Dawn Lehman	Charles Roeder

Stronger and faster are two words that describe a new type of bridge design that UW CEE researchers are working to implement into practice. In light of current events, particularly a pedestrian bridge that collapsed near Florida International University in Miami in mid-March, the research is especially timely.

The UW CEE research team, including professor Dawn Lehman and professor emeritus Charles Roeder, designed a new concrete-filled steel tube (CFST) frame system, which provides increased strength and stiffness when compared to traditionally used reinforced concrete or structural steel systems and also allows for accelerated bridge construction. The new bridge design concept has recently been included in updated bridge design manuals.

The CFST design is stronger than traditional methods, as the steel tube is at the optimum location to resist bending, thereby maximizing strength and stiffness while minimizing material requirements. The steel tube also confines the concrete infill, which results in increased strength and strain capacity in the concrete. The concrete infill, in turn, restrains the steel tube from local and global buckling. Component testing of the new system at the University of Washington indicates that the components have higher flexural strength, flexural stiffness and shear strength (up to 2.5 times the shear strength of a conventional, reinforced concrete component).

Another advantage is that CFSTs allow for accelerated bridge construction, as the steel tubes serve as formwork and reinforcement to the concrete fill, negating the need for reinforcing cages, shoring and temporary formwork. By accelerating the construction process, traffic and other interruptions are minimized, which is especially advantageous in urban areas. Current accelerated bridge construction designs encompasses a variety of structural systems and construction techniques, many of which do not have a CFST framework. 

Components of a CFST system.

Despite their construction and structural advantages, the use of CFSTs has historically been limited due to dated design specifications and a lack of standard connections. In the U.S., for example, design specifications are prepared by separate groups for structural steel and reinforced concrete structures, which results in conflicting design standards. With the goal of implementing standardized connections, the researchers developed and tested three new connections in the UW Structural Research Laboratory, all of which provide excellent seismic performance. The most economical connection is an embedded tube connection, where a ring is welded to the end of the tube and the tube is embedded in the adjacent concrete. In addition, welded dowel and extended dowel connections were tested.

The research has resulted in recent changes to the American Association of State Highway Officials bridge design specification as well as state departments of transportation, which supports the increased use of CFST in bridge piers and pile and drilled-shaft foundations.

The research is funded by the U.S. Army, Caltrans and Washington State Department of Transportation.