Work Speeds Ahead on Replacing Century-Old Bridges With Tunnel

Lessons Learned: 20 Years After a Tragedy in NYS

Wed April 18, 2007 - Northeast Edition
Laurie Mercer

On the New York State Thruway, April 5, 1987, two Saratoga County women were on their way to a baby shower. Three Niagara Mohawk employees were coming home from a bowling tournament in Syracuse. An 18-wheeler was hauling a load of wood pulp headed for Orange County.

Sid Brown, a news photographer with the Daily Gazette in Syracuse just happened to be nearby at the Schoharie Creek Bridge photographing a father and son team of firefighters. His camera was temporarily stowed while he took the names of his subjects.

What happened next was a “high screaming noise caused by the bending of steel. It was a real strange sound,” Brown said. “We looked up. We saw two cars go into the river heading west. Then we saw the truck heading east.” They were helpless to stop what happened next.

The 540-ft. (164.6 m) span began to drop in pieces 80 ft. (24.4 m) into the flooded Schoharie Creek below.

In all, 10 people drowned; one body was never recovered. Within hours, then Governor Mario Cuomo arrived at the accident scene and demanded that every bridge in New York State be inspected immediately. The unprecedented failure of a major interstate bridge stunned officials. But on the heels of personal loss, the wheels of reform began to gain momentum.

How Was It Built in the Early 1950s?

Investigating engineers eventually concluded that a series of design, construction and maintenance failures led to the collapse, and that four key factors contributed to the undermining of pier footings through something called “scour.”

Schoharie Creek flows from the foot of Indian Head in the Catskill Mountains to the Mohawk River. The bridge is one of several bridges constructed in the early 1950s by the New York State Thruway Authority (NYSTA) for a 559-mi. (900 km) superhighway, running across New York State in the Mohawk Valley northwest of Albany.

The bridge design, submitted in January 1952, consisted of five simply supported spans with nominal lengths of between 100 and 120 ft. (30.5 and 36.5 m) Concrete pier frames supported the bridge spans with abutments at each end.

The pier frames were constructed of two slightly tapered columns and tie beams. The columns were fixed within a lightly reinforced plinth, which was positioned on a shallow, reinforced, spread footing. The spread footing was supposed to be protected by a deep layer of dry riprap. This never happened. The superstructure was made up of two longitudinal main girders with transverse floor beams. The skeleton of the 8-in. (20.3 cm) thick bridge deck consisted of steel stringers.

The Schoharie Creek Bridge was completed around October 1954. Nearly a year later the bridge took a heavy hit in a 100-year flood. Experts said the damage done at that time may very well have had a bearing on the collapse of the bridge three decades later.

Among the lessons learned, the as-built plans do not reflect the true condition of the bridge. One discrepancy — the as-built plans show that sheet piling would be left in place to protect the piers. In reality the sheet piling had been removed after the bridge’s construction.

More Pier Modifications

In 1957 plinth reinforcement was added to each of the four piers to correct the problem of vertical cracking. Shortly after construction, the bridge pier plinths began to form vertical cracks because of high tensile stresses in the concrete plinth. The original bridge designs called for reinforcement to be placed in the bottom portion of the plinth because the designers had confidence that the concrete tension could resist the bending stresses without reinforcement.

Structural weaknesses in the design increased when reinforcement was added incorrectly. In one college case study of the accident the author wrote, “To be properly anchored the tension reinforcement must be extended past the supports — in this case, into the columns. Obviously, this was not done.”

He said there would have been difficulty to extend the reinforcement through the columns without replacing them. Yet again, weakness leads to disaster. “Ironically, because the added plinth reinforcement was not adequately anchored, it may have contributed to the brittle and sudden nature of the subsequent collapse, by supporting the plinth until most of it had been undermined.”

By fall 1957 these and other problems, including bearings out-of-plumb, roadway approach slabs that settled, poor drainage on the roadway, and deficient supporting material of west embankment dry stone, had been corrected.

But the time bomb had begun to tick.

Spring Floods Bring Disaster

Like the 100-year flood the bridge endured in the 1950s, the morning of April 5, 1987, saw a spring flood with rainfall and snowmelt producing conditions of an estimated 50-year flood. The accident began with one car and one tractor-semi trailer on the bridge as pier three toppled into the flood-swollen creek. This quickly caused the collapse of spans three and four while three more cars, unable to stop in time, fell into the gap.

Pier two and span two fell nearly 90 minutes after span three dropped. Pier one and span one shifted two hours after that.

Six days later a large section of the Mill Point Bridge, about 3 mi. upstream from the accident, also collapsed. Fortunately the NYSDOT had already closed the bridge fearing that flooding had also eroded that bridge’s foundation.

What Happened Next?

A cofferdam was built around the site to be dewatered and excavated, both for investigation and to construct a replacement bridge. Two separate investigative teams were directed to pinpoint the causes for failure. The primary smoking gun turned out to be extensive scour under pier three. Scour is “the removal of sediment from a streambed caused by corrosive action of flowing water.”

Four key factors contributed to the severity of the scour. First, the shallow footing used could be undermined. Second, the foundation of pier three was bearing on erodable soil. High velocity floodwater had penetrated the bearing stratum. Third, the area around the footing due to excavation was supposed to be backfilled with riprap stone to the entire depth of the excavation. This was not done. Backfill instead was erodable soil topped off with dry riprap. Finally, riprap protection, inspecting, and maintenance were faulty.

Scouring, in fact, began just after the bridge was built. Riprap placed at construction was probably washed away in the 1955 flood and never replaced. Once the backfill was exposed, peak flows removed backfill material, which was replaced by sediment settling into the scoured holes.

In the contract issued in 1980 for maintenance work, all reference to new stone riprap had been deleted by a non-engineer state employee who decided, after viewing the site from shore, that it was unnecessary. The original bridge design called for leaving the sheet piles—used to keep water out of the excavation area during construction — around the piers. Specified riprap would then fill the area between the pier footings and sheeting, but sheet piles were not left in place.

Finally there was a loss of support capacity, primarily due to scour. The upstream end of pier three fell into a scour hole approximately 9 ft. (2.7 m) deep as the bridge began to collapse.

Even though the Schoharie Creek Bridge had been inspected annually or biennially since 1968, an underwater inspection of pier footings had never been done.

An Infamous Accident Prompts Reforms

The upside of the Schoharie Creek Bridge collapse was a series of reforms designed to prevent similar incidents. The disaster helped fuel a Thruway toll increase and the passage of a state Transportation Bond Act for more maintenance and repairs. Moreover, a call for far more rigorous bridge inspections was answered.

Keith Giles, senior vice president and Albany branch manager of the consulting engineers Greenman-Petersen, was superintendent of bridge maintenance for the Thruway Authority at the time of collapse. Giles, quoted in the Albany Times Union said, “The Schoharie Bridge collapse led to significant improvements in bridge management, including inspection, maintenance and operations and design and construction.”

When the Schoharie Creek Bridge was built, the tools of the day were not adequate for predicting scour. Today it is important to accurately predict the effects of scour and to design bridges to resist those effects. Three important lessons were learned:

• Correct selection of a critical storm as a guide for the design of bridges crossing water.

• Regular inspection of superstructure, substructure and underwater features.

• Erosion protection around piers and abutments susceptible to scour.

Why Do Some Disasters Resonate?

In addition to the deaths of 10 people, closing and detouring the critical east-west interstate for repairs had economic consequences. Moreover, close to $45 million went to reconstruction and related costs, in addition to millions paid in legal settlements reached with the victims’ families.

Even though 20 years have passed, the impact of the Schoharie Creek Bridge collapse remains visible in bridge construction and inspection systems made stronger from another bridge’s collapse.

In addition to better engineering and inspection, bridge designs now routinely have redundant features to help avert disaster if another piece of the structure fails. CEG