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.

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.

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

"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:

"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."

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.

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.