Welcome to this themed issue of Bridge Engineering focusing on major bridge replacement projects in North America. Major bridges over navigable waterways are interwoven into the fabric of North America; roughly half of the continent is accessible through inland navigation, and population density is well correlated with this network. A significant number of major bridges were built 75 to 100 years ago (Caprani and De Maria, 2020), when populations and traffic were roughly one-third of what they are today. Many of these bridges have reached the end of their service life and are seriously under capacity, whether to vehicular traffic they support or navigation traffic that they impede.
The goal of this themed issue is to provide a broad overview of some recent major bridge replacement and rehabilitation projects in North America. The eight papers selected include six bridge replacements, a major bridge rehabilitation to enhance navigation and a new border crossing between the United States and Canada. In terms of typologies, the papers comprise three arch bridges, one suspension bridge and four cable-stayed bridges, with main spans ranging from 150 m to 850 m (492 ft to 2788 ft).
Transportation underpins the economic fabric of North America, with major bridges playing a crucial role. The projects presented herein cover the entire continental geography. From the high-seismic west coast to major river crossings in the central region and on to the hurricane-prone eastern seaboard, the designers have developed elegant solutions to the unique challenges each project presented them with.
Major bridges possess a certain iconic character due to the combination of scale and structural system. The incongruity of the east bay (through truss) and west bay (suspension) spans of the San Francisco Oakland Bay Bridge is a prime example where the replacement of the east bay truss spans (Nader and Maroney, 2023) needed to be addressed. The unusual self-anchored suspension bridge is a direct outcome of the need to deliver a unique iconic structure much more in harmony with the iconic bridges in San Francisco Bay. In contrast, the raising of the Bayonne Bridge (Spoth and Haight, 2023) is something of the opposite set of circumstances; rather than creating a new iconic bridge it was necessary and appropriate to preserve an enduring legacy of Othmar Amann's ingenuity and ‘a major work of structural art’ (Billington, 1985: p. 140). The surgical reprofiling of the roadway within the iconic through arch is as large a scale programme of bridge preservation as has been completed in North America.
Many major river crossings are invariably associated with deep soft soils, and generally poor foundation conditions. Just getting out of the water is often the most challenging and expensive part of a major bridge project. Several of the papers herein discuss unique deep foundation systems, including some of the longest drilled shafts and the deepest driven piles in North America (Wynperle et al., 2023). In other cases, past industrial use has resulted in contaminated subsurface conditions, where excavation must be limited to the minimum extent possible, resulting in the pile caps constructed at the ground surface (Martin et al., 2023). For in-water foundations, water-line pile caps have become commonplace to both minimise costs and construction impacts to benthic habitat. These trends have pushed new designs towards large-diameter deep foundation elements and precast pile caps which require more complex logistics and larger installation equipment (Paradis, 2023).
Bridge replacements represent a particular challenge with the need to minimise disruption to vehicular and navigation traffic throughout construction. These constraints have made the projects featured in this issue decidedly more complex, often requiring the use of unique erection techniques. Float-in and float-out strategies, as used effectively for the Broadway Bridge (McCombs and Larson, 2023), are a perfect example of the value of accelerated construction techniques resulting in a five-fold reduction in vehicular and navigational impacts. Even the demolition of the existing bridges has presented major design and deconstruction challenges, which we believe would be worthy of its own series of papers.
Through-truss bridges represent the largest and most structurally deficient category of long-span bridges in North America (Farhey, 2018). While trusses have generally performed well, they are sensitive to localised damage at the connections, are a particularly challenging structural system to rehabilitate and have less intrinsic redundancy, which can lead to larger maintenance outlays and, ultimately, a reduced service life. In many parts of North America, major truss bridges are programmed for replacement and there is more available funding for infrastructure replacement now than there has been over the last 30 years. Most major bridges are today designed for longer service lives, with many bridge owners moving to a minimum design life of 100 years or more; with designs attentive to component replaceability and redundancy, truss bridges are becoming a less likely option. While trusses still predominate long-span rail bridges, there are growing signs that other structural forms are becoming viable alternative solutions, as showcased by the Amelia Earhart bridge replacement (McCombs et al., 2023) and the Samuel De Champlain cable stayed bridge (Nader and Mailhot, 2023). The intrinsically higher redundancy of multi-stayed cable systems, coupled with a higher ease of erection and inherent replaceability of single structural elements, makes cable-stayed bridges and network arches much more appealing structural systems than the truss bridges, as demonstrated in this themed issue.
To summarise, as the inventory of North American bridges is renewed over the coming decades, we believe more and more cable-stayed and network arch bridges will be built, alongside unique suspended spans and major transformation for existing structures. We believe that this themed issue achieves its aspiration of becoming an early collection of major bridge replacements in North America, and we hope this issue can become a point of reference for North American designers when faced with future challenges of existing major bridge replacements around the continent.
We wish to thank all authors who directly contributed to this themed issue, as well as all the reviewers and the editorial team for having made this themed issue possible. We hope you will find these papers to be both useful and inspiring. We would like to invite the readers to directly contribute to this themed issue by submitting their discussions on the content of the papers, or by directly contacting the authors of this editorial with suggestions for future projects you would want to us to develop further.
