Abstract
On September 11, 2006, “unusual” deflections of an overhead bridge-mounted sign support truss along IR 75 in Dayton, Ohio were reported. Investigation by Ohio DOT road crews revealed the complete fracture of two truss members near one of the truss supports. While total collapse did not occur, such an event could have catastrophic consequences along a major arterial roadway like IR 75. This concern, along with the fact that a similar failure occurred near Cleveland, Ohio in 2004, prompted an investigation that is the subject of this manuscript. Results of the investigation are based upon a combination of in situ field monitoring of traffic-induced bridge vibrations at the location of the failed sign support truss, finite-element simulation of the expected dynamic response of the original truss in such an environment, the length of service of the truss at the time of its failure, the volume of truck traffic on the bridge in question during that time of service, and metallurgical examination of the failed components of the sign truss. The conclusion of the investigation is that extremely high-cycle fatigue of the chord-web diagonal welded connection was the cause of the truss failure. The in-service effective stress range of the AASHTO Category ET connection in question was most likely below the currently specified AASHTO constant amplitude fatigue limit for the detail, but the enormous quantity of response cycles (approaching or even exceeding 1 billion cycles) such a bridge-mounted sign on a heavily traveled route accumulates over a service life of 30 or 40 years, exceeds anything currently considered in the design codes for such structures. The implications of this fact on the current inventory of such structures in similar installations and of the same age range, i.e., installed with the original interstate routes, largely in the 1960’s, are obvious.
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Acknowledgments
Ralph Van Kirk and his crew at District 7 very ably made the field monitoring on a very busy bridge possible and provided us, as well, with numerous design documents for the bridge and sign structure. Jim Roth of the ODOT Division of Operations, Office of Traffic Engineering provided valuable background information used for the modeling and analysis of the sign structure. Professor John Lewandowski and his students in the Department of Material Science and Engineering at the Case School of Engineering expertly performed the metallurgical examination and analysis.
References
AASHTO. (2001). Standard specifications for structural supports for highway signs, luminaire, and traffic signals, 4th Ed., American Association of State Highway and Transportation Officials, Washington, D.C.
AASHTO. (2004). AASHTO LRFD bridge design specifications, 3rd Ed., American Association of State Highway and Transportation Officials, Washington, D.C.
Dowling, N. E. (2007). Mechanical behavior of materials: Engineering methods for deformation, fracture, and fatigue, 3rd Ed., Prentice-Hall, Upper Saddle River, N.J.
ODOT. (2007). Ohio Department of Transportation traffic survey data, ⟨http://www.dot.state.oh.us/techservsite/offceorg/traffmonit/default.htm⟩ (March 1, 2007).
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© 2009 ASCE.
History
Received: Oct 16, 2008
Accepted: Feb 9, 2009
Published online: Sep 15, 2009
Published in print: Oct 2009
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