MassDOT's Bridge Section maintains a statewide database that includes technical data on over 8000 bridges/culverts in Massachusetts. This database is intended to include, at a minimum, all those bridge/culvert structures in the Commonwealth that carry, or pass over, a highway open to the public. As part of MassDOT's effort to evaluate the historical significance of all those bridge structures that fall under its purview, MassDOT's Historic Bridge Specialist has screened the statewide bridge database to identify all known examples of the eight bridge structural types thought to possess the greatest potential for historical significance. Comprehensive inventories of all of the known examples in the database of these eight structural types are now nearing completion. A preliminary list of Massachusetts bridges that have already been identified as individually listed in or as eligible for listing in the National Register of Historic Places is now available at the following link:
For a brief description of each of the eight structural types selected for comprehensive inventory (and a list of those other structural types represented by National Register-listed or - eligible bridges in Massachusetts), please scroll down the page or click on the following links:
The movable bridge type, in general, includes any form of bridge having one or more spans capable of being raised, turned, lifted or retracted from its normal position to create an opening for the passage of marine traffic. When the comprehensive inventory of the movable bridges in the MassDOT database was completed in 1985, there were 44 examples then in existence, representing three major sub-types:
- Bascule - a movable leaf pivots vertically away from the channel on an axis point (whether fixed or rolling)
- Retractile - a movable span pivots horizontally to open the channel
- Swing - a movable leaf is drawn away horizontally from the channel on railroad-like tracks
The primary structural elements of a timber truss bridge are jointed wooden structures that are essentially open-webbed beams - i.e. a solid beam has been reduced to a rigid framework composed of a series of stiff triangles, known collectively as a truss. The 17 timber truss bridges identified in MassDOT's comprehensive inventory of this structural type, completed in 1986, included:
- Through Trusses: the traditional "covered bridge" where the trusses are laterally braced overhead and are covered by a roof
- Pony Trusses: a truss bridge with insufficient height to allow the use of lateral bracing overhead
Bridges of both configurations were built using trusses in a variety of patterns, including the Howe, Town/lattice, Burr arch/truss, and variations of the traditional multiple kingpost/multiple kingrod type.
These bridges are similar to timber truss bridges, except that they are built entirely of metal instead of wood. 201 metal truss bridges were identified in MassDOT's comprehensive inventory of this structural type, completed in 1988/89, representing a wide variety of 19th and 20th century truss designs. The earliest of these metal bridges were constructed of wrought and/or cast iron; by 1900 all of the metal trusses were being built of steel. Some of the earliest of the metal truss bridges in the database, dating from the 1870's and 1880's, followed truss patterns developed and patented by innovative Massachusetts engineers like Thomas W. Pratt and Charles H. Parker.
The iron, and later steel, rib arches of the 19th and 20th centuries expanded the range of the arch structural type, by substituting lightweight metal ribs for the heavy stone barrels of traditional masonry arch design. Only 13 metal rib arch bridges were identified by the Historic Bridge Inventory, with examples ranging in date from 1896 through 1950. These 13 bridges, however, represent a surprisingly complete cross-section of the various approaches to metal rib arch design, and include examples of deck, through, and half-through (rainbow) roadway placement; of the fixed, tied, 2-hinged, and 3-hinged arch forms; and of the solid, braced (trussed), and spandrel-braced rib types.
A suspension bridge has a deck that is supported in a nearly horizontal position by being suspended from chains or cables that pass over towers and are anchored into the riverbanks at the extreme ends. Only one historic suspension bridge is included in the Historic Bridge Inventory - the 1910 Essex-Merrimac Bridge between Newburyport and Amesbury. This bridge follows an essentially modern suspension bridge design, utilizing steel wire cables and a deck-stiffening truss, but was built to replace the historic "Chain Bridge," a 19th century landmark much beloved by local residents.
Concrete arches, most dating from the early years of the 20th century, combine steel reinforcing (to take tension) with concrete (to take compression) in a traditional arch structural form. The MassDOT Historic Bridge Inventory, now being completed, includes some 150 concrete arch bridges. Most are conventional barrel arches with filled spandrels, but there are a handful of the more dramatic open-spandrel rib arch designs as well. And while most examples appear to have utilized standard deformed bar steel reinforcement in typical patterns, some interesting early examples of the Kahn, Melan, and Luten reinforcing systems have been identified.
Stone arch bridges are the most numerous historic bridges in Massachusetts, and these structures, if well designed and well built on solid foundations, can practically last forever. Currently there are 265 identified stone arch bridges in the Historic Bridge Inventory, with more likely to be added as the survey of this type progresses. The oldest stone arch bridge in the MassDOT database is the 1764 Choate Bridge in Ipswich. The newest, in the town of Douglas, was designed by the Massachusetts Department of Public Works, and erected in 1926.
Stone slab bridges, of which 56 are currently known in the MassDOT Historic Bridge Inventory, represent an ancient structural type - a simple beam (in this case a slab of stone) spanning over a waterway. The oldest known stone slab spans in the database, the three approach spans flanking the later (1849) stone arches of the Adams Street Bridge in Dorchester Lower Mills, were probably built in 1765. The inventory of this bridge type is still in process and it is presumed that numerous additional examples of this modest structural type remain to be identified.
Most of the other structural types in which bridges have been, and continue to be, built in the United States are represented in Massachusetts. If the historically significant examples among these other types are less numerous than those for the eight listed above, there are at least few National Register-listed or -eligible examples of the following structural types in the database: iron/steel girder-and-floorbeam; iron/steel stringer; steel rigid frame; concrete rigid frame; timber stringer; and steel cable-stayed.