My experience with bridges has given me a new respect for all bridges.


I arrived at the Schoharie Bridge on the NY State Thruway approximately 1 hour after it collapsed and since that day my respect for bridges was heightened. The report in the previous blogs now confirms that the bridge infrastructure in the US is desperate condition. It also appears that the Canadian bridges are suspect as well. About 1 year after the NY experience, a bridge that I crossed every day on a main highway had a sign posted that directed trucks to use the left lane when crossing the bridge. I was an executive at a major trucking company and could see the dangerous practice of lane changes into a passing lane. After daily calls to the Ontario Ministry of Transport to get an answer to the directive I was stonewalled. Especially when I discussed the probable deterioration of the structure. After 2 weeks of my calls the sign was removed and I have not seen any sign of repairs. That was decades ago.

Report on the Scoharie Bridge Collapse

Schoharie Creek Bridge collapse

From Wikipedia, the free encyclopedia

Schoharie Creek Bridge

Total length
155 metres (509 ft)

Opened
October 1955

Collapsed
April 5, 1987

The Schoharie Creek Bridge was a New York State Thruway bridge over the Schoharie Creek near Fort Hunter, in New York State. On April 5, 1987 it collapsed due to bridge scour at the foundations after a record rainfall. The collapse killed ten people.

The failure of the Schoharie Creek Bridge motivated the improvement in the development of bridge design and inspection procedures.[1]

The final design for the bridge was approved in January 1952 by the New York State Department of Transportation, (previously named The New York State Department of Public Works). The design described a 155 meters (509 ft) crossing consisting of five simply supported spans with nominal lengths of 30.5 meters (100 ft), 33.5 m (110 ft), 36.6 m (120 ft), 33.5 m (110 ft), and 30.5 m (100 ft). The bridge was supported with pier frames along with abutments at each end. The pier frames were constructed of two slightly tapered columns with tie beams. The columns were fixed in place within a lightly reinforced plinth positioned on a shallow, reinforced spread footing. The spread footing was to be protected with a dry layer of riprap.[2]

The superstructure consisted of two longitudinal main girders with transverse floor beams. The skeleton of the bridge deck (200 millimeters (7.9 in) thick) was made up of steel stringers.

Construction began on February 11, 1953 by B. Perini and Sons, Inc.

Service[edit source]

The bridge was partially opened during the summer of 1954 before construction was fully completed

The Schoharie Creek Bridge (NY 1020940, New York State bridge identification number), began full service beginning in October 1954.

In the spring and summer of 1955, the pier plinths began to show vertical cracks ranging from 3 to 5 mm (0.12 to 0.20 in), as a result of high tensile stresses in the concrete plinth. In 1957, plinth reinforcement was added to each of the four piers.

Almost a year later on October 16, 1955, the bridge was damaged by a flood.

Collapse[edit source]

On the morning of April 5, 1987, during the spring flood, the Schoharie Creek Bridge collapsed. A snowmelt combined with rainfall totaling 150 mm (5.9 in) produced an estimated 50 year flood.

Pier three was the first to collapse, which caused the progressive collapse of spans three and four. Ninety minutes later pier two and span two collapsed. Two hours later pier one and span one shifted. A National Transportation Safety Board investigation suggested that pier two collapsed because the wreckage of pier three and the two spans may have partially blocked the river, redirecting and increasing the velocity of the flow of water to pier two.

Six days later, 5 km (3.1 mi) upstream, a large section of the Mill Point Bridge collapsed. The bridge had been closed since the flood as a precaution that its foundations had also been eroded.[3]

Casualties[edit source]

At the time of the collapse, one car and one tractor-semitrailer were on the bridge. Before the road could be blocked off, three more cars drove into the gap. During the following three weeks, nine bodies were recovered from the river. The body of the 10th victim was recovered in the Mohawk River in July 1989.

Causes of failure[edit source]

It was concluded that the bridge collapsed due to extensive scour under pier three. The foundation of the pier was bearing on erodible soil, consisting of layers of gravel, sand and silt, inter-bedded with folded and tilted till. This allowed high velocity flood waters to penetrate the bearing stratum.

The area left around the footing was not filled with riprap stone, but instead was back-filled with erodible soil and topped off with dry riprap. Riprap protection, inspection, and maintenance were determined to have been inadequate.

The investigations showed that the scouring process under the piers began shortly after the bridge was built. At the time of the collapse, the upstream end of pier 3 fell into a scour hole approximately 3 meters (9.8 ft) deep. Investigators estimated that about 7.5 to 9 meters (25 to 30 ft) of the pier was undermined.

The design for the Schoharie Creek Bridge originally called for leaving sheet piles in place (which are used to keep water out of excavation areas during construction). The riprap would then have filled the area left between the pier footings and the sheeting. However, this sheeting was not left in place.

Another reason for the collapse was the weight of the riprap. The design specification called for riprap with 50 percent of the stones heavier than 1.3 kilonewtons (290 lbf), and the remainder between 0.44 and 1.3 kN (99 and 290 lbf). Investigators found that heavier riprap weights of 4.4 to 6.7 kN (990 to 1,500 lbf) should have been specified.

Other considerations as to the cause of the collapse included design of the superstructure, quality of materials and construction, inspection and maintenance. Investigations found that these factors did not contribute to the collapse.[4]

Twelve hours before the Schoharie Creek Bridge collapsed, due to heavy rainfall, the rush of water through the Blenheim-Gilboa Pumped Storage Power Project 40 miles (64 km) upstream hit a historic high. To cope with the overload, the dam released water into the Scholarie Creek according to the rate at which it was entering the reservoir from upstream. This was a possible contributing factor to the failure.[5]

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