Case Study #1: West Cornwall Bridge,
Cornwall-Sharon
(click on image for larger view)
Bridge: Covered timber truss built in 1864
Problem: Deteriorated trusses and floor, posted for four
tons
Solution: Install steel deck as secondary structural
system
West Cornwall's covered bridge has a long history of problems. In 1939
the bridge was posted for a ten-ton limit after a state inspector
observed, "Only the grace of God now prevents the structure from
collapse." A few years later an oil truck (weighing twenty tons) plunged
through the floor. Flood waters in 1955 nearly swept the bridge away. In
1961, a huge ice jam built up around the pier and abutments; only
dynamiting the ice saved it from certain destruction.
A 1961 law required
the State to maintain the covered bridge even if it was withdrawn from
service, and local citizens opposed building a new high-level bridge
because of its impact on the village. Responding to their concerns, the
State developed a solution that retained the old bridge.
The bridge was
raised up about two feet, and a steel orthotropic deck was inserted to
carry the traffic independently of the timber truss. The deck consists of
two large side beams and a 3/8-inch thick top plate reinforced by 9-inch
U-shaped ribs, all welded together. The new structure is barely visible
on the exterior, and a new wood-plank floor conceals it on the inside of
the bridge. The timber truss was maintained intact, including the entire
lower chord. The 1973 project won the Federal Highway Administration's
outstanding historic preservation award.
The bridge remains one-lane
wide, and vertical clearance is only eleven feet. However, an
unrestricted crossing is just four miles to the south.