Orthotropic Steel Decks
Orthotropic steel decks have been employed worldwide, particularly in Europe, Asia, the Far East, and South America. However the use of orthotropic steel decks in the United States has been fairly limited, such that their use represents a very small percentage of the bridge inventory. The modern orthotropic welded deck bridge construction was developed by German engineers in the 1930’s, and the first such deck was constructed in 1936.
In general, an orthotropic steel deck consists of a flat, thin steel plate which is stiffened by a series of closely spaced longitudinal ribs that run parallel to traffic, placed orthogonal to transverse floor beams. The deck stiffness is considerably different in the longitudinal and transverse direction, hence orthotropic steel decks are structurally anisotropic. According to Troitsky (15), the name orthotropic comes from the fact that the ribs and floor beams are orthogonal, and the elastic section properties are different, or anisotropic, in both directions; thus the system became known as orthogonal-anisotropic, which shortens to orthotropic. Typically, an orthotropic steel deck is made integral with the supporting superstructure component, such as the top flanges of girders or floor beams. Figure 14 shows general sketches of an orthotropic steel deck integral with a plate girder.
In an orthotropic steel deck system, live loads are transferred through the wearing surface and top steel deck plate, to the longitudinal ribs, and then to the transverse floor beams. The load in the floor beam is then transferred to the main load carrying system, such as a longitudinal plate girder. Per Article 9.8.3.1 of the AASHTO LRFD (7th Edition, 2014), the deck plate is to act as a common flange of the ribs, the floor beams, and the main longitudinal components of the bridge. With proper maintenance, experience has shown that a steel superstructure system that employs the use of an orthotropic steel deck can have a long service life.
The use of orthotropic steel decks has been limited in the United States for a variety of reasons, but mainly due to issues related to the complex design and fabrication associated with them, as opposed to the use of a conventional reinforced concrete deck slab. Orthotropic steel decks also require a different set of inspection and maintenance techniques. Orthotropic steel decks have had problems related to fatigue cracking and performance of wearing surfaces in the past, which have made bridge owners wary of specifying the use of these decks. Many of these problems are related to limitations of design and analytical methods used in the past, as well as reliance on trial and error with regard to detailing and fabrication.
The construction and fabrication techniques employed are very important to the successful use of orthotropic steel bridge decks. Orthotropic steel decks typically require detailed construction specifications and special quality control procedures during fabrication. Current designs typically are not standardized, and thus repetition does not currently help to improve construction and fabrication techniques.
Orthotropic steel decks can be constructed more rapidly and provide a significantly longer service life than conventional concrete deck slabs. Construction tends to be more rapid, since most of the components are assembled in the fabrication shop. Future deck replacement is typically not required for orthotropic steel decks, but based on past experience, the wearing surface typically requires replacement every 20 to 30 years.
Article 9.8.3 of the AASHTO LRFD (7th Edition, 2014) provides guidance on design and detailing for orthotropic steel decks that should currently be followed by bridge designers. However, it should be noted that the Federal Highway Administration (FHWA) funded a project to develop a new manual focused on the design, fabrication, and construction of orthotropic steel deck bridges (17). The Manual for Design, Construction, and Maintenance of Orthotropic Steel Bridge Decks (18) covers relevant issues related to orthotropic steel deck bridges, including analysis, design, detailing, fabrication, testing, inspection, evaluation, and repair. Based on extensive recent research efforts, this manual addresses the limited guidance currently provided with regard to fatigue design. This manual also addresses issues related to the quality control of both construction and fabrication, as well as establishes sound detailing concepts based on current experience in order to help develop standardized details.