By THOMAS RYLL / March 20, 1997
In 1960, four massive cylinders of solid steel -- 20 inches in diameter and 67 inches long -- were removed from the Interstate 5 Bridge for a tuneup.
Each piece, originally cast in about 1916, was cut and welded. The work was meant to ensure a long future for each cylinder.
Instead, at least one of the pieces, called a trunnion shaft, was fatally flawed by the process, according to an engineering firm's report.
The shaft began to crack.
Because the trunnion is part of the system that moves one of the bridge's 272-foot-long lift spans, the crack could be expanding every time the bridge opens and closes.
For months, engineers have been debating how to repair the trunnions.
While many elements of the plan are uncertain, this is for sure: For at least three weeks, possibly this September, one side of the Interstate 5 Bridge will be useless to commuters. It is the old, 1917-vintage side, the span now carrying the bridge's three northbound lanes.
For those three weeks, all traffic -- north- and southbound -- will be crammed onto the southbound span.
Most likely, two of the southbound span's lanes will be used for southbound traffic.
That will leave one, and only one, for all northbound vehicles.
There will be no fancy reversible-lane feature to accommodate northbound rush-hour traffic's desperate need for more than one lane.
It has been nearly 40 years since only a single I-5 span existed to carry cross-river traffic.
Now, the combined vehicle count across the Columbia River on the I-5 and Interstate 205 bridges is more than 200,000 each day. And while many motorists will no doubt either stay home or shift to I-205 during the I-5 work, hundreds will be stuck in what could be monumental traffic jams.
"This is an important decision," said Katy Tobie, an Oregon Department of Transportation community affairs coordinator, referring to the timing of the bridge repair. "The impacts will be significant on both sides of the river."
But not nearly as significant as the impact that would occur if the flawed trunnion were to fail, allowing one of the bridge's four 700-ton concrete counterweights to fall more than 100 feet to the bridge deck below.
"You could actually wipe out the southbound structure," said one engineer.
Each of the bridge's twin lift spans has two counterweights. When the spans are in place for use by highway traffic, the counterweights hang near the tops of the bridge's four towers.
Thirty-two steel cables, each about 185 feet long and 2 inches in diameter, are fastened to each end of the lift spans. The cables, which weigh 1,600 pounds apiece, travel vertically to loop over a 12-foot-tall pulley known as a sheave.
Each lift span has four sheaves carrying 16 cables. With the exception of one aircraft warning light, the sheaves are the highest points on the I-5 Bridge.
The cables that travel over the sheaves are fastened, 16 per side, to the east and west ends of the counterweights.
Each sheave rides on a steel shaft supported at each end by what is known as a bearing block. The assembly -- shaft, blocks, bearings -- is called a trunnion.
The problem that will mean headaches for commuters lies in the trunnion at the northeasternmost corner of the bridge.
Like the other three trunnions on the northbound span, it was rebuilt during bridge renovation in 1960. The newer bridge span to the west opened in 1958, and the original 1917 span was closed for the work.
A 1995 report by a DGES Inc., an Olympia consulting engineering firm working for the Oregon Department of Transportation, traces the cracking of the trunnion shaft to machining and welding done in 1960.
At that time, the shafts were converted from simple journal-style bearings -- about as sophisticated as those on a child's wagon -- to roller bearings, which do a better job of reducing friction. The machining and welding changed the characteristics of the steel, and it began to crack.
In the late 1970s, a similarly designed railroad bridge in Illinois suffered a trunnion shaft failure.
As a result, ultrasonic testing was employed during a 1987 inspection (one report says 1986) of the I-5 Bridge. Tests disclosed the possibility of a crack in the shaft.
However, because the shaft is fully enclosed and cannot be visually examined, it was not until testing using a newer technology known as acoustic emissions monitoring confirmed the crack's presence in 1994.
In early 1996, a re-examination showed the crack was continuing to grow.
At that point, says one report, "No determination could be made that the crack was growing faster or slower than predicted."
Now, engineers predict that the shaft could fail sometime between 1999 and 2019, "with the most probable year being 2009." The report goes on to say, "Crack growth occurs during raising and lowering of the lift span. Failure will most likely occur during one of these operations."
Not, however, if engineers can help it. The plan calls for assembling a 200-foot crane on a barge or platform just upriver of the north-span counterweight/trunnion assembly. The crane would be used to lift new sheaves, shafts, bearings and cables into place.
"The objective is to replace the shaft before 1999, the first year the probability of failure becomes significant," says the Oregon report.
Engineers want to do the work this September or October. Officials have drawn up three , one of which calls for a straight 21-day closure. Two other options call for on-again-off-again closures, which thus far have little support.
"I say, let's take all the misery at one time," said Gerry Smith, regional administrator for the Washington Department of Transportation, which splits the bridge operation and maintenance cost with Oregon. That was seconded by Don Owings, a WDOT engineer who attended a planning meeting earlier this week. "My own opinion is that we close it and do it," he said.
