that helps us predict how much life we have left on our
bridges,” Harper said.

CTA officials hope over
time to expand the monitoring to at least some other bridges and to add Web
cameras to create a visual record, including the common problem of trucks
hitting viaduct support columns and taking out pieces of concrete, or getting
wedged under the crossing, which has a clearance of only 12 feet 10 inches.

"The broader purpose
for the monitors is not to tell that a failure is imminent, but to focus on
parts of the structure that are most vulnerable in order to identify developing
problems,” said David Kosnik, a researcher at the institute, which has
installed monitoring devices at more than 80 locations in the U.S.

Widespread use of
monitoring equipment will depend in part on keeping the cost as low as
possible, said Daniel Marron, chief research engineer at the institute.

"We try to use
off-the-shelf components," said Marron, who estimated the Devon-Sheridan
cost about $35,000.

The researchers learned
that a tall truck recently struck the Devon-Sheridan bridge. The truck damaged
instrument wires, knocking out one of the sensors.

Harper said CTA officials
plan to install cameras at the bridge to chronicle such mishaps. The next step
would be to deploy sensors and cameras at other viaducts across the system.
After Devon-Sheridan, the next bridge in rough shape is at Hollywood, Harper
said.

The Infrastructure
Technology Institute recently received a $3.3-million federal grant to monitor
transit bridges for the CTA and New York’s Metro North Railroad.

"At this and other
locations, we can help a public agency that has serious budget constraints to
safely extend the service life of its infrastructure," Schofer said.