The recent collapse of the Interstate 695 Francis Scott Key Bridge in Baltimore, Maryland, is a tragic reminder of how vulnerable major highway bridges over navigable waterways are to failure due to extreme event loads. Crossing over one of the busiest shipping routes in the United States, after the Neopanamax container ship MV Dali struck one of the bridge piers, the impacts rippled throughout the city, bringing both vessel cargo movement and vehicular commuter traffic to a halt.
However, this was not the first such major event. By the end of the 1970s, those who monitored the safety of world waterways observed an uptick in the frequency and severity of vessel collisions with bridges. The annual occurrence of ship-bridge collisions worldwide increased from 0.5 between 1960 and 1970 to 1.5 between 1971 and 1982 (NRB 1983), a troubling trend for what had been rare occurrences. Ultimately, it was the 1980 Sunshine Skyway Bridge collapse in Tampa, Florida, that spurred national transportation and research agencies to partner and launch studies into these catastrophic, costly, and often tragic events. It became clear a void existed in engineering guidance for bridges that spanned navigable waterways and were vulnerable to ship and barge collision. In response, the American Association of State Highway and Transportation Officials embarked on establishing appropriate design guidelines.
Moffatt & Nichol staff played an important role in the development of what was to become the 1991 American Association of State Highway and Transportation Officials (AASHTO) Guide Specification and Commentary for Vessel Collision Design of Highway Bridges. The landmark publication provided the bridge design community with the ability to evaluate the risk of vessel collision and estimate the associated magnitude of impact forces. AASHTO has since updated the Guide Specification in 2009 and 2010 to integrate lessons learned from the use of the original 1991 document. The vessel collision requirements of the guide specification are now part of the AASHTO Bridge Design Code itself and are mandatory for design. Today, these AASHTO provisions are an indispensable tool for evaluating vulnerability of highway and railroad bridges to potential catastrophic collapse due to ship and barge collisions for both relatively simple and complex bridge and navigable waterway geometries.
Moffatt & Nichol has since continued to expand and refine the company’s bridge practice focus on bridge-vessel safety and has been a contributor to Federal Highway Administration, The World Association for Waterborne Transport Infrastructure (PIANC), and International Association for Bridge and Structural Engineering guidance and training programs to address bridge-vessel collision design. Moffatt & Nichol has brought this global skill set to bridge design and protection projects across the globe.
However, as practitioners it is essential to be ever vigilant to changing conditions. When it comes to vessel collision, the risk analysis, factors, and conditions—such as vessel traffic volume and characteristics, channel geometry, water elevation and scour, impact loads—are unique to each bridge/highway structure and subject to change over time. As has been seen from the 1960s forward, commercial vessel size continues to increase, impacting design geometry and air draft. In recent years, changing weather patterns have brought record rainfall and storm surge, affecting navigation channels, as demonstrated when, during a two-week period in January 2016, floodwater conditions caused six separate barge collisions with highway and rail bridges crossing the Mississippi River, resulting in temporary closures for structural inspections.
For an in-depth look at the application of AASHTO vessel collision principles, as presented at the 2018 PIANC-World Congress, visit
https://conference-service.com/pianc-panama/documents/agenda/data/full_papers/full_paper_46.pdf
For questions or additional information related to our bridge services, please contact us at publicrelations@moffattnichol.com.
