Imagine steering your car onto an 85-ft. high exit ramp when suddenly it looks different. It appears to be “out.” Before you know it, you feel the road disappear beneath the wheels of your vehicle. It feels like the towering highway is collapsing as you begin driving over the ruptured lane. Suddenly, your car loses power after lunging over a 2-ft. drop. One section of the span has come loose and is wedged perilously against the edge of a tall, concrete pier. Your headlights go dark. Your radio goes silent.
That was what happened to a motorist traveling on the northbound lane of an access ramp that links Albany, NY’s, Interstate 787 to the Dunn Memorial Bridge on July 27. Thankfully, that motorist became the one — and only — traveler to drive over the shattered roadway before authorities were notified.
After being informed of the situation, the New York State Department of Transportation (NYSDOT), the State Emergency Management Office and Albany police immediately shut down the ramp — the highest in the 35-year-old tangle of roads. Unsure of the extent of the damage, officials also closed the entire cloverleaf and most of the streets that go under the elevated highway until it could be determined what caused that section of the access ramp to separate.
No Time Wasted
Just days after the collapse, workers completed a temporary reinforcement job that involved securing the two segments atop the concrete pier with a steel apparatus.
“They attached two segments of two girders that meet and made them one so the damaged span wouldn’t slip off,” Paul Wells, chief engineer of NYSDOT, reported. “It [the span] was in a precarious position. It was important to stabilize it so we didn’t lose it and the area below remained safe.
“We could see what happened up there, but it was important for us to know the cause. We thought this was a pretty sound bridge,” said Wells.
The eight-lane bridge opened on Dec. 1, 1970, replacing a four-lane span built in 1933. At its last inspection in November 2003, the bridge was found to be in “good” condition. It received a rating of 5 on a scale of 1 to 7, with 7 being equivalent to a new bridge. Based on a two-year schedule, it would likely have been inspected again this past fall.
In August, NYSDOT released inspection reports showing that bridge bearings beneath the broken ramp segment had been rated poorly in that November 2003 inspection. The condition of the bearings was rated at “2” on a seven-point scale. That grade is used to shade between 1 (totally deteriorated or in failed condition) and 3 (serious deterioration or not functioning as originally designed).
What Went Wrong?
Initially, experts said the problem bearings could have been a factor in the break. Other potential contributors — including design, traffic loads, weather and maintenance — also were examined.
Acting NYSDOT Commissioner Thomas J. Madison Jr. noted that the column involved was the highest in the bridge and ramp network. It was on a relatively sharp curve and the breach followed a period of very warm weather. Although heat can cause pavement to expand and break up, in this case the section of the bridge appeared to have dropped, indicating the problem might involve more than heat expansion. The roughly 2-ft. shift at the top of the pier suggested it was bent or tilted.
Michael Rourke, a professor of civil engineering of Rensselaer Polytechnic Institute, opined that movement of the section that remained in place could have pushed the segment that fell from the supports. That movement could have resulted from heat expansion or it could have dropped when the pier tipped away. Another possibility, Rourke said, could have been a recent disturbance of soil near the pier’s base.
For starters, a pair of steel towers were built 15 ft. north and south of the break to help support the ramp, which came off its footing and fell to the top of the concrete pier. To build those towers, Erector-set-like steel pieces were unloaded from flatbed trucks and piled underneath the overpass. A backhoe dug a trench around the pier in which a concrete pad was poured. The towers were constructed on top of that. Hydraulic jacks were then placed on top of the towers to help elevate the 300,000-lb. ramp sections back into place. Within days after the temporary support towers were secured engineers slowly raised the fractured ramp up from the concrete pier.
Once the towers were lifted into place the girders were secured to the metal towers. Officials believed the girders would prevent the bridge from further shifting and buy time for repairs. They would not, however, be strong enough to allow motorists to drive over the bridge.
The closure of the bridge forced the approximately 36,000 motorists who use it to drive between Rensselaer and Albany every day to seek alternate routes. Approximately 15,000 daily drivers use the expressway between the bridge and the Empire State Plaza.
The ramp remained closed while the towers were erected. The left lane of the ramp from I-787 southbound to the Dunn Memorial Bridge and Rensselaer, which passes under the broken ramp, also was closed because a section of one of the towers passed through a hole in the deck and pavement. The ramp reopened to traffic on Nov. 18. NYSDOT officials reported that minor repairs still needed to be made. As a result, the ramp again was shut down from Dec. 5 to 12.
Following the collapse, NYSDOT engineers reported that the top of the pier was tilted within two days of the fracture, but until the ramp sections were lifted, it could not be determined how much permanent damage had been sustained. The 400-ton, steel-reinforced concrete pier bent approximately 16 in. to the south, said NYSDOT Spokesman Peter Graves.
As a result, “Once the jacking operation was complete, our structural engineers … concluded that it simply was not salvageable. In the interest of safety, the best course of action was to replace it,” Graves said.
The force of the shifting ramp caused the reinforcing steel imbedded in the concrete pier to bend. Once the reinforcing steel yielded, it lost its ability to take on weight and, as a result, there was no way it could be repaired. Engineers believe the pier didn’t give until after the ramp broke.
Cause Finally Determined
Months after its collapse, New York State transportation officials announced that an unusual combination of conditions, including a bent concrete pier, faulty bearings and extreme temperature swings, resulted in the failure of the busy elevated ramp.
In a statement issued by acting State Transportation Commissioner Thomas J. Madison Jr., NYSDOT, said, “The probable cause, in a phrase, was a set of factors — unique, unforeseen and unprecedented — that converged simultaneously.”
In spite of the low marks the bearings received during inspections over the past 10 years, it was apparent from the announcement that they were not — on their own — enough to cause the July 27 break. While repairing or replacing the tilted bearings could have prevented the ramp from breaking, NYSDOT engineers said it wasn’t perceived as urgent because other conditions that were key contributors to the failure were unknown.
NYSDOT’s investigation also revealed several previously unknown circumstances that contributed to the ramp’s failure and has led to revised procedures for inspecting bridge bearings.
Under the new guidelines, bearings in such poor condition as those located where this ramp failed will receive a red flag to incite immediate attention.
NYSDOT has bolstered other bearings in several locations, including eight sections of the elevated ramp network that connects I-787, the Empire State Plaza and the Dunn Memorial Bridge. Also included on the list are the Alexander Hamilton Bridge between the Bronx and Manhattan and a bridge carrying the Robert Moses Parkway over the Fire Island inlet on Long Island.
Even so, Madison said the circumstances leading to the ramp failure were unique to the interchange — built between 1970 and 1973 — and the concrete pier that supported several sections.
In its report, NYSDOT investigators detailed a series of developments during the ramp system’s life that led to a shift that created a 19-in. vertical gap between two sections of road at the interchange’s highest point.
• The first indication that the ramp’s rocker bearings were behaving strangely occurred during a 1985 inspection. Bearings on the segment north of the pier had become misaligned and tilted to the north. Rocker bearings are designed to rock back and forth to accommodate bridge sections’ natural expansion and contraction with heat and cold.
• The curved bottom surface of the tilted bearings corroded over the years, impairing their ability to roll back upright when the temperature or other conditions changed.
• Over time, the tilted bearings pushed constantly on the concrete pier, which gradually flexed to the south.
• Ultimately, bearings on the section to the south of the pier also began to tilt more, putting pressure on the pier and bending it farther.
Experts suspect that fluctuating temperatures along with some unusual traffic load caused the concrete pier’s final flex southward. The bearings supporting the ramp segments tipped over and the segment to the north of the pier crashed down. The edge of the highway panel caught on the edge of the pier, narrowly avoiding a total collapse.
The concrete pier met all the design specifications when it was erected and the concrete was durable. The reinforcing steel inside, however, turned out to be deficient for the conditions created by the heavily traveled ramp.
The bearings had been rated at a level of 2 since 1995. Nonetheless, officials have said the bearings appeared to be stable. Replacement was planned for 2010.
Rocker bearings — standard parts in many bridges built in the early 1960s and 1970s — are no longer commonly used. The concrete pier being built to replace the damaged one will be topped by elastomeric bearings, flat structures with layers of steel and synthetic rubber.
’Road’ to Repair
Work on dismantling the old pier began at the end of August. Long drills were used to bore holes into the top section. Large mechanical chipping hammers were employed to break the tower’s concrete into smaller pieces. The new pier was poured on site and constructed in sections.
As a precautionary measure, several ramps around the Dunn Memorial Bridge received backup bolstering to avoid another collapse while NYSDOT officials explored why the first section of the ramp dropped. New steel pieces were installed to guard against collapse if bearings in another location should fail. That way, if, for some reason a bearing shifted, the short piece of girder would be in place to hold up the span.
“The work should not be perceived as an indication that investigators were focusing on problems with the rocker bearings that support the ramps as a likely cause of the rupture,” officials proclaimed. “These extraordinary precautions were taken because the shift occurred.”
The work performed was similar to what was done on three different piers days after the rocker bearings tipped. Reinforcement was added at five locations on the ramp exiting the bridge to I-787 North as a result of a more detailed follow-up inspection to those that were conducted in the week and days that followed the July 27 shift.
Crews welded short pieces of H-beams to the steel plate on top of the piers. Rather than be allowed to move too far and tip the rocker bearings, the span rests on the H-beam. Engineers said the measure would work as a stopgap but if bridges are not allowed to move, other problems will develop.
The five locations receiving reinforcements are the Dunn Bridge ramp westbound to I-787 northbound; the South Mall Expressway eastbound to I-787 northbound; the South Mall Expressway eastbound to I-787 southbound; the ramp from I-787 northbound to the Dunn Bridge eastbound; and another pier on the ramp from I-787 northbound to the Empire State Plaza.
The estimated cost of the stabilization, investigation and repairs to the broken ramp is between $1 million to $1.5 million.
NYSDOT plans to replace 67 of the low-rated rocker bearings with elastomeric bearings on six piers next year. Bearings on nine additional piers in the ramp system will be replaced in 2007. Total cost is estimated at $3.5 million. CEG