There will be more to the stunning Apple Campus 2 than meets the eye. One of the most fascinating—and important—elements of the building will be 3 levels under your feet.
AC2 will sit on top of 700 base isolators. These elements, which are structurally attached to the foundation, are designed to prevent structural and architectural damage during a seismic event.
“The whole idea behind base isolation is that during a seismic event it releases the building from the ground motion,” says John Worley, the lead structural engineer on AC2.
In other words, during a seismic event, the foundation is anchored into the ground and will move with the ground motion. The isolated building will experience only slight movement.
Base isolation is a well-known structural design approach, but AC2’s massive size and signature round shape made the process of designing and testing these base isolators a challenge for John and his team of 10 engineers. Good thing they started early.
“Base isolation was settled on from the get go,” says John. “This [early design decision] has allowed us to do some pretty aggressive things from a non-structural design perspective—the design of the glass, very large stone walls with small joints, things like that. We had to do a lot of studying to understand just how this building will move in an earthquake.”
First, John and his team created a computer simulated model of the building on top of the base isolators. Then they ran the model through 16 types of ground motions.
“Concurrent with our computer simulations, we had 3 prototypes of the base isolators fabricated so we could verify their behavior in real life,” says John. “Testing on those 3 told us very specifically how these things behave, their stiffness and the period of vibration. This fed into our analysis.”
Each steel base isolator is approximately 7 feet in diameter and weighs around 15,000 pounds. “These things aren’t just off the shelves," says John. "Apple had them customized for low friction, which minimizes ground accelerations transferred to the structure while allowing the building to move much more than a typical base-isolated building."
Just how much can AC2 move? Up to 4 and 1/2 feet in any direction.
John adds: “If this building moves 4 and 1/2 feet, all kinds of hell is breaking loose around us. It would be a catastrophic earthquake. But even in that case, we would expect people to be safe inside and some minor damage in the architecture—very minor structural damage.”
When the doors open at AC2, it will be the largest base-isolated building in the world.
Apple has also contracted filmmaker Carlos Carcas to document the creation of Apple Campus 2. Apple's future headquarters in Cupertino will cover 176 acres, exceed 2.8-million square feet, and will accommodate over 12,000 staff.