Discussion of “Damage Localization and Finite-Element Model Updating Using Multiresponse NDT Data” by Masoud Sanayei, Erin Santini Bell, Chitra N. Javdekar, Jennifer L. Edelmann, and Eugene Slavsky

The authors discuss fatigue strength of welded rib-to-web junctions at the deck supports with reference to redecking of the Williamsburg and the Bronx-Whitestone bridges, where low available depth necessitated the use of special details, and recommend the use of the “modified nominal stress approach” for the design of such connections. However, experience with thousands of existing orthotropic deck bridges throughout the world shows that applied live load stresses are not the only cause of fatigue cracking and that factors related to fabrication may be of equal, or possibly, greater significance. It has been reported that in many cases, cracks in orthotropic decks developed shortly after fabrication and welding, or early after the deck installation, before the live loads had been fully applied. This led to the conclusion that fatigue cracking occurs where the effects of heavy wheel loads coincide with faulty details or inappropriate fabrication procedures. PSP 2005 . Therefore, consideration of the effects of live load stresses alone addresses only one aspect of the fatigue problem. The authors imply that the type of weld used for the rib-toweb connection is accounted for by Detail Category assigned to the fillet, the partial and the full penetration welds. However, many other factors related to welding such as weld size total heat input , welding method manualautomatic , welding speed and sequence, weld imperfections, weld terminations, etc. may be more important. Dimensional fabrication tolerances and the degree of constraint of the plate elements to be welded are further factors that must be considered in the assessment of fatigue resistance. Tensile forces developing at welds due to weld shrinkage upon cooling could be a direct cause of cracking. These forces are directly related to loose fabrication tolerances and the geometric constraints resisting the shrinkage. Welding and other processes causing melting or plastification of steel, such as flame cutting, heat strengthening, cold pressing of ribs, etc. causes metallurgical micro-structure changes of grain size and crystalline structure, which may cause embrittlement. Residual tensile and compressive stresses also strongly affect the structural behavior of the steel material. Fabrication factors affecting fatigue resistance may be widely different for the various details of an orthotropic deck and depend on equipment and techniques used by the fabricators. These factors are unpredictable, may be different on various projects, and are generally unknowable to the designer. Design codes do not provide specific guidance for welding and fabrication tolerances