Executive Summary

Orthotropic bridge decks are efficient, lightweight steel deck systems, typically fabricated by welding “ribs” to the underside of a flat steel plate. The potential benefits of orthotropic steel deck (OSD) systems are compelling. For example, in the early 1990s, the deteriorating concrete deck on Montreal’s Champlain Bridge was replaced with an orthotropic steel deck that weighed 25% less than the original deck.

In the last code revision cycle (2019), the orthotropic bridge deck design provisions in CSA S6:19, Canadian Highway Bridge Design Code, were replaced with references to the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications to capture the current state of knowledge and to address needs in the CSA S6 provisions until more Canadian-specific guidance could be developed.

As a result, CSA S6:19, Clause 10.16 requires that the design of OSDs satisfy the requirements of Clause 9.8.3 of the AASHTO specifications while still using loads and factors from CSA S6:19 for the serviceability, fatigue, and ultimate limit states (SLS, FLS, and ULS). Due to differences in the applied factors and loads, there are instances where using this approach is not only difficult but may not be appropriate from a structural safety or economic perspective. For example, in fatigue design, it is not clear when to switch from the AASHTO provisions for analysis to the CSA provisions to account for differences in Canadian truck traffic. Similarly, the current approach may over-penalize Canadian bridges because it requires using the AASHTO deflection limits in conjunction with the heavier Canadian design truck. These issues raise concerns that may discourage designers in Canada from using orthotropic deck solutions for new projects.

This report summarizes the results of a study that investigated these issues and presents recommendations for revisions to CSA S6 for the 2025 code cycle. The main outputs of this research include a literature review, an industry survey, applications of the existing design provisions to a sample deck system, a short investigation of Canadian truck weight data from two provinces, and recommendations for revised provisions to be included in the 2025 edition of CSA S6.

Industry experts, including members of the Project Advisory Panel, with prior experience in orthotropic deck design, reported that high fabrication cost, long-term issues with the wearing surface, and fatigue cracking are the most common factors that dissuade designers from considering OSDs as a design alternative. Furthermore, experts identified several challenges experienced during the design and construction of previous decks, including ensuring an adequate penetration level for the rib-to-deck plate welds with no blow-through, difficulty selecting appropriate deck dimensions, confusion in navigating between the CSA and AASHTO provisions, and satisfying the imposed deflection limits. Designers and fabricators who participated in the industry survey also shared advice on best practices for designing optimum OSD solutions from a cost and performance perspective.

Deflection and fatigue analyses were also conducted for this study. Deflection analyses of a sample deck system showed that two-way deflection is approximately 40% higher than one-way deflection. These analyses also confirmed that the current approach results in heavier deck designs for Canadian bridges because it requires using the AASHTO deflection limits in conjunction with the heavier Canadian design truck. However, this requirement may be justified, given the apparent presence of heavier trucks in the available Canadian highway traffic data. A fatigue analysis was performed using AASHTO Level 3 design for the same sample deck system. It involved application of the hot-spot stress method using a finite element (FE) analysis. A comparison of the weld toe stresses using the CSA and AASHTO trucks and load factors showed that the Canadian design truck and load factors result in stresses that are 55% and 35% higher for finite and infinite fatigue life design, respectively. As a result of this exercise, instances were identified where it may not be clear to the designer when to use factors or provisions from CSA S6 or the AASHTO specifications. Gaps in the AASHTO procedure, such as specific instructions on how to perform notch stress analysis of the rib-to-deck plate weld root, were also identified and solutions were proposed where possible. This report concludes with proposals for revisions to CSA S6 and for future research to improve CSA S6 beyond 2025.