Wheels in motion
By: Garrett Andrews in Scrolling Box August 29, 2017 5:32 pm
Bobby Parker, a pile buck with Hamilton Construction, examines a weld while working on a project to replace four steel wheels that help raise and lower the Broadway Bridge. (Sam Tenney/DJC)
In the early days of the 20th century, civil engineers were in an arms race to find the best ways to open a bridge.
One novel method was tried out by engineer Ralph Modjeski for Portland’s Broadway Bridge, completed in 1913. A Rall mechanism employs giant, 8-foot-diameter steel wheels that roll along tracks high above the center of the bridge.
To explain the mechanism’s complex movement to the uninitiated, it helps to have visual aids, said Jon Henrichsen, Multnomah County’s division manager for bridges.
“It’s one weird, elliptical motion; it’s much easier to show with pictures, or, better yet, a video,” he said. “This is a very complicated bridge – way more complicated, really, than it needs to be. You can build a simpler bridge that’s easier to maintain and more reliable.”
Like an evolutionary dead end, the Rall-type bascule bridge was en vogue for only about a decade before simpler, more efficient and economical systems took over. The Chicago-type bascule would become the standard of the style, an example being the Burnside Bridge. But in 2017, the Broadway Bridge and its unique design are still vital to Portland and the 32,000 people who cross the span daily. And now, after a century of rolling back and forth, its wheel-based lift mechanism needs a tune-up.
The four massive wheels have turned more conical than round, and cracks are showing up.
“There’s just a lot of issues with these,” Henrichsen said. “It’s time to replace them.”
The county has contracted with Springfield-based Hamilton Construction to replace the Broadway Bridge’s one-of-a-kind Rall wheels and the tracks and bearings, and rehabilitate the trunnion, control struts and associated structures. It’s a complex undertaking that requires a highly specialized skill set, and it presents its own engineering challenges – how to lift the bridge to swap out the wheels? How to make them?
To change the wheels, the Hamilton team is lifting the entire bridge, one 1,000-ton leaf at a time. Bridge engineer Ed Wortman likens the problem to jacking up a truck to change its tires, but in this case, no place on the bridge is hard enough to set the “jack.”
“It wasn’t designed to be lifted up into the air as we’re doing now,” Wortman said.
An 88,000-pound Rall wheel measuring eight feet in diameter is one of four on the Broadway Bridge that is being replaced as part of a $13.5 million project that will last through December. (Sam Tenney/DJC)
The solution created by OBEC Consulting Engineers of Eugene is a lift system that uses the stone and concrete piers under the bridge for bases, and thick steel plates welded to the bridge. Two 900-ton capacity jacks will work together to push thick posts upward.
This will all go on while bike and vehicle traffic and the Portland Streetcar are moving across the bridge.
“They’ll only lift it just enough to get those wheels out,” Wortman said. “Then it’s just like changing the tire on a car.”
Wortman was a bridge engineer for Multnomah County starting in the 1990s, but he began working on bridges in 1960. He’s worked part-time since his retirement in 2007, and was on site this month checking progress on the project.
“We always thought we were going to have to change these wheels, and we thought the designer probably made some accommodation for it,” he said. “Then we started looking at it, and it turned out, it wasn’t that easy.”
Ultimately, Wortman said, the decision to use the design was a financial one – it was slightly cheaper than the other two alternatives. Despite any trouble, it actually has done its job – creating the widest possible opening for ships – quite well, he said.
“It was a complicated system, but clever – but also, hard to maintain,” he said. “It’s pretty special. There’s not another bridge like it in the world.”
Each wheel supports about two million pounds of weight. But those wheels have been rolling back and forth for more than a century. The wheels are solid hunks of steel that start out about 30 to 40 percent larger before being pounded into shape. In 2017, not much metal is forged west of the Mississippi River, but VEGA contracted with Peninsula Iron Works to make the forgings.
The 88,000-pound wheels were forged – not cast, as were the originals – at Peninsula Iron Works. The original wheels had cored holes in them like spokes in a bicycle wheel, which helped make them considerably lighter. They were pounded out on the shop’s vertical boring mill, which was manufactured in 1968 but features modern automated CNC controls. They were flipped three times, and shipped on trucks with special permits.
“This is probably more accurate than they were done historically,” said James Johnson, president of Peninsula Iron Works. “Your big advantage today would be in how the materials were manufactured, compared to a casting.”
Johnson and his brother are the third generation to operate Peninsula. The fabrication shop performs 3,000 jobs per year. For this one, they were subcontracted under Vigor, which prepared plans and shipped the materials for the forgings. At more than 100 years old and the only shop like it on the West Coast, Peninsula aided the Allied war effort in World War I and mills serving the Pacific Northwest lumber industry. Its handiwork can be found nearly everywhere in the Portland area – including on all five movable bridges.
Once the new wheels are on, much testing will be needed to ensure they’re aligned with each other and the tracks are oriented perfectly. Among the updates incorporated into the project is breaking up the big metal struts that connect to the control room and pull the bridge open. Breaking them into three pieces, instead of one, makes for easier adjustments.
There will be one, final 48-hour closure over a weekend when final testing will be conducted. Once the old wheels are off, they will likely be shipped to a recycler, though the county has been in talks with the Regional Arts and Culture Council about using one in a public art installation, Henrichsen said.
In Modjeski’s day, bridges had much shorter life spans – maybe 20 years for the Broadway, Henrichsen said. The original operators of the bridge didn’t even grease its gears.
“He’d probably think we were crazy to still be operating this bridge,” Henrichsen said of Modjeski.
But a complete bridge replacement project would cost between $300 million and $400 million, and significantly impact commuters.
“After all these years, it still works well,” Wortman said. “So you can’t question the decision to build it in the first place.”
Broadway Bridge Rall wheel replacement
General contractor: Hamilton Construction
Cost: $13.5 million
Construction start: July
Expected completion: December
New Broadway Bridge Rall Wheel being turned at PIW on 20' diameter Vertical CNC lathe.
Original Rall Wheels prior to removal, Broadway Bridge, Portland, OR. Picture from the web.
