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NHDOT explains 10‑ton posting for Plymouth’s Green Bridge after inspection finds undersized truss member
Summary
New Hampshire DOT engineers told the Plymouth Select Board that a 10‑ton legal posting for the US‑3 (Green) Bridge over the Baker River stems from a rare as‑built error and added dead load; DOT said limited emergency and school‑bus crossings can be reviewed annually, and listed options — removing pavement, strengthening members or leaving the posting in place pending funded rehabilitation.
New Hampshire Department of Transportation bridge officials told the Plymouth Select Board that the state’s recent decision to post the US‑3 “Green Bridge” over the Baker River at 10 tons resulted from a combination of a rare as‑built fabrication error and later added pavement that together reduced the bridge’s live‑load capacity.
Bill Saffion, chief of NHDOT’s existing bridge section, said the bridge — built in 1930 and last rehabilitated in 1985 — contains truss members that were fabricated undersized because plan dimensions were transferred incorrectly during construction, and that the 1985 work added an extra inch of deck and two inches of pavement. “The compounding effect of these two errors means that the 1985 rating overestimated the available capacity,” Saffion said, and later noted, “the 10‑ton posting of the bridge here is due to a member that has no section loss; it is an as‑built condition, not an as‑inspected condition.”
Saffion and David Scott, Bureau Administrator for Bridge Design, walked the board and public through how DOT rates members, accounts for dead loads (permanent weight such as pavements) and compares available member capacity to statutory legal and certified legal vehicle loads (citing RSA 266:18 and 266:18‑A). DOT explained that an E‑series restriction (now shown in the bridge inventory) excludes larger certified vehicles, and that the 10‑ton controlling value affects both legal and certified legal classifications in this particular case.
The presentation described several immediate follow‑up steps DOT has taken and options for the town:
- Limited, annually reviewed exceptions: DOT said it has assessed specific town vehicles and a school bus and determined the bridge could safely carry those vehicles under strict conditions (single crossing, <5 mph, and an annual reassessment). Officials stressed the review must be repeated every year because condition may change.
- Do‑nothing/maintain posting: leave the 10‑ton posting in place and pursue enforcement by Troop G when over‑limit crossings are reported.
- Reduce dead load: remove or reduce asphalt on the bridge and approaches to lower permanent dead load, while recognizing that pavement and membranes protect the concrete deck from deicing salts and that removing pavement can create safety and drainage issues unless carefully engineered.
- Strengthen or rehabilitate members: commission design work and cost estimates; DOT said project 4407 is programmed with a $3.3 million budget for condition work (ad date Nov. 4, 2031), but that replacement is not currently funded and rehabilitation options must be weighed against the 10‑year plan and budget constraints.
Saffion said the 2024 consultant inspection and subsequent 2025 load rating updated earlier assumptions; an additional inch of pavement found during a 2025 curb‑reveal survey confirmed an average of three inches of pavement, which DOT incorporated into the rating and found still consistent with the 10‑ton control. He cautioned that the undersized member was not in a location subject to heavy deicing corrosion, so the issue arose from an as‑built design/fabrication mismatch rather than section loss from corrosion.
Town officials and the fire chief pressed DOT about emergency‑response implications; DOT reiterated it will work with municipal emergency‑response officials to assess allowable crossings for specific vehicles, but recommended operators follow posted limits in general and report repeated or deliberate over‑weight crossings to Troop G for enforcement.
What happens next: DOT said it will develop engineering options and cost estimates and that any change in posting or a rehabilitation plan would require engineering, budgeting and (if applicable) permitting. DOT also noted that if the town seeks to alter the driver pattern (for example, single‑lane controls) DOT could evaluate whether that changes posting outcomes.
The presentation closed with a request from DOT that the town continue to document any observed over‑limit use so the agency can pass that information to state police for enforcement and track potential impacts to the structure.
The Select Board heard the briefing and asked DOT to continue annual reassessments of any vehicles granted exceptions and to return with scenarios and cost estimates for rehabilitation or deck‑weight mitigation measures.
