The seismic retrofit of two existing, 40-story egress staircases within a massive high-rise required great care in design and sequencing to keep the occupied building safe during construction. Holmes’ retrofit and fire safety strategy kept the egress staircases fully operational throughout construction.

Our solution set a precedent for mitigating existing stair vulnerabilities in a vast portfolio of buildings by achieving the following:

1. Accommodated inter-story drifts as expected from the Non-Linear Response History Analysis (NLRHA), without damage or collapse of stairs.
2. Minimized disturbance to tenants of the fully occupied building.
3. Kept egress undisturbed through coordination with the San Francisco Fire Department.
4. Kept stairwells operational 24/7 during construction.
5. Resulted in significant cost savings and delivered the project ahead of schedule.

The existing stairs consisted of top and bottom stringers rigidly attached at the floor levels, and an intermediate landing suspended from the existing floor framing. Holmes’ retrofit left top and bottom connections undisturbed to avoid shoring the stringer runs and interrupting the path of egress. Earthquake-induced movement at the intermediate landing was accommodated by cutting the landing in half and re-supporting it on low friction pads and support framing. The support framing attached to extensions of the existing suspended rods, eliminating the need for shoring entirely. The built-in shoring was designed so that saw cutting of the landing could be halted in the middle of construction if emergency egress was necessary.

An onsite mock-up of the retrofit was conducted for a single landing at each stairwell. The mock-ups exposed unforeseen conditions such as the extents and locations of abatement including asbestos-containing materials, sheetrock, fireproofing and lead paint. Lessons learned from the mock-up saved $500,000 USD and delivered the project ahead of schedule.