An investigation into a cracked insulating glass unit (IGU) at the sinking Millennium Tower in San Francisco has determined the break was caused by an exterior impact. According to the “Cracked Glass Unit 36B Investigation” report by Allana Buick & Bers (ABBAE), uploaded by NBC Bay Area, the investigation showed a lack of structural defects that could have caused that type of damage.

“The curtainwall connections also do not show abnormal movement that could cause an IGU to break,” reads the report.

The determination was based on the most recent investigation and previous curtainwall and glazing observation conducted between October 2016 and February 2018.

The firm notes in the report that the damaged IGU is located in the master bedroom of unit 36B on the north-facing side of the large tower. The breakage occurred on September 1, 2018. The exterior pane was cracked while the interior pane of glass remained intact.

The report quotes the homeowner of unit 36B describing the incident.

“We were abruptly awoken by a massive bang at 2:30 a.m. in the morning. This sound was so massive in scale, it sounded like a huge, thick vault door had just slammed shut,” it reads.

According to ABBAE, the cracks appear to initiate from the left side of the IGU, approximately mid-span of the left jamb. The firm observed impact-like damage and cracks emanating from that point.

“At the time of our initial observation, the cracks had spread throughout the rest of the glass. In the days following our initial site visit, the cracks continued to spread because of the fact that the glass is heat strengthened,” reads the report. “We did not rule out building settlement as a potential cause of breakage. Our initial concern was that due to the heat strengthened nature of the glass, the cracks would continue and potentially pieces of broken glass may fall down.”

ABBAE hired a local glazing contractor to apply thick security film over the glass as a protective measure. The window-washing rig was not functioning at the time but the glazier was able to reach out from an adjacent operating window opening to apply the film, according to the report.

Replacement glass has since been ordered for the unit and further analysis will occur after the current glass is removed.

To reach its conclusion the architectural engineering firm:

  • Interviewed the homeowner;
  • Observed the glass and surrounding curtainwall assembly;
  • Used long distance photography taken from an adjacent building;
  • Demolished interior finishes to observe the structural slab edge connections at units 26B and 37B;
  • Took interior survey measurements for the floor level and height differential;
  • Used 3D scanning performed by a third party to confirm measurements; and
  • Made visual observations from the swing stage on the exterior of the cracked glass.


ABBAE conducted its previous investigations to determine if the building settlement had any noticeable impact on the exterior curtainwall.

“During these exterior drops, we observed some issues with module offsets as well as the ‘coating finish’ on the aluminum mullions. During these visual surveys, we conducted about a dozen drops with the window-washing rig on various elevations to identify these issues. We observed module offsets and recommended further invasive testing to ascertain the causation of the shifting panels … Based on our drops as well as the drone survey, we observed that offset panels were isolated on a few stacks, on certain floors only,” reads the report.