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Long-Term Behavior Of Bridge Girders Focus Of Aging Infrastructure Research
October 2015

When looking at a typical prestressed concrete bridge girder, there may not be any visually noticeable deflections from traffic or any other loading, but this does not tell the entire story. What may not be easily observed is evident to and accounted for by the engineer who designed that girder. One of the reasons for this discrepancy is that most of these deformations occur slowly over time. While casual observation may not be able to perceive any of these deformations, the long-term durability of a structure is greatly affected by them.

Photo courtesy of the American Concrete Institute.

“Long-term durability is a major issue for today’s infrastructure. In order to create concrete bridges with longer service lives and better performance, we must better understand the long-term behavior of these members,” said David Garber, PhD, a project researcher and now an assistant professor at Florida International University.

Prestressed concrete bridge girders rely on a pre-compression force applied by tensioned steel strands for good durability performance and long life. Due to the material properties of both steel and concrete, the effectiveness of this pre-compression force, referred to as prestress loss, will naturally decrease over time and must be accounted for at the time of initial construction by the design engineer. Underestimating the prestress loss can lead to cracked girders with shorter life-spans; overestimating can lead to wasted material and other constructability issues.

A better understanding of the long-term behavior of prestressed concrete bridge girders was the focus of a research project conducted at The University of Texas at Austin (UT) under the leadership of Oguzhan Bayrak, Ph.D., a professor at The University of Texas at Austin, assisted by José Gallardo, adjunct professor at Technological University of Panama, and Dean Deschenes, laboratory manager at Simpson Gumpertz & Heger.

In order to gain a better understanding of this long-term behavior, an experimental study was conducted in which 30 full-scale bridge girders were constructed and instrumented. The constructed girders were then shipped to several different environmental exposure sites and stored there for up to three years. During this period, the behavior of these girders was measured using vibrating wire strain gauges embedded in the test specimens.

Since bridge girders do not just sit idle when they are actually in use, the next phase of the research program was to load and test them at the Ferguson Structural Engineering Laboratory (FSEL) at UT. “Strain monitoring in the field can only get us so far,” said Dr. Garber, “In order to truly understand the behavior of an aging girder, structural testing should be conducted in a controlled, laboratory setting.” As a result, all of the bridge girders were shipped back to FSEL for flexural service load testing.

“A designer comes into a project typically knowing where the bridge will be built, but generally not knowing how it will be built,” says Dr. Garber. “He or she is then only able to specify the concrete strength to the fabricator, which is not sufficient for adequately estimating the long-term behavior of a structure with a high level of confidence.”

The next step for the reseach team will be to identify ways to improve the methods for designers to better predict and estimate the long-term behavior of prestressed concrete girders.

Article reprinted from materials provided by the American Concrete Institute (ACI).

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