Civil Engineering Reference
In-Depth Information
(1987)
The Schoharie Creek Bridge was constructed in 1953 and carried the New York State
throughway across the Schoharie Creek. It consisted of five simply supported spans
with a total distance between the abutments of 165 m (540 ft) (Figure 1-2). The
bridge carried two lanes of traffic in each direction approximately 24 m (80 ft) above
the creek bed (Figure 1-3). On April 5, 1987, during the worst flooding experienced
in years, the bridge collapsed (Figure 1-4). Four cars, a truck, and ten lives were lost
as a result. The sudden rerouting of the highway disrupted commerce on both sides
of the creek and focused attention on the collapse of an otherwise undistinguished
structure.
Investigations confirmed that scour of the pier was the primary cause of
failure. The primary defense of the Schoharie Creek Bridge against scour was the use
of dry rip-rap. This consisted of a large field of quarry stones shaped as right
rectangular prisms to inhibit rolling in flood waters. In early 1955 vertical cracks
were observed in the pier plinths. A heavily reinforced concrete element 0.9 m (3 ft)
thick was cast on top of each plinth in 1957 to control further cracking. Inspections
were conducted in 1983 and 1986, but on the second occasion high water prevented
detailed inspections of the bottoms of the piers.
During the April 1987 flood scour removed the support from the southern
portion of the plinth, thereby generating tensile bending stresses in the top of the
plinth leading to eventual fracture through the plinth and cross footing that served to
connect the two support columns (Figure 1-5). An immediate loss of support between
spans 3 and 4 was inevitable.
Lessons Learned
The failure due to scour of this bridge emphasizes the necessity for ensuring that
bridge footings are deep enough to avoid the loss of support capacity arising from
scour around the foundation. The presence of flood waters during the 1986
inspection inhibited a thorough inspection of the bridge. In hindsight, it appears to
have been irresponsible for those involved not to have re-inspected when the flood
had receded and it would have been possible to have undertaken a more
comprehensive examination of the footings of the columns. The continuing problem
with scour causing bridge collapses prompted interest in improved technology for
underwater inspection.
It is important to accurately predict the effects of scour, and to design bridges
to resist those effects. Lessons learned include:
1.
Proper selection of a critical storm for the design of bridges crossing
water.
2.
The need for regular inspections of the superstructure, substructure, and
underwater features of the bridge.
