ORTHOTROPIC STEEL DECK BRIDGES IN THE U.S. Brian M. Kozy1 and Justin Ocel2 Abstract This paper summarizes some of the recent developments in the U.S. on the subject of orthotropic steel deck (OSD) bridges. The Federal Highway Administration published a manual to provide up-to-date technical guidance on the proper design, construction and maintenance of OSDs for bridges and the AASHTO bridge design specifications have been greatly revised and expanded. The rib-to-deck weld continues to be an area of difficulty in OSD construction; due to competing desire for fabrication economy and fatigue longevity in the detail. Preliminary results from FHWA research on this detail are presented. Background Many of the world’s notable major bridge structures utilize the orthotropic steel plate system as one of the basic structural building blocks for distribution of traffic loads in decks and for the stiffening of slender plate elements in compression. Examples include the new San Francisco Oakland Bay Bridge, Self-Anchored Suspension Span in California and the proposed Strait of Messina Bridge in Italy. Stiffened steel plates have been used for many years in a wide range of steel construction applications. They are particularly prevalent in the ship building industry and for hydraulic applications such as tanks, gates, and locks. The first orthotropic steel deck (OSD) bridge was developed by German engineers in the 1930's and the first such deck was constructed in 1936. In the United States, a similar system was built and often referred to as a “battle deck” because it was considered to be as strong as a battleship. In recent years in the United States, there has been focused interest in bridge design concepts that are modular, prefabricated, and rapidly constructible. To provide updated guidance and encourage the use of OSDs, the U.S. Federal Highway Administration (FHWA) published the Manual for Design, Construction, and Maintenance of Orthotropic Steel Deck Bridges (Connor, et al. 2012). An OSD typically consists of a steel deck plate with welded stiffeners or ribs parallel to each other in the longitudinal direction. Transverse cross beams are typically used to support the ribs and provide stiffness in the transverse direction. The transverse cross beams typically serve as floor beams transferring the deck loads to the main structure. These floor beams are often integrated with the deck structure where the top flanges of the floor beams are often the deck plate itself. The stiffening ribs can be open shapes such as plates, inverted T-sections, angles, and channels or closed box-type ribs with different geometric shapes; trapezoidal closed ribs are 1 Principal Bridge Engineer, Structural Engineering Team Leader, Office of Bridges and Structures, Federal Highway Administration, 1200 New Jersey Avenue S.E., Washington, DC 20590 2 Structural Steel Research Program Manager, Turner Fairbank Highway Research, Federal Highway Administration, McLean VA the most common. Figure 1 gives an illustration of a typical trapezoidal closed-rib OSD panel. The first orthotropic steel deck with closed ribs was constructed in Germany in 1954. Compared to open stiffeners, the closed ribs have many advantages. First, closed ribs can transfer the traffic load much more efficiently in the transverse direction. As a result, closed ribs can have wider spacing than open ribs. This results in fewer ribs that results in lighter weight, and less welding compared to open rib systems. Second, closed ribs can provide much higher flexural and torsional rigidity in the longitudinal direction allowing longer spans between transverse elements. In other words, fewer cross-beams are required, which further reduces the deck self- weight and the number of welds associated with the cross-beams. Lastly, since single-sided welds are used to attach the closed ribs to the deck versus double-sided welds for open ribs, the number of rib-to-deck welds is reduced by half. However, the one-sided welds required for closed-ribs can cause quality control and inspection issues which can be costly. Figure 1. Typical closed-rib OSD panel To overcome the challenges of one-sided welding and prevent premature fatigue failure, more careful consideration is needed to design rib-to-deck welds, and ongoing research is being done. Many of the earlier vintage orthotropic decks with closed ribs experienced fatigue cracking problems. There was a lack of knowledge about fatigue and a lack of guidance in the structural design codes. The complex stress state present at the rib-to-deck welds makes fatigue design even more difficult. The quest for lighter self-weight led to relatively thin deck plates in the structural design. However, many of the designs with thinner deck plates were vulnerable to high local stress effects from wheel loads. The contribution of the wheel-load effect was not fully considered in early deck designs and many bridges experienced fatigue cracking problems. Compared to main structural members, orthotropic steel decks tend to have a higher incidence of fatigue problems because of the local effects of wheel loads. Wheel loads cause large local stress variations, stress reversals, and an increased number of stress cycles that must be considered in fatigue design. Design Methodology According to AASHTO LRFD The applicable limit states for the design of OSDs include Strength, Service, and Fatigue, according the AASHTO LRFD Bridge Design Specifications (AASHTO 2014), here forth referred to as just “AASHTO”. All limit states need to be considered for complete design, but generally it is the Fatigue limit state that will control the majority of design details. Strength Strength design must consider the following demands: rib flexure and shear, floorbeam flexure and shear, and axial compression. The rib, including the effective portion of deck plate, must be evaluated for flexural and shear strength for its span between the floorbeams. The floorbeam, including the effective portion of the deck plate, must be evaluated for flexural and shear strength for its span between primary girders or webs. The reduction in floorbeam cross- section due to rib cutouts must be considered by checking flexure and shear where the portion of web is removed. When the panel is part of a primary girder flange, the panel must be evaluated for in-plane compressive strength based on stability considerations. Testing has shown that OSD panels can have tremendous reserve strength for lateral loading beyond yield point due to membrane stiffening. This reserve, however, is dependent upon the boundary support conditions. For simplicity, the approach to strength design should conservatively limit stresses to the specified minimum yield strength or critical buckling stress. Service The Service limit state must be satisfied by limiting overall deflection for the deck plate to be less than the span length divided by 300, rib deflection less than their span length divided by 1000, and relative deflection between adjacent ribs less than 2.5mm (0.10 in.). These deflection limits are intended to prevent premature deterioration of the wearing surface. Another applicable service limit state is the Service II limit state for the design of bolted connections against slip in the overload scenario. This should be considered for the design of bolted rib and floorbeam splices. Fatigue AASHTO introduces two fatigue limit states: Fatigue I for infinite-life design and Fatigue II for finite-life design. Due to fact that OSDs are governed by wheel loads (in particular the rib- to-deck connection), they experience multiple cycles of stress from every truck passage and thus will most often be designed for infinite life. However, finite life design may produce more cost- effective proportions when the traffic volume is not excessively high. Design Load For OSDs, it must be recognized that the AASHTO LRFD-specified 145 kN (32 kip) truck axle in the HL-93 load model actually represents a tandem consisting of two 71 kN (16 kip) axles spaced at 1220mm (4ft.). Thus, each wheel of the 71 kN (16 kip) axle is properly modeled in more detail as two closely spaced 45 kN (8 kip) wheels, 1220mm (4ft.) apart to accurately reflect an actual Class 9 tractor-trailer with tandem rear axles (see Figure 2). The single axle simplification is acceptable for main members not directly subjected to axle loads, such as girders, floorbeams, truss members, etc. However, research has shown that for elements directly loaded by wheels, such as expansion joints, the OSD plate, etc, this assumption is inappropriate. Figure 2 – Refined Fatigue Design Truck Footprint (all dimensions in mm) Load Factors For AASHTO fatigue design of OSD components and connections, the fatigue load factors are taken as 1.50 for Fatigue I and 0.75 for Fatigue II. There is an exception to this where it is increased to 2.25 for Fatigue I when checking connections to the deck plate and details around the floorbeam cut-out. The increased Fatigue I load factor is based on stress range spectra monitoring on both the Williamsburg Bridge and the Bronx Whitestone Bridge, which indicate that the standard Fatigue I load factor, which was developed for girders, floorbeams, truss members and other “global” components is unconservative for the design of certain OSD components. These studies indicate that the ratio of maximum stress range to effective stress range is approximately 3.0, which is larger compared to standard bridge girders. This is likely due to a number of factors such as occasional heavy wheels and reduced local load distribution that occurs in deck elements, as opposed to a main girder. Cycles per Truck The frequency of loading is critical for finite life design in OSDs. The Average Daily Truck Traffic (ADTT) and cycles per truck passage (n) both influence the total number of cycles for design. For components and connections of the OSD subjected to direct wheel loads, the number of cycles for design is governed by the number of axles expected to cross the bridge.