Before contractors can reinforce and repair the cracked West Seattle Bridge, the city first needs to confirm it won’t fall.
Fast-moving diagonal cracks on the girder walls, which branched out by 2 feet in early March, aren’t the only source of worry. Consulting engineers last month also noticed crack patterns along the ceilings and floor with the hollow girders, according to one of 14 records the city posted online this week.
Engineers from the firm WSP saw the cracks were quickly converging — and if the cracks were to meet, the bridge could fall.
“Our biggest concern has become [that] the extent and rate of cracking near the quarter points of the main span could lead to collapse in the near future if strengthening is not implemented quickly,” said a March 20 message from WSP engineer Greg Banks to Matt Donahue, director of road structures for the Seattle Department of Transportation (SDOT).
Banks also cites a professor from the University of Toronto who agreed patterns were “consistent with reaching a collapse mechanism. It is not anticipated that a collapse mechanism is imminent.” A hand-drawn diagram depicts 12 locations of cracking, all diagonal and parallel.
Some cracks advanced within 11 inches of the top of the girders, which run lengthwise under the bridge deck.
A report on collapse risk, requested by Mayor Jenny Durkan, is already in draft form and should be done within a couple of weeks, Donahue said Wednesday.
The department hasn’t released a detailed work order or itemized cost for the collapse-risk analysis, to be done under WSP’s February 2017 contract for citywide on-call design services.
“We will also model worst-case scenarios so that we can identify early warning signs and implement additional safeguards to catch any issues before it is too late,” SDOT spokesman Ethan Bergerson said.
A few cracks have proliferated even since March 23, the day the bridge was abruptly closed.
“We have discovered one new crack, and a few other cracks have grown by a few inches,” Bergerson said last week.
However, Donahue said Wednesday, that based on his near daily visits to the girders, the additional cracking occurred almost entirely on March 23, between his morning phone call to recommend the emergency closure and when the bridge was barricaded between 7 p.m. and 8 p.m.
“It happened within several hours in a day that the cracks propagated,” he said, reinforcing his view the traffic shutdown worked effectively to avert more damage.
“Right now, my degree of confidence about the bridge being able to support its weight is unchanged,” he said.
He said SDOT hasn’t yet devised its strategy for short-term “shoring” to brace the bridge until repairs can be done over several months. The bridge’s 590-foot-long concrete arch towers 140 feet over Duwamish Waterway, where any marine blockages need Coast Guard permission.
Other newly released documents show that as far back as 2014, experts couldn’t clearly diagnose why cracks were appearing in a structure only 30 years old. They even mentioned repair ideas — not because they feared disaster, but to preserve the structure’s life span of 75 years or so.
A key 2014 technical memo by John Clark, a consulting engineer with decades of experience, points to many questions.
Cracks proliferated symmetrically in the twin girders, 112 feet shoreward from the middle of the bridge. Those hot spots can now be easily seen by travelers, because SDOT in 2019 slathered amber-tinted epoxy patches on the exterior, including superficial cracks, to seal out rainwater.
“The observed fact that cracking has occurred in four similar locations indicates that the cause may be something related to design assumptions or procedures assumed in design being different from what has actually occurred,” Clark acknowledged in the 2014 memo.
But he said the cracking “does not influence the ultimate load capacity of the bridge.”
Clark, who was part of the bridge design team in 1980, mentioned possible fixes such as carbon wrap, epoxy grout and additional steel strands to cinch the concrete segments tighter from within. But he advised that carbon wrap was expensive.
Donahue on Wednesday pointed to concrete “creep” as a factor in the cracking. Concrete typically shrinks over 10 to 20 years, forming some cracks even on a solid bridge.
“While they designed it appropriately to the code of the time in the ’70s and ’80s, the creep is greater than it should be,” he said.
Over three decades the stresses became mismatched and generated cracking near certain places where internal steel strands were tightened to compress the concrete segments.
John Stanton, a University of Washington civil-engineering professor, said in a March interview there might have been concrete shrinkage followed by a slackening of steel bands that help the girders resist downward forces.
“I don’t think there’s any reason to suggest that the West Seattle Bridge concrete is bad,” he added. “Any such problem would have shown up long ago.”
Another possible culprit, the addition of a fourth eastbound lane, appears in a Feb. 21 letter from WSP to Donahue. The eastbound side leaving West Seattle is routinely packed in morning peaks, including buses weighing about 50,000 pounds with passengers, while the westbound side rarely fills.
“Field observations have indicated that the cracking is somewhat worse on the south box girder (eastbound)…” the letter says. SDOT Director Sam Zimbabwe has previously mentioned the added lane as a factor.
A special inspection in December 2018 by Denver-based firm BDI found that “classic shear cracks” were already forming and appeared to be extensions floor cracks.
The records also reveal the 2001 Nisqually earthquake as a possible source of cracking, mentioned in reports from Clark in 2013 and 2014. SDOT research found the bridge swayed three inches lengthwise in the quake, Clark wrote.
The 2018 inspection by BDI went further to say it’s highly probable cracks initially formed “as a result of torsional deformation during a significant earthquake.”
Considering how cracks appeared symmetrically, and were somewhat baffling, why wasn’t SDOT alarmed enough last decade to pursue repairs?
“It was,” Donahue said, “which is why we upped our inspections frequency in the first place, we went back and talked to John Clark and did extra inspections on the bridge, continued to put extra care and attention to the bridge.”
The city injected epoxy into the cracks and installed gauges shortly after the first cracks appeared in 2013, as well as began inspecting yearly and then monthly in late 2019.
The cracking, he said, “wasn’t anywhere near as extensive as it is right now.”