Bridges

The residual welding stresses in unrestrained steelwork generally reflect the two-dimensional state of stress and can approach the uniaxial yield stress of the parent steel or weld materials, irrespective of whether a direct tensile stress is applied. Residual welding stresses are relevant to the considerations of plate and beam buckling but are ignored in the sectional design of steelwork on the bases that they are self-equilibrating and can dissipate when plastic flow in the steelwork is possible.

Heavily-constrained steelwork generally reflects the incidence of multiple steel plates, and the overlapping (or close location) of multiple welds, often from different cardinal directions. In this case: stresses may reflect the three-dimensional state of stress and include a ‘hydrostatic’ component. Steel yields on account of dilatory behaviour due to shearing stresses. ‘Hydrostatic’ stresses do not contribute towards this shearing behaviour and are unrelated to yielding. When ‘hydrostatic’ stresses are significant: steel may fracture by brittle cleavage at the ultimate tensile strength without prior yielding or warning – at which time the principal tensile stresses may be greater than the Tresca or Von Mises ‘equivalent stress’, and/or greater than the yield stress which is derived from the uniaxial test of a steel coupon. Brittle failures of several North American bridge structures at the locations of heavily-constrained welded details have been attributed to the development of large ‘hydrostatic’ stresses, as well as notch-like, or crack-like planar discontinuities within the steelwork.

This paper discusses these failures and the possibility that the risk of constraint-induced fracture is not understood by many Australian engineers. This paper also identifies appropriate design procedures to reduce the risk of constraint-induced fracture, including design responses developed by BG&E Pty Ltd for the recent design of major steel network arch bridges for the Sydney Gateway project in Sydney, Australia.