This morning SDOT briefed the City Council on the situation with the West Seattle Bridge (see the article I posted over the weekend for an explanation of what closed the bridge). Here is some additional information from that briefing, as communicated by SDOT staff.
- The federal standard requires bridges to be inspected for cracks every two years.
- In 2013 during a standard 2-year inspection, SDOT found atypical cracking, and hired an engineering firm to study it. According to SDOT, the results were “not particularly conclusive,” but recommended more frequent inspection and monitoring. SDOT installed crack gauges to help them track the cracking going forward.
- From 2014 to 2019 SDOT made annual inspections (instead of every 2 years). They saw that the cracks were beginning to grow, but not at an atypical or concerning length.
- Last year The Federal Highway Administration introduced a new classification of trucks, and as part of that required new load-ratings for bridges by 2022. The calculations for load-rating require more advanced analysis for a bridge that is seeing atypical cracking, so SDOT, knowing it would need more time, decided to start the new load-rating for the West Seattle Bridge right away instead of waiting until 2022.
- Last fall as they were gathering data for the load-rating analysis they saw the cracking patterns change, which caused them to increase to monthly inspections. They began to look at mitigation for the cracking (though not mitigation for traffic on the bridge).
- By February of this year, SDOT’s engineering consultant was recommending eventually reducing the traffic to 2 lanes in each direction (from the current 3 westbound and 4 eastbound). That was not a recommendation for immediate action, so the department started pulling together what a reduction in lanes would look like from a traffic management plan.
- On Friday March 19th, the engineering consultant changed its recommendation to closure of the bridge.
- At 9am, Monday, March 23rd, SDOT Bridge Group Supervisor Matt Donahue, a certified bridge inspector with 20 years of experience, took his team into the bridge to inspect the cracking. He was so alarmed by what he saw that he called SDOT leadership — on his cell phone from inside the bridge — and said that they needed to close the bridge that day. Donahue told the Council this morning that the kind of growth in cracking in the bridge is “completely unacceptable” in that type of reinforced-concrete structure. He said that it was the amount one would expect to see in years, not weeks. He also explained that angular cracks like the ones in the West Seattle bridge are called “shear cracks” — and they continue until failure, which can happen very quickly.
- The Council was briefed that morning, Mayor Durkan approved the closure at 11am, the announcement was made to the public in the afternoon, and the bridge was closed at 7pm.
- The low “swing bridge” is currently not in top shape either; SDOT has been looking at it closely over the last several months. The control system and gates are scheduled for replacement, and the department is evaluating its load rating too. In the meantime, since the swing bridge is almost directly underneath the tall bridge, SDOT has had to make a plan to shut it down too in case the tall bridge begins to show signs of collapse — though the department thinks the likelihood of that happening is low now that traffic has been removed from the bridge, lightening its load.
- The city is reserving the lower swing bridge for emergency vehicles, freight and transit use only. Even then it is seeing 15,000 trips a day; its rated capacity is 20,000.
- SDOT has rolled out its short-term traffic mitigation plan, including a now-operational temporary signal at Highland Park Way and Holden. It has also installed several traffic-count stations, collecting data to be used to further adjust traffic-mitigation measures as necessary. In the mean time, the department has begun work on medium- and ling-term mitigation plans
- The first step to repair the bridge is to put in temporary shoring so that it is safe for a contractor to be on the bridge to do repairs. Donahue said that the most likely repair will involve carbon-fiber wrapping for the cracked segments, combined with additional post-tensioned steel tendons. An extra complication is that repair activities must stay out of the federal maritime travel “box” on the river below. If they need to enter the box, then they will need to obtain a federal permit — which will take more time. Also, maintaining social-distancing for the workers involved in the repair will further complicate the effort.
Council members publicly flogged SDOT Director Sam Zimbabwe over their frustrations in not being briefed on the issues with the bridge until the day it needed to be closed, and they pressured him to commit to when he could give them a timeline for the short-term solution. Zimbabwe resisted giving immediate answers, saying that only one week after the closure the department doesn’t have enough information to give either a timeline or an estimated budget. He said that in three to four weeks he expected to have a much clearer idea of both. He also promised to stay in “tight communication” with the Council.
Council members did, however, thank SDOT staff for taking quick action last Monday to protect the safety of Seattle’s residents.
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