Design and Detailing Considerations
In accordance with Articles 9.8.3.4 and 9.8.3.5, refined methods or approximate methods of analysis are acceptable to determine force effects in orthotropic steel decks. Approximate methods of analysis for both open rib and closed rib decks may be based on the Pelikan- Esslinger method, per Articles C9.8.3.4.1 and 9.8.3.4.3c of the AASHTO LRFD (7th Edition, 2014).
Per Article 9.8.3.5.2, orthotropic steel decks are to be designed for the service, strength, and fatigue limit states. Strength design should consider the design of the deck related to rib flexure and shear, floor beam flexure and shear, and axial compression (panel buckling) of the deck. Deflection limits should be satisfied for the Service I limit state, so as to prevent premature deterioration of the wearing surface. The Service II limit state is required for the slip critical design of any bolted connections. Fatigue in orthotropic steel decks requires careful consideration as orthotropic steel decks experience numerous stress cycles.
Fatigue cracking has been observed in orthotropic steel decks, resulting from vintage weld details and desire to minimize weight. Early analytical tools and procedures were limited in their ability to quantify the stress states at these details and the early experimental fatigue database was limited (17). Detailing and fabrication practices also relied on experience gained through trial and error. The design of critical details in orthotropic steel decks is typically controlled by cyclic live load and the fatigue limit state. Many of these details that have experienced cracking are due to fatigue, and can be sensitive to the fabrication process. In particular, cracking has been experienced at rib to deck welds, rib to floor beam welds, and at welded deck splice plates. However, it should be noted that an orthotropic steel deck system is highly redundant, and cracking related to fatigue most often does not represent a serious threat to the strength or integrity of the structure as a whole (17). Furthermore, the FHWA manual, previously discussed, provides further guidance regarding the analysis, design and physical testing related to fatigue in orthotropic steel decks.
Detailing requirements are provided in Article 9.8.3.6 of the AASHTO LRFD (7th Edition, 2014). The minimum deck plate thickness shall not be less than 0.625 in. or four percent of the larger spacing of the rib webs, while the thickness of closed ribs should not be less than 0.1875 in. Furthermore, the interiors of closed ribs are to be sealed by welds in order to prevent the ingress of moisture and air into the closed rib. In accordance with Article 9.8.3.6.2 of the AASHTO LRFD (7th Edition, 2014), the one-sided weld between the web of a closed rib and the deck plate shall have a target penetration of 80 percent, with 70 percent minimum and no blow-through, and shall be placed with a tight fit of less than a 0.02 inch gap prior to welding.. Additional guidance regarding the deck and rib details is provided in Article 9.8.3.6.4.