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A window on the 36th floor of the sinking Millennium Tower may have cracked due to the tilting of the building and could be an isolated break — or a sign of a widespread problem, an architectural engineering firm told the city in a report Friday.

The report, filed late Friday with the city’s Department of Building Inspection, says someone will rappel down the building early next week to photograph the crack from the outside and to tape up the glass as a safety precaution.

The engineering firm, Allana Buick & Bers, contracted by the building’s residents, has so far been able to evaluate the window only from inside the tower and said it needs to assess the exterior of the window and building to determine what caused the crack.

On Saturday, inspectors will use drone photography and high-powered telephoto lens photos from adjacent buildings to examine the exterior.

“The fact that this piece of glass may have failed due to the building tilt may be a localized defect or potentially be more widespread,” the report said. “Further investigation of the building exterior is required to understand and assess the issue.”

Back to Gallery Millennium Tower window crack could signal bigger... 6 1 of 6 Photo: Courtesy of City and County of San Francisco Department of Building Inspection 2 of 6 Photo: San Francisco Department of Building Inspection 3 of 6 Photo: Lea Suzuki / The Chronicle 4 of 6 Photo: San Francisco Department of Building Inspection 5 of 6 Photo: John King / The Chronicle 6 of 6











The crack kindled anxieties this week among condo owners and others that the broken window could be symptomatic of deeper problems with the 58-story building. Since opening in 2009, Millennium Tower has sunk by around 18 inches and tilted to one side, prompting serious concerns about the building’s structural integrity.

The firm’s report reached a starkly different conclusion than Millennium management, which emailed the tower’s residents Thursday and called the crack a one-off problem.

After two days of inspection and investigation by the tower’s construction experts, the crack was deemed “an isolated issue” by Millennium’s construction experts, according to the email, which was obtained by The Chronicle on Friday.

The email also mentioned previous window breaks the tower has experienced, “which experts determined were unrelated to the building settlement.” Architects said that windows can at times crack because of changes in temperature, defects in the glass and other factors.

The tower’s managers, who did not respond to requests for comment Friday, said in the email to residents that they planned to keep investigating what caused the glass to crack, “including a look into the assembly and installation of the window.”

The building’s management also promised to replace the window, after having safeguarded it from popping out or shattering.

We “are certain this investigation will help determine the cause of this crack and ensure this condition will not affect other units,” the email said. That will include a look at the window from the outside, using a drone, according to a separate email to homeowners obtained by The Chronicle.

The city’s Department of Building Inspection was notified of the crack Tuesday. The department then ordered Millennium management to examine the window and submit an engineering report. Spokesman Bill Strawn said the department will evaluate the engineering firm’s report Monday.

Jerry Dodson, a Millennium resident, was skeptical of the management team’s claim that the cracked window was an isolated incident. Dodson and 19 other Millennium residents are suing the tower’s developer, Millennium Partners — as well as the city inspection department, the city attorney and the Transbay Joint Powers Authority — alleging that the builder and city officials knew that the high-rise was sinking but didn’t disclose it to buyers.

The noise when the crack rippled through the window “was heard four floors down,” he said.

“It’s a bit hasty to conclude that it’s not related to a building that’s tilting toward Mission Street by 18 inches.”

For local residents not worrying about the fate of their sky-high homes, the cryptic crack on the 36th floor was another entertaining installment in the Millennium Tower saga.

“It’s something we talk about when we walk past it, every single time,” said Justin Ford, who works at the software research firm Duetto near Union Square. “It’s a giant conversation starter.”

The crack is on the tower’s Mission Street side, invisible from the sidewalk far below. But the sidewalk itself was enough to stop Renato van der Halen and Marcio Celuppi in their tracks. The strain of the tilting tension can be gleaned by a 12-foot-long buckle in the concrete, as well as a multitude of smaller cracks.

Van der Halen, who lives in Noe Valley, was taking his guest from Houston to see the new transit center when they made the detour to Mission Street: “I’ve heard of the leaning tower,” Celuppi confirmed. “A group of friends back home were talking about it.”

As they paused for another look upward, Celuppi was asked his thoughts.

“They must be crazy to stay,” he said, referring to the residents. “If you have enough money to live there, you must also have enough money to live somewhere else.”

Dominic Fracassa and John King are San Francisco Chronicle staff writers. Email: dfracassa@sfchronicle.com, jking@sfchronicle.com Twitter: @dominicfracassa, @jkingsfchron