Minnesota concrete bridge design detail holds strong under testing

Prestressed concrete is one of the most common materials used today for bridge girders. Since the 1990s, one detail of Minnesota’s prestressed concrete bridge design has differed from industry standards. To determine whether this detail was performing well, the Minnesota Department of Transportation (MnDOT) enlisted the help of researchers from the University of Minnesota.

Prestressed concrete girders are made by casting the concrete around tensioned cables. When the concrete hardens, the cables are cut from the forms, transferring their forces to the concrete. The aim is to reduce the likelihood of cracking in the girders, which helps to create a durable system.

“When prestressed girders are cast, steel stirrups are inserted vertically along the length of the girder,” says Catherine French, the principal investigator and CSE Distinguished Professor in the Department of Civil, Environmental, and Geo- Engineering.

“MnDOT has specified the use of U-shaped stirrups in its prestressed concrete girders, but national guidelines specify stirrups with bent legs to anchor the stirrups. We wanted to ensure that the U-shaped stirrups are working just as well as the nationally specified stirrups in this application.”

Researchers conducted a two-phase experiment to test the performance of the MnDOT stirrups. First, researchers conducted a test to measure the strength required to pull the stirrups out of concrete blocks, simulating the anchorage conditions in the girders. Next, two full-scale girders (four girder ends) were fabricated and tested to study how well the U-shaped stirrups reinforced the beams for shear, a force that can cause cracking along the sides of the beams at approximately 45-degree angles.

The results of all of the tests showed that the MnDOT-specified U-shaped stirrups performed just as well as would be expected from the nationally specified stirrups with bent legs in prestressed concrete girders.

“As a result of this research, we know that the existing Minnesota bridges built with U-shaped stirrups will perform as we expect bridges built with bent-leg stirrups to perform, and that the state can feel confident about building similar bridges using this design detail in the future,” says Professor Carol Shield, the co-investigator of the research.

Reprinted with permission from the May 2015 issue of CTS Catalyst, a publication of the University of Minnesota Center for Transportation Studies.