Steel tubes have inherent advantages as structural members, including exceptional torsional stiffness and resistance, and improved local buckling resistance compared to conventional steel open-section members. These advantages are not easily or widely exploited in steel bridge design due to the lack of comprehensive design provisions for steel tubes in bridges.
Closed rectangular section box sections, most often welded from plates, have a long history in steel truss bridges. These box section members are typically connected to each other, or to open-section members, using conventional bolted gusset plates. Well established design provisions for these truss bridge conditions exist. The design provisions that are needed relate to cold-formed or hot-formed steel tubes, with or without longitudinal seam welds, that are connected with limited use of welded or bolted gusset plates. The provisions should address both round and rectangular tubes.
These steel tube design provisions require research in the following areas: (1) tubular member property specifications primarily focused on material mechanical properties and geometric tolerances; (2) limit state resistance formulations for tubular members; (3) definition of load effects for connections of tubular members; and (4) limit state resistance formulations for connections.
(1) In the area of tubular member property specifications, it is recognized that steel tube material specifications are “product” specifications, which address both material properties and product properties, such as dimensions. Concerns to be addressed include the consistency of mechanical properties (e.g., fracture toughness) around the tube and testing methods. Dimensional tolerances, including wall thickness tolerance and overall dimensional tolerances, should be addressed.
(2) The AASHTO LRFD Bridge Design Specification (BDS) and the AISC Specification for Structural Steel Buildings include formulas for the resistance of tubular steel members at various limit states. Hollow steel tubes and concrete-filled steel tubes should be considered. These formulas should be reviewed and differences should be rectified to provide resistance formulations that are fully consistent with the AASHTO LRFD BDS and with the tubular member property specifications that are adopted in (1).
(3) Work is needed to review past research and existing AASHTO LRFD BDS provisions, AISC provisions, and provisions from other steel design standards for connections to tubular steel members, to establish the load effects to be used for limit state design checks. Tube-to-tube direct connections, and plate-to-tube connections also need to be considered. Hollow steel tubes and concrete-filled steel tubes should be considered. Unstiffened connections are usually more efficient/economical, and for unstiffened connections, deformation of the tube wall may result in non-uniform local distributions of stresses in welds or forces in bolts. For fatigue of welded connections, local structural stresses (sometimes called “hot spot” stresses) may need to be considered. So for each limit state design check, the load effect (or the “demand”) needs to be established and corresponding limits on the geometry of the tubes at the connection (for which this established load effect is an accurate measure of the critical connection response) need to be established.
(4) With the load effects for limit state design checks established from (3), the corresponding resistance for steel bridge limit states, and associated design checks, need to be established for connections to tubular steel members. Past research, and existing AASHTO LRFD BDS provisions, AISC provisions, and provisions from other steel design standards should be reviewed. Tube-to-tube direct connections, and plate-to-tube connections need to be considered. Uncertainty in resistance needs to be considered, to establish appropriate resistance factors.
Finally, with items (1) through (4) complete, the results should be written in specification language with commentary, consistent with the AASHTO LRFD BDS.