Case Study #2: Riverside Avenue Bridge,
Greenwich
(click on image for larger view)
Bridge: Cast-iron and wrought-iron 1871 through truss
Problem: Inadequate load-bearing capacity (posted at 11 tons), deteriorated sidewalk
Solution: Install new I-beam bridge within historic structure
Because it is adjacent to a railroad station, Riverside Avenue Bridge carries substantial traffic. Bypass of the historic structure with a separate, parallel bridge would have required taking property from the surrounding residential neighborhood and disrupting the existing street pattern.
The solution, designed by PRC Engineering of Stamford, Connecticut, was to build a new beam bridge entirely within the old truss. By constructing two intermediate piers between the tracks, the depth of the new structure was held to 24 inches. The truss's floor beams (themselves a 1925 alteration) were partly cut away to create clearance for four I-beams, which carry a new concrete-slab deck. The truss's original lower joints were retained intact. The new bridge, which is structurally independent of the truss, provides a roadway width of 21 feet, with 14 feet of vertical clearance under the truss's portals.
Other aspects of the 1989 rehabilitation, which cost $1.5 million, affected the appearance of the bridge but not its historic integrity. On the east side, a new free-standing sidewalk and stairway structure connects with the train platform below, and roadway barriers along the inside of the truss provide protection for the bridge's easily damaged cast-iron uprights.