Seattle's steel drawbridges will be tested, measured and watered more often in summertime, after August heat caused a 76-minute traffic shutdown at the Ballard Bridge on Wednesday night.
Seattle’s steel drawbridges will be tested, measured and sprayed with water more often in summertime, after August heat caused a 76-minute traffic shutdown at the Ballard Bridge Wednesday night when it wouldn’t close completely.
Crews doused the 101-year-old bridge as a precaution for the next day, following a long operation to lock the decks into correct position, said Matt Donahue, roadway structures manager at Seattle Department of Transportation (SDOT).
The crossing serves 65,000 vehicle trips daily.
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The former SDOT protocol was to examine hot bridges and consider dousing them on the third consecutive day of 85-degree-plus temperatures, to reduce thermal expansion of bridge components. Now SDOT will water the bridges on the first or second day of high temperatures, Donahue said.
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This follows incidents last summer when SDOT repeatedly blocked the University Bridge during a heat wave and soaked it, to prevent the two steel leaves from binding in the center. The usual half-inch gap had narrowed to a quarter inch.
The Ballard Bridge was blocked Wednesday from 6:44 p.m. to 8 p.m., after the south drawspan deck didn’t fully descend, and wound up 1 inch higher than the north deck. This failure was caused by thermal expansion and friction among steel parts, Donahue said. He heard a noise like the squeal of a boiling tea kettle.
Car traffic could have crossed, but that inch posed other problems, he said.
The Ballard Bridge’s central locks, where rods under the south deck insert into tubes in the north, couldn’t be set into place as normal. That misalignment disrupted the programmable logic control software that governs the pace and angles of moving drawspan gears. And heavy loads such as buses or semi-trucks might bounce the unlocked bridge leaves at the center, Donahue said.
Bicyclists and pedestrians also would have faced safety hazards from a one-inch bump.
Donahue, himself stuck in traffic at Interbay, walked to the bridge control tower and directed his team to promptly troubleshoot, test, and fix the span.
He said bridge technicians eventually overrode the control system, and drove a truck to the center, to push the leaf downward — while the setting sun allowed the bridge to cool somewhat.
SDOT hasn’t yet located the exact spot where steel parts scraped or blocked each other. That will take more time and money, he said. The city owns century-old steel drawbridges at Ballard, Fremont and the University District, and operates a new King County bridge at South Park. Washington state owns the Montlake and First Avenue South bridges.
Steel absorbs heat faster than it cools, Donahue said, causing a cumulative expansion. “At night it will start to cool down, but it never gets back to normal,” he said.
Four of the first eight days this August were above 85 degrees; 11 days were that hot in August 2017.
In Chicago, a fireboat sprayed a steel bridge for a half-hour when the span reached deck temperatures over 100 degrees in late June. The interlacing, finger-shaped edges of the drawspan decks swelled and stuck together, so the DuSable Bridge couldn’t open for passing boats. Popular Science magazine compared the problem to arthritis, where “the joints had become too inflamed to move properly.”
Portland travelers have suffered on occasion when overhead catenary wires that power Portland’s electric-powered light-rail trains have sagged under extreme heat, forcing trains to slow or suspend service. But the 106-year-old Steel Bridge, which carries four light-rail lines, hasn’t needed to be cooled despite recent hot summers.
Other bridges over the Willamette River also haven’t required watering in the last couple decades, said Mike Pullen, transportation spokesman for Multnomah County.
“Once we had to use dry ice, to shorten the metal and disengage parts of a bridge that were coming together,” he said. However, that incident at the Morrison Bridge downtown was caused by a misalignment not by heat.
Global warming may lead to more bridge expansions and more cooling baths. But Donahue said he’s more focused on Seattle’s aging structures, which deform over several decades.
He said Seattle should devote $80 million to maintenance yearly instead of the current $17 million, or the city might someday wind up closing worn-out bridges for safety.
Already in the Duwamish area, one lane of a concrete span above railroad tracks at Fourth Avenue South has been shut down because SDOT lacks the money for retrofits, he said.