University of Nevada, Reno Four spans with abutments and piers were connected to three shake tables.

Researchers now are sifting through the data gathered from a Feb. 15 shake test on a large-scale four-span bridge model at the University of Nevada, Reno. It is the first time a full-bridge structure model was used, say officials. The test also showcased a wireless bridge monitoring sensor that could represent the next-generation niche market.

Three 50-ton-capacity shake tables subjected a 110-ft-long concrete bridge to forces simulating a severe earthquake as part of a $7-million program to test large-scale bridge models. The goal is to  fine-tune simulations and improve bridge designs. It is part of the National Science Foundation’s George E. Brown Jr. Network for Earthquake Engineering Simulation (NEES).

University of Nevada, Reno professor M. Saiid Saiidi, project director, says that the test allowed researchers to study the interaction between abutments and piers.  “We have three piers, and the interaction among them was clear,” he notes. “The quake was both transverse and longitudinal. When the column failed, the failure was unlike any I’d seen.”

Jean Dixon/University of Nevada, Reno Onlookers watch bridge model at university test.

Under bidirectional stresses, one 1-ft-diameter column experienced severe lo-cal damage, with concrete breaking into pieces throughout it. Yet in another column on the same pier, this did not happen, Saiidi says.

In previous tests, shaking was done in one direction and only on a single-column or two-column pier. “This test was closer to reality,” he says. But researchers must decide whether they can extrapolate the results for real-life situations, he adds. Though months of evaluation lie ahead, one finding was that the bridge, built to current seismic design codes, did not collapse under forces of about 7.5 on the Richter scale, greater than the Northridge earthquake of 1994.

University of Nevada, Reno

University of Nevada, Reno

University of Nevada, Reno Using high-frequency waves, sensors transmit data and advice.

Three more tests on similarly scaled bridges will take place over the next two years, with researchers from various universities planning to focus on specific technologies. For example, Florida International University will study a carbon or Fiberglas pier tube during one of the tests, says Amir Mirmiran, professor and chairman of the civil and environmental engineering department.

While California has used carbon wraps as a retrofit for existing bridges, FIU will test a tube as part of a new bridge column. Concrete and rebar would be built within the tube, to be made either of carbon or Fiberglas polymer, says Mirmiran. “Glass is not as strong as carbon but provides more flexibility. It meshes better with the concrete,” he notes.

Sensible Sensors

Scores of sensors were employed on the Feb. 15 test and 11 of those were a patented new type of wireless monitor developed by Garo Kiremidjian, president of Sensametrics, and Anne Kiremidjian, professor of structural engineering and geomechanics at Stanford.

The battery-operated units contain microprocessors that store and analyze up to 2 gigabytes of data from stresses on the bridge and send alerts and assessments back to a bridge official’s computer or hand-held device. Unlike conventional sensors, they do not require cabling along the bridge.

Wireless monitoring sensors are a fairly new technology. CTLGroup, Skokie, Ill., has used WiFi and cellular communications protocols and equipment in the last decade for buildings, bridges and highways, says Vice President Adrian Ciolko. “Wireless communications aren’t without their significant technical challenges, but on large projects, they sure can make life easier.”

Jean Dixon/U of Nevada, Reno Section of span ready for shake test.

Each autonomous Sensametric wireless transmitter unit works like a 2.4 GHz radio on high-frequency protocols and can be set to analyze periodically or to “sleep” —thus saving power— until a catastrophic event occurs, like an earthquake or a barge collision, say the Kiremidjians. The sensors also monitor ongoing stresses from ambient noise, traffic, weather or construction activity. “Once, you used the data to see how the bridge failed,” says Anne Kiremidjian. “But now you can use it for ongoing monitoring and to provide alerts and assessments—not just of catastrophes, but of effects from construction.”

Units form a wireless “mesh” network and can be synchronized with accelerometers, temperature sensors or strain gauges, Anne Kiremidjian says. They can hook to a video system so that if an overweight truck crosses a bridge, its license plate can be caught. If an extreme event occurs, such as major cracks or displacement, a nearby unit sends an alert to the user with information on the likely cause, extent of damage and recommendations for further action, such as shutting down the bridge or restricting traffic.

The Feb. 15 test “enabled us to test the system in a very harsh environment and gave us a lot of confidence that we can resolve the problems” with radio interference by adding more routers, Anne Kiremidjian says. The new sensors may be tested on one of the future shake-table tests, and several state transportation agencies have shown interest, she says.