Reinforced-Concrete Beam and Girder
Although not as picturesque as the covered bridges, the stone, steel, and concrete arches, or the metal trusses, reinforced-concrete beam and girder nonetheless play an important role in the history of bridge building in Vermont. Most bridges that are traversed on Vermont's roadways today are reinforced concrete or steel. The structure is quite simple - a horizontal support across an open span - and the bridges are easy and inexpensive to construct. The evolution of the highway system in Vermont is closely linked to the evolution of the beam and girder bridges. As automobile traffic increased after WWI, the beam and girder bridge was accepted as the best bridge type for the increasing traffic. As well, after the 1927 flood, a large number of beam and girder bridges were constructed. Reinforced-concrete beam and girder bridges can be divided into two categories: standard (1917-1940) and conventional (1940-1975).
Standard Reinforced-Concrete Beam and Girder Bridges (1917-1940)
This was the first phase of high-way bridge construction. T-beam, slab, through girder, and box culverts were constructed during this phase.
T-Beam: The T-beam bridge type, seen below, was developed during the early 1920s.
Slab bridges were used for very small spans and were constructed as early as 1912 in Vermont. Originally the concrete slab bridge only spanned 5 to 20 feet, but by 1938, slab bridges were built to a maximum of 25 feet. Solid side rails with recessed panels were common on slab bridges.
Through Girder: Also called a side girder, the through girder bridge consists of massive, paired girders, one on each side of the deck. Most were constructed between 1922 and 1925.
The box culvert is the smallest and most ordinary of the concrete bridges. These box culvert bridges were constructed with floors, sidewalls, abutments, and decks of concrete, forming what looks like a concrete box.
Decorative Railings: Many of the bridges constructed during this phase had decorative railings, and many were scored to give the appearance of stone. Cable railings, seen in the bottom image, became quite popular during the 1930s.
Conventional Reinforced-Concrete Beam and Girder Bridges (1940-1975)
This second phase of high-way bridge building demonstrated improvements over the standard bridges built between WWI and WWII. Steel beam and girder bridges were favored at this time, but concrete continued to remain competitive. Rigid-frame, continuous, prestressed, and post-tensional girder bridges were constructed during this phase. Many of these bridge types are not easily distinguishable on the surface but are, instead, characterized by interior construction: i.e. the pre-stressing of the beams. However, post-war reinforced concrete bridges are generally larger than their early counterparts, particularly those dating from the late 1950s and 1960s interstate construction phase.
Rigid-frame bridges are not technically beam and girder construction, but rather they consist of one single, unified frame, as seen in the image below. Vermont began to construct this type of concrete bridge in 1942. Notice the very shallow arch that forms on the underside of the deck. This is characteristic of the rigid-frame bridge.