Concrete Filled Steel Tubes for Deep Foundations of Transportation Structures-Paper

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Concrete-filled steel tubes (CFSTs) combine a steel tube and concrete core to create strong, stiff, and ductile structural members. Although widely used internationally, they have seen limited use in U.S. bridge construction, partly because design standards are divided between steel and concrete systems and standard, constructible connections have been lacking.<br /><br />University of Washington research, supported by transportation agencies, has examined CFST performance and practical connections for bridge foundations and piers. CFSTs are particularly useful for piles and drilled shafts in weak, liquefiable soils, where foundations face substantial lateral and seismic forces. Compared with conventional reinforced-concrete solutions, they can reduce member size, material use, and field labor. Prefabricated tubes also support accelerated bridge construction by avoiding extensive rebar-cage placement and temporary formwork.<br /><br />Full-scale connection tests evaluated three approaches: embedded-ring, welded-dowel, and reinforced-concrete-cage connections. Each can provide effective force transfer under gravity and seismic loading. Embedded-ring connections showed comparatively high stiffness and strength, while adequate embedment and footing dimensions were essential to prevent concrete punching or tube pullout. Debonded welded dowels improved deformation capacity and limited damage to the pier cap.<br /><br />A separate 22-specimen shear-testing program found that CFST shear resistance comes primarily from the steel tube, with additional contribution from the concrete fill and any internal reinforcement. The test results showed that existing equations can substantially underestimate shear strength; the proposed expression provides a more accurate estimate.<br /><br />The research has informed changes to bridge design provisions and supports broader adoption of CFSTs in U.S. bridges. Overall, standardized design guidance and practical connections can make CFST foundations and piers an economical, resilient option that enables faster construction.

Keywords

concrete-filled steel tubes

CFST

bridge foundations

bridge piers

seismic loading

prefabricated construction

steel tube

concrete core

shear strength

embedded-ring connection