Home » An Engineer Raises Questions About Bridge Deck Structural Integrity
We would like to commend you for the excellent and timely article titled Engineers Seek Ways to Warn of Failures ‘Waiting to Happen.' One example where full disclosure of information about a failure could have contributed to better understanding of the problems and might prevent similar errors in the future was orthotropic decks failure on the Bronx-Whitestone Bridge (2005) which occurred within one year after their installation on the bridge. Cracking already occurred during testing of the prototype panels for this project, however, refined analysis by experts, having full knowledge of serious fabrication defects of the test panels, predicted a “75-year fatigue life” for the bridge decks. Having been involved in research, development and design of orthotropic deck bridges over the past five decades, we inquired about specific information about the extent and details of cracking on the bridge, however we were advised that this information was “confidential”. The repairs were very costly and took over a year to complete.
From published information about the prototype panels details, and based on available information about failures of European and Japanese orthotropic decks, we concluded that the cause was not “the contractors’ fault”, as was implied, but was due primarily to faulty design, based on elastic “fatigue theory” inapplicable at the welds, while disregarding the much more important effects of fabrication (mainly very large residual tension forces caused by excessively large weld sizes used by designers with the purpose of reducing the “stress range” due to applied truck loading). We advised the AASHTO Bridge Committee and the FHWA about our conclusions and, as “Friends of the Committee”, we prepared, as requested by its Chairman, our proposed revisions of design specifications for orthotropic decks based on pragmatic basis and not on inapplicable academic theory. Our suggestions have been endorsed by several overseas authorities in this field, and we presented them with ample corroborating evidence, including our earlier publications on this subject.
Yet, the FHWA and their engineering consultants decided to prepare the intended second edition of the “Design Manual for Orthotropic Deck Bridges” based on the theory that was demonstrated to be totally misleading in the BWB investigation. (The first, 1963 Edition of the Manual was prepared for the AISC by our office). The FHWA also proposed their revisions of the AASHTO specifications for orthotropic decks based on analytical approach, which would compel the use of unnecessarily large weld sizes, thus increasing, rather than reducing, the decks’ susceptibility to cracking. The AASHTO Bridge Committee is now proposing to adopt these specifications at the coming Committee meeting at mid-May.
In the meanwhile, during the past year, two versions of orthotropic panels, designed for redecking of the Verrazano Bridge in New York by similar theoretical methods, ominously cracked in testing. Again, our requests for specific information on these failures were denied because of “confidentiality.” It appears that the costly experiences on the Bronx-Whitestone Bridge could be repeated, again, on a much larger scale.
In our publications and messages to the FHWA and the AASHTO we insisted that structural failures should be openly discussed in order to draw proper lessons from such cases. We also recommended mandatory independent reviews and checking of major bridge construction and rehabilitation projects (as is now required in the U.K), and pointed out that such reviews could have prevented many failures, including the 2007 tragic I-35W bridge collapse in Minnesota (see our Letter to the Editor, Civil Engineering, Jan. 2008). This failure was subsequently extensively and openly discussed, an official NIST report was published, and, eventually, a cautionary note on design of truss gusset plates was inserted in the AASHTO LRFD Bridge Specifications. In our specifications proposal for orthotropic decks we suggested that a similar warning about the use of inappropriate analytical approach for assessment of their safety against cracking should be given.
We fully agree with your conclusions that failures offer valuable learning opportunities, and that “public safety trumps secrecy.” We trust that, eventually, rational design and construction rules and specifications for orthotropic decks, in accordance with current state-of-art, will be adopted.
Roman Wolchuk Principal, Roman Wolchuk Consulting Engineers Jersey City, N.J.
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