Three characteristics of steel tubular arch bridges are being discussed. The brace members, connecting the main chord tube to an upper concrete or composite deck can be arranged either as triangular truss members, or as rigid node vertical or slightly sloping frame members. As can be expected, the former arrangement results in lower bending of the arch chord and lower compression near its centre. However, the number of brace tubes for triangular arrangement is twice as much as for the frame system. In addition, the nodes of the truss system necessarily join at least 4 tubes and are of the K-type. The latter are more complicated to design and to fabricate than T-nodes, used in the frame arrangement.
Fatigue resistance of welded tubular nodes is rather low. Should the chord diameter be sufficiently large, internal diaphragm stiffeners can be provided, thus increasing considerably the fatigue resistance. Both numerical and experimental research demonstrates that an optimal location of the diaphragms can be found. However, this optimum location does not necessarily correspond to the minimum of stress concentrations.
Most springs of steel tubular arch bridges are rigidly connected to a concrete abutment. Generally prestressing bars are used to achieve this. Unfortunate experience has shown that this connection may still be too flexible to prevent axial rotation due to torsion. The alternative of using various connectors has been implemented in few bridges. The torsion resistance of such a connection has been assessed in 3 scaled tests. The results have given more insight into the mechanism of torsion connection and results in experimental values of the torsion stiffness. The preliminary conclusion of this research indicates that stud connectors show larger stiffness than strips, which is a rather unexpected outcome.