While the new skylight-lit atrium and the beautifully restored and preserved finishes steal the show visually, as craftspeople, what really excites us is how all the pieces come together. The new structural system within the historic Gracemont building at the Bush School in Seattle stands out as a masterpiece of planning, coordination, ingenuity, and execution.
Structural Steel
How can the building’s complex steel and diaphragm connections be installed with minimal disruption to the existing structure? First, cut eight holes in the roof of the building! Then, crane large steel beams down through the roof vertically and turn horizontally for placement (instead of piecing together a welded beam in place). Finally, lower full-length steel columns vertically through the roof and down the inside of the exterior walls past three floors to be placed precisely on large concrete footings with anchor rods in the basement.
In total, we installed 70,000 pounds of structural steel. Hoisting structural elements in from above resulted in minimal impact on the existing building and didn’t require slicing and rejoining long pieces of steel. The newly installed structural steel seamlessly integrated and reinforced the floor structures as intended.
Collector Strap System
We used an innovative concrete approach around the majority of the perimeter walls of the building on the 2nd and 3rd floors (and up the gables) and called them “concrete pockets”. This concrete acts as the glue to stabilize and tie the exterior walls of the building to the floors with a collector strap system to reinforce the exterior walls and keep them from failing in an earthquake event.
Rafn carpenters chipped away inside layers of existing hollow clay tile from the inside of the building, leaving the exterior brick facade undisturbed. Then, Western Specialty Contractors installed helical ties through the chipped-away areas into the solid masonry.
Rafn carpenters then built forms anchored to the surrounding plywood floor, which included “bird’s mouth” openings where we poured concrete into the wall. We then mixed concrete at each placement site and poured/pumped it into the formwork.
Once the concrete had cured and the forms were stripped away, there was now concrete into which we installed threaded rod epoxy anchors. Rafn carpenters then infilled the floor with solid blocking and continued installation of the horizontal strapping system with straps up to 8′ long, reaching away from the walls toward the center of the building.
Our carpenters did a great job placing and connecting a vast amount of collector straps throughout the building. They installed solid wood blocking, 1/4″ thick steel plates, and over 1,000 angle brackets with over 10,000 fasteners to tie the whole system together.
This work occurred in the 2nd floor system above historic plastered ceilings, the 3rd floor system, and the roof structure. Rafn craftspeople took great care to place each heavy element gently so as not to cause damage or disruption to the plaster directly below.
Elevator Shaft Shotcrete
As a significant upgrade to the accessibility of the building, we installed a new elevator and elevator shaft from the basement up to floor 2 (30+ feet). After the shaft was formed, Conco applied spray-in concrete (Shotcrete) from the inside (8″ thick) all in one day. This saved a substantial amount of time, as cast-in-place concrete would have taken much longer to install. Additionally, we were able to have the Shotcrete placed against the backside of historic finished walls without any issues by structurally bracing and reinforcing the walls, keeping them from stressing as the concrete was applied.
We were also able to maximize the use of the space above the elevator shaft by tightening our measurements and tolerances to make the shaft as compact as possible. This enabled us to keep the floor above the elevator from having to be raised at all, creating an easily accessible storage space for the school to use (minus where the HVAC system was installed).
There was a lot of collaboration with our architectural partners at SHKS Architects and our structural engineering partners at Degenkolb Engineers throughout the project. Ultimately, all but the structural steel elements are hidden away in the walls and under the floors for a clean look and a strong building.
Mario Cuadros
Mario was Rafn’s Senior Project Engineer on the Gracemont Hall renovation project for the Bush School in Seattle. This complex renovation and seismic retrofit project restored and preserved the historic building while giving it new life. The team restored beautiful exterior mansory, ornate plastered ceilings and walls, and historic woodwork throughout the building. New modern mechanical, electrical, plumbing, and fire sprinkler systems, in conjunction with energy upgrades to the exterior envelope, ensure occupant comfort and energy efficiency. And critical safety elements added to the building’s structure create a safe building at the center of the school’s Upper Campus.