adapted from AASHTO’s Guide Specifications using a vulnerability assessment calculation. The AASHTO Guide Specification and Commentary for Vessel Collision Design of Highway Bridges , 1 st Edition (1991) introduced the vulnerability assessment calculation, and AASHTO included this calculation in its updated 2009 Guide Specifications with 2010 interim revisions. Neither FHWA nor AASHTO require a bridge owner to complete a vulnerability assessment for a bridge designed before the release of the 1991 guidelines. Additionally, there is no requirement nor code that establishes a risk threshold for existing bridges per AASHTO.
MDTA Activities following the Key Bridge Collapse
Following the allision with the Key Bridge and prior to the NTSB’s interim report, MDTA engaged the consulting engineering firm Moffatt & Nichol (M&N) to conduct an initial evaluation of the risk of vessel collision with the Bay Bridge (eastbound and westbound bridges) in accordance with AASHTO methodologies for risk analyses of the main channel of the Bay Bridge. This analysis developed options for risk reduction and provided concept design of protective structures. The cost of the evaluation was approximately $600,000 and was fully funded by MDTA capital funds. Historic traffic data for vessels transiting under the Bay Bridge was obtained from USCG shipborne Automatic Identification System (AIS) database for 2023 and analyzed to include transits under the Bay Bridge by containerships, bulk carriers, tankers, car carriers, general cargo ships, cruise ships and barges. AIS information relates to encoded Very High Frequency signals broadcast from vessels containing information such as vessel position, maneuvering characteristics, loading conditions, and navigational status. The AIS database includes various parameters describing each vessel such as: Maritime Mobile Service Identity identification number, vessel name, vessel type, status, speed, position, heading, beam (width), length overall, draft, deadweight tonnage, and timestamp when data was collected. The analysis was conducted in accordance with the procedures outlined in the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications, 10 th Edition (2024), which applies primarily to new bridges, and in accordance with the procedures contained in the AASHTO Guide Specification and Commentary for Vessel Collision Design of Highway Bridges , 2 nd Edition (2009 with 2010 interim revisions), which includes recommendations on the evaluation of existing bridges. The 2009 AASHTO Guide Specifications are significantly more comprehensive in the analysis and design of new bridges for vessel collision than the abbreviated requirements of the 2024 AASHTO LRFD Bridge Design Specifications (Code). The objective of the vessel collision risk assessment was to evaluate the effectiveness of proposed vessel collision risk reduction measures at the Bay Bridge; namely, to evaluate the annual frequency of collapse in the current condition, analyze the effects of operational measures on the risk of collapse, and develop protection structure configurations that achieve the recommended annual frequency of collapse.
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