A pallet loaded with heavy timbers broke through a construction platform on the new Nalley Valley viaduct Saturday, narrowly missing motorists on South Tacoma Way, 45 feet below.
No one was hurt, but witnesses who called 911 shortly after 5 p.m. said there were close calls.
About four dozen 4-by-4s, each 10 feet long, rained down onto the roadway, the state Department of Transportation said Tuesday.
“We don’t know what went wrong yet,” said Claudia Bingham-Baker, a spokeswoman for the department. “But something obviously did.”
Mowat Construction, the general contractor on the job, is conducting a “root cause analysis” and will report to the Transportation Department when the review is completed, Bingham-Baker said.
As an immediate precaution, she said, Mowat redistributed 4-by-4s stacked elsewhere on overhead girders to avoid another collapse.
Many of the concrete girders for the new mainline of state Route 16 have been set in place atop support pillars, but the concrete roadway deck has not yet been poured. In the meantime, crews spanned the girders with wood beams and 3/4-inch plywood sheets, and they are using the temporary floor to store lumber and construction equipment.
Bingham-Baker said it is too early to say with precision what happened, but the span strength of the 3/4-inch plywood apparently was not adequate to support the load.
All such details are calculated and specified in the design, she said, and at this point it is unclear where the error occurred.
Bingham-Baker said she could not say when Mowat will finish its analysis.
“We don’t have a deadline per se,” she said, “but it will be soon, because they obviously don’t want it to happen again.”Rob Carson: 253-597-8693