(Crew already on low bridge – photo via @lungodog on Twitter)

UPDATED 4:59 PM: The city has announced an emergency closure of the high-rise West Seattle Bridge “until further notice,” starting at 7 pm tonight. Here’s our video of the Skype briefing during which this was announced:

Here’s the SDOT news release. Below, our as-it-happened coverage of the announcement, and further updates:

=================================

2:55 PM: Just announced via a sudden online media briefing: The West Seattle Bridge has been developing cracks over time, says the mayor, with “further deterioration” detected recently – so “there are concerns that the high-rise bridge cannot safely support vehicular traffic at this time” so they need to assess it further. So tonight “the West Seattle high-rise bridge will be closed to all traffic … until we can do further assessment” to determine how the bridge can be safely operated. The low bridge will remain open to transit and first responders, and freight. “I’m grateful they found this” before something bad happened.

SDOT director Sam Zimbabwe says the bridge will close at about 7 pm “until further notice.” The cracking is in the concrete, he says. They’ve been noticing cracking for several years but it’s now to the point where they feel they have to remove “live-load traffic.” He reiterates that traffic outside of the categories mentioned will not be able to use the low bridge – they’ll have to use other paths out such as the 1st Ave. S. Bridge.

Council President Lorena González says a public briefing on this will be scheduled.

We’ve asked several questions via the web chat feature and they’re now answering. Why so sudden? Zimbabwe says some new federal load-rating requirements last year led to a “more detailed assessment” while deterioration continued. “There really wasn’t another choice at this point.”He says it’ll be weeks before they know the type of repairs they need – but then the repairs could last months.

Regarding details on the damage, SDOT engineer Matt Donohue is answering off-camera. He says a “post tensioning steel strand” in the bridge supports is where the problem is. What kind of repairs are needed? Zimbabwe says some shoring for starters, followed by likely “some kind of fiber wrap” among other things. “They take some time to design and construct.”

The timing with the drop in traffic is just coincidental, says Zimbabwe. We asked about other transportation options – this is so sudden they don’t have that worked out yet.

They will be out with maps soon to show the closure. From WS east, detour signage will start in The Junction, Admiral, Delridge, 35th, etc., appearing starting tonight. I-5 traffic will not be allowed onto the Spokane St. Viaduct (the east half of the high bridge) except for transit, freight, and emergency vehicles, to “avoid confusion.”

Zimbabwe says it was no one thing that led to this decision – but for example some of the vehicles and volumes on it are more than it was designed for. Couldn’t load restrictions enable reopening? Not in this case, he said. Plus, he notes, the bridge design means “80 percent of the load is the bridge itself.”

3:26 PM: The briefing is over. We recorded it on video (shakily off a Skype feed but it’ll have to do unless the city has a copy available) and will add that after we upload it.

ADDED 7:42 PM: SDOT crews are out blocking off access points. Also City Councilmember Lisa Herbold has sent a statement: