Sky-high inspection of troubled tower

A window-washing rig was lowered gingerly down Millennium Tower’s western face Tuesday morning, carrying an engineering inspector on a mission to examine the cracked window on the sinking building’s 36th floor.

The external inspection, which took about 90 minutes, represented one of the last major items on the building management’s to-do list before it submits a report to San Francisco’s Department of Building Inspection detailing what caused the window to crack on Sept. 2.

Rachel Miller, an attorney representing building management, said the report, compiled by the structural engineering firm Allana Buick & Bers, should be completed next week.

City officials want to know whether the cracked window in

unit 36B is a one-off issue — caused by something like temperature fluctuations — or a symptom of deeper problems at the troubled building. Since it opened in 2009, the tower has sunk by around 18 inches and tilted to one side, prompting a barrage of finger-pointing and lawsuits over who’s to blame.

Millennium Partners, the tower’s developers, claim that the Transbay Joint Powers Authority caused the building to sink and tilt because it pumped out millions of gallons of groundwater from the soil during construction of the Transbay Transit Center next door.

The joint powers authority maintains that the developer’s building methods are to blame.

While the Department of Building Inspection has repeatedly insisted that Millennium

There are persistent concerns that the crack could be caused by issues with the building’s curtain wall.

Tower remains safe for occupancy, there are persistent concerns that the crack could be caused by issues with the building’s curtain wall, sometimes referred to as the building’s skin.

Curtain walls, which include the building’s windows, are common features of high-rise construction and typically don’t bear any weight apart from their own.

In 2016, while investigating a Millennium tenant’s complaint of strange odors in her unit, Allana Buick & Bers investigators found that portions of the building’s curtain wall panels had “shifted due to the excessive settlement of the building.” Those shifts resulted in air gaps that could allow fire and smoke to leap between adjacent floors, the firm stated in a report, creating an additional safety risk.

The window in 36B has been taped internally and externally to prevent it from shattering, but the city is worried about the risk of other windows cracking, with the possibility of glass raining down onto the sidewalks. As an additional safety measure, the Department of Building Inspection required the tower’s managers to construct overhead scaffolding that wraps around the building from its main entrance on Mission Street around the corner to Fremont Street.

Until recently, Millennium management didn’t expect to be able to conduct the external inspection of the window until next month. In a Sept. 13 letter to city officials, attorneys for the tower said the windowwashing rig had an “inoperable” safety brake that needed to be replaced before it could be used. The replacement part was initially going to be manufactured and shipped from Germany, meaning it wouldn’t have been usable until around Oct. 19.

But Miller said Tuesday that management had the replacement part manufactured locally, which moved up the inspection.

“I think this is an example of how hard (Millennium management) is trying to meet DBI’s demands,” she said.

Photos by Noah Berger / Special to The Chronicle The Millennium Tower, soaring above downtown San Francisco, was already sinking and tilting — and then a 36th-floor window suddenly cracked, causing a new round of alarm.

An inspector in a windowwashing rig is lowered to examine the mysterious crack that suddenly appeared in a window in unit 36B of Millennium Tower. With that window taped securely into place, the worry is that more windows could break, and this time rain down glass on the sidewalk and street.