Strong Research, Stable Structures

Keeping Oregon’s Infrastructure Up and Running

During regular inspections in 2000, Oregon Department of Transportation (ODOT) personnel noticed something disturbing about the vintage concrete bridges all over Oregon. From the Powder River in the northeast to the Rogue River in southern Oregon, the bridges were cracked.

Chris Higgins and his team of research students build their own concrete girders at the Large-Scale Structural Testing Laboratory. The College of Engineering is working toward expanding the lab to include a strong wall and other testing features.

Of the 1,800 concrete girder bridges listed in the state database roughly 500 showed visible cracks, many of those on Interstate 5 and Interstate 84, the main north-south and east-west corridors in the state.

“From just about anywhere in the state, you can’t travel east or west, north or south without crossing one of these bridges,” says Chris Higgins, a professor in OSU’s Dept. of Civil, Construction, and Environmental Engineering. “It has a large scope and is an expensive problem.”

ODOT took immediate action. The agency imposed load limitations on several bridges, closed others, and commissioned Higgins and his team at the College of Engineering to help the state analyze the severity of the cracks, assess the remaining capacity, and predict the remaining life of these bridges.

“I was pleased to partner with OSU in development of the testing plan, and to be part of the technical advisory committee for the research project,” says Ray Mabey, deputy director of ODOT’s OTIA III State Bridge Delivery Program. “Some of the research methods and results of this study will be recognized nationally.”

The majority of the bridges affected were built in the 1950s. At the time, engineers were employing very lean and efficient designs, specifying only that material which was necessary. When they began using standardized deformed reinforcing bars, a new technology at the time, they deviated from their past successful practice when detailing the reinforcing within the bridges, Higgins says.

Prior to cracking, the concrete carries the load on the bridge. Once cracked, the load is transferred to the steel rebar imbedded in the structure. As trucks became heavier and more frequent, the load on the bridges increased and large diagonal cracks formed. Because the bridges contain less steel than required by current codes, the structures were of concern to engineers.

“We undertook field work and full-scale laboratory testing and analysis to determine how the bridge members were behaving,” Higgins says. “We studied the load effects and found that it was a matter of both shear and moment acting on the section. The problem was a conspiracy of two force effects, and not a lone gunman.”

Higgins and his research team completed field data collection on four Oregon bridges, and built 44 full-size concrete girders to 1950s specifications for testing at the Large-Scale Structural Testing Laboratory housed in the College of Engineering’s O.H. Hinsdale Wave Research Laboratory. The laboratory contains a strong-floor that enabled the team to carry out full-scale experimental tests on the four-foot thick girders, and the first-ever tests of real-size bridge girders failing under moving loads.

Higgins’ team developed a reliability-based assessment methodology for ODOT to use on a bridge-by-bridge basis, and provided ODOT tools to determine the
residual load carrying capacity.

“Once you make that assessment you can decide what needs to be done, if anything, to ensure the safety of the structure,” Higgins said. “All this work is about protecting the safety of the traveling public, making the best use of public transportation investments, and keeping Oregon up and running.”