California College of the Arts Expansion

The EBFs were formed using 12-inch-square glulam members, ebony-stained to match the steel. Each was attached to a knife plate using steel dowels, and the plates connect with a single pin to the horizontal perimeter beams wrapping the buildings. Engineers designed the pins so the timber members are only subjected to axial loads, simplifying the connection by providing rotational freedom.

To meet San Francisco’s challenging seismic requirements, engineers decoupled the gravity and lateral systems. The mass timber gravity systems—3-ply CLT panels with glulam beams, girders, and columns—were carefully detailed with consideration for lateral seismic drift. The EBF lateral systems are slightly offset from the gravity structures and wrapped around the building perimeters, adding robustness and ductility. In addition, seated glulam beam and column base connections were designed with slotted holes to avoid inhibiting the rotations of the members, which could happen with lateral building displacement. Because CLT diaphragms were not prescriptively recognized in the code at the time the project was permitted, concrete floor toppings are used as the diaphragms and are tied to the horizontal perimeter beams. This groundbreaking seismic design underwent a rigorous peer review to validate the approach.

The glulam was preservative pressure treated in accordance with American Wood Protection Association (AWPA) Use Category 3B (UC3B), intended for exterior applications where the wood is not in direct contact with the ground. Dowels and fasteners were installed only from the inside of the building, avoiding exposed holes on the exterior. The braced frames were also fabricated with drip edges at the ends, and a saw cut for the connecting knife plate was added to the underside of each brace to prevent water from becoming trapped.