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The Tyngsborough Bridge

A view of the Tyngsborough Bridges from Middlesex Road Senior Housing Complex/Rony Camille for

Tyngsborough, MA is probably best known today for its green painted single-arched iron bridge over the Merrimack. Constructed in 1931 as a replacement for an earlier wooden planked structure, this bridge has become the town’s emblem and more practically, a major river crossing for residents of Massachusetts and New Hampshire alike. Having fallen into serious disrepair, the green bridge was replaced by a temporary structure while work was completed to restore it.

A view of the Tyngsborough Bridge. Main (Left) and Temporary (Right). Photo Furnished by Nick DiNatale |

Municipality: Tyngsborough
Bridge Owner: Massachusetts Highway Department
Facility On Bridge: State Route 113
Feature Under Bridge: Merrimack River
Date Built: 1930 Date Rebuilt:
Overall Length: 656′ Overall Width: 41.5′

Structure Description

Designer: MassHighway in-house Design Unit
Builder: Simpson Bros. Corp., Boston Bridge Words
Main Unit: # of Spans: 1 Length of Each: 1 @ 547′
Approaches: # of Spans: Length of Each:

Structure Type:

Erected as 3-hinged steel trussed rib through arch; center hinge fixed upon completion. Arch ribs are 20′ deep tapered in end panels to a point at end hinges. Pratt-type trussing in the arch ribs with 34 panels. The 2 crown panels have crossed diagonals and heavy horizontal struts, which are pinned at the centerline. Upon completion of the bridge and with all dead load in place, the crown hinge was fixed by means of riveted plate connections between the opposing ends of the upper and lower chords. Pinned steel I hangers carry the strongly cambered deck. Deep portal frames; heavy upper lateral system, with sway bracing at panel points.

History of Bridge:

Replaced a 4-span iron whipple truss bridge built 1873.

Significance of Bridge

The 2nd oldest of 5 positively identified steel rib through arch bridges in the MassHighway database, and one of only 13 known steel rib arch bridges of any configuration in the database. One of only 2 of the 13 to utilize an open, braced rib design (the Boston University Bridge, Boston is the other). The Tyngsborough Bridge’s 547 foot main span is the longest simple steel arch span in the MassHighway database, and the 5th-longest single span among all bridges listed in the database.


Information compiled from MassDOT.