A 2,000-pound concrete slab was reported loose and at risk of failure on top of a skyscraper that's under construction in the Rincon Hill area of SoMa. The building, at an address listed as both 41 Tehama and, alternately, 33 Tehama — as per an advertisement on the side of the building — is to be a more than 35-story structure when complete. The potential failure was of a small structure on what's now the top of the building, between the 35th and 36th floors, where concrete walls beneath a boom began leaning, causing the boom to lean with them.

See the daylight on the top floor on the far right of 41 Tehama? That's the loose concrete slab. @CBSSF pic.twitter.com/ZZy7roEueu — Andria Borba (@andria_borba) February 15, 2017

The nearby site of 543 Howard was one of the first buildings to be evacuated at the direction of the Fire Department's public information officer.

Loose concrete on 30th floor of 41 Tehama. 543 Howard being evacuated. It's a mess here. Find a different route to Bay Bridge folks. @CBSSF pic.twitter.com/c9E5i8zem3 — Andria Borba (@andria_borba) February 15, 2017

At the scene of 41 Tehama where a crane carrying a 2k slab failed. Evac orders pic.twitter.com/CKYGqVfNvf — Jonah Owen Lamb (@jonahowenlamb) February 15, 2017

In 2014, the Business Times wrote that the tower at 41-Tehama was a 418-unit project.

***EVACUATIONS***41 Tehema 30th floor loose concrete slab set 2k Lbs loose potential to fail AVOID AREA 41 Tehema being evacuated 254 Hrs pic.twitter.com/kAuoR0Otzn — San Francisco Fire (@sffdpio) February 15, 2017

Media point Second and Howard additional EVACUATIONS 235 second st and 505 Howard https://t.co/KaAOcikP7S — San Francisco Fire (@sffdpio) February 15, 2017

Here's @sffdpio with an overview of 41 Tehama Street, a building under construction. Malfunctioning crane putting 2K concrete slab at risk. pic.twitter.com/u5H20MKwy9 — Michael Bodley (@michael_bodley) February 15, 2017

543 HOWARD IS BEING EVACUATED 41 Tehama shelter in place https://t.co/2xRvXkDUYa — San Francisco Fire (@sffdpio) February 15, 2017

#021517EV1 UPDATE ***EVACUATIONS*** 543 HOWARD 41-44-56-58 TEHEMA IMMEDIATE EVACUATIONS POTENTIAL CEMENT SLAB FAILURE pic.twitter.com/RawntfUzYT — San Francisco Fire (@sffdpio) February 15, 2017

SFFD PIO 41 Tehama experienced wall failure this afternoon. Engineering specialist being flown in. @CBSSF pic.twitter.com/bREKz1mhhq — Andria Borba (@andria_borba) February 16, 2017

#SF fire officials say a crane fell atop this building at 41 Tehama and have evacuated nearby blds. pic.twitter.com/27lpUHh1to — Jonah Owen Lamb (@jonahowenlamb) February 15, 2017

"Basically, there are four walls supporting a smaller crane," a Chronicle reporter at the scene summarized, indicating what officials later clarified was a boom, not a crane. "And one is unstable. If it falls, it could pull the smaller crane with it."

In an update to that explanation to the Chronicle, SFFD Officer Jonathan baxter says that the crane or boom in question began falling into a concrete wall that firefighters were trying to reach before it can pull down the wall. "If the crane falls, it could take the concrete wall down," said Baxter.

Here's a later, clearer explanation of that:

NOW: Asst. Chief Tom Siragusa explains what happened in more detail. Strut failed on small crane when setting concrete walls on 30th floor. pic.twitter.com/6KSpOJwDY3 — Michael Bodley (@michael_bodley) February 16, 2017

"This is all precautionary stuff," Baxter emphasizes. "We are looking at a potential slab falling. If that slab falls, we are looking at danger below. We are not sure where yet."

@michael_bodley here's a shot of the small crane from my building on new montgomery pic.twitter.com/Y7nvECRFJC — zkull (@WordOfZac) February 15, 2017

Evacuation underway in downtown #SF bc of small crane failure atop construction project pic.twitter.com/ay8VBAVEw4 — Jonah Owen Lamb (@jonahowenlamb) February 16, 2017

Evacuations were for 543, 531, 527, 505, 547, 555, 557 Howard; 41, 44 Tehama; 235 Second. For once, during the evacuations, the Fire Department wasn't telling media choppers to get out of the way, but instead soliciting their help in seeing the problem.

Media helicopters we would like your live feed to help us out please contact the pio pic.twitter.com/VfCxXEXWKn — San Francisco Fire (@sffdpio) February 15, 2017

"Cantilevering" is the technical term for the wall. Should be straight. Is leaning out over the street. Crane on roof supported by wall. https://t.co/psIyQa1VYX — Michael Bodley (@michael_bodley) February 16, 2017

SFist has just received a statement from the developer:

"This afternoon, an incident occurred at the 33 Tehama site between levels 35 and 36 where an interior forming system had a partial hydraulic failure while being raised to the next level. The interior forming system and the concrete placement arm have been secured and are being evaluated by engineers to bring it back to level. No injuries and or damage has been reported. Please note the boom is located in the center of the top level. This is NOT the crane attached to the outside of the building. Previous reports that a 2,000 pound slab of concrete is in danger of falling are also false. 33 Tehama is being developed by Hines and Invesco Real Estate. Lendlease is the general contractor."

This post has been updated throughout.