Building Better Bridges

Project: Hollow bridge columns
School: Missouri University of Science and Technology
Location: Rolla, Mo.
Principal Investigator: Mohamed ElGawady

The idea that hollow bridge columns could perform better than solid concrete ones seems counterintuitive, but according to research conducted at Missouri University of Science and Technology that’s the case: Hollow columns covered by a GFRP coating could extend the lifespan of a bridge.

Research funded by the Missouri Department of Transportation and the Mid-America Transportation Center compared the performance of hollow-core GFRP, concrete and steel bridge columns to conventional rebar-reinforced solid concrete columns. Mohamed ElGawady, associate professor of civil, architectural and environmental engineering, developed a hollow bridge column consisting of an outer GFRP tube with a steel tube inside. During construction, self-consolidating concrete typically used for bridge columns is poured into the gap between the inner steel and outer GFRP tubes, forming a concrete wall shaped like a doughnut. The steel tube is entrenched into the reinforced concrete footer with an embedded length 1.6 times the tube’s diameter, while the GFRP tube ends at the footer.

Using pipe with fiber orientation at ±53º, ElGawady compared the performance characteristics of solid concrete columns with a hollow column design featuring a high-performance epoxy and glass fiber matrix typically used in the oil industry and then with tubes made from a lower cost iso-polyester FRP matrix used for drainage pipes. “We tested different numbers of layers and matrix systems of FRP to produce columns that would cost-effectively respond to local requirements,” says ElGawady. “For instance, columns incorporating 3/8-inch epoxy FRP tube have more strength and durability for use in high-traffic zones, while columns with 1/8-inch polyester FRP tube would be more appropriate for low traffic zones.”

Missouri S&T Bridge Columns

Top-down view of bridge column designs. Left: The hollow column designed by Missouri University of Science and Technology features a GFRP exterior tube and steel interior tube filled with concrete. Middle: The tubes prior to the addition of concrete. Right: A traditional solid concrete and rebar column. Photo credit: Sam O’Keefe/Missouri University of Science & Technology

The elements of the hollow column work together to act as a complete engineering system to achieve significantly higher strain, strength and ductility compared to the solid reinforced concrete column. The inner steel and outer GFRP tubes function as a continuous confinement for the concrete shell, while the steel tube also adds flexural and shear reinforcement. The concrete shell delays localized buckling of the steel tube. The outside GFRP tube improves corrosion resistance and provides environmental protection. Additionally, characteristics of the GFRP tube improve both collision and seismic performance.

Cost estimates for the hollow columns are higher than for a reinforced concrete column. “While the upfront cost is higher, the total life cycle cost should be much less, particularly considering reduced repair and maintenance and improved performance,” says ElGawady. The hollow column uses 60 to 75 percent less concrete material, reducing the column’s weight by a minimum of 45 percent, thereby lowering transportation costs to the construction site.