Fatigue Design of Orthotropic Steel Bridges

Many of the world's signature long-span steel bridge structures utilize the orthotropic steel plate as one of the basic building blocks in the structural system, for distribution of traffic loads in decks and for the stiffening of slender girder plate elements in compression. Orthotropic bridge behavior is complicated since the deck plate distributes wheel loads in two directions, with different structural rigidity in the longitudinal and transverse directions; and involves integral behavior of the deck with the floorbeams and girders. U.S. engineers are often unfamiliar with these types of structures and existing published manuals are limited in their ability to guide complete engineering design. One of the primary reasons for advancement of orthotropic deck details in recent years is from application of modern, refined techniques for engineering analysis and design. Emphasis is now placed on the fact that the design of these structures is mostly controlled by fatigue limit states. Complex details necessary to make these structures work require advanced fatigue evaluation techniques which must rest on accurate stress range calculations with the use of refined Finite Element Analysis (FEA) or prototype testing. Such an approach to design verification has broad implications to bridge engineering practice in the U.S. and special application of the AASHTO Specifications. This paper provides a summary of a recommended approach to fatigue evaluation of an orthotropic steel bridge for design based on the current state of the practice.