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Designing “Around” Base Isolation

As an expansion joint cover manufacturer our focus is movement. Although there are several types of movement, seismic forces can be the most catastrophic to a structure and proper design is critical to preventing loss of human life and infrastructure. The following is a brief description of base isolation and how it works. We have also included a simple checklist of what to look for when designing a seismic joint system for a base isolated structure.

What is base isolation?

Base isolation is a system that will fully decouple or separate a structure from the earth. These systems are designed to dampen the quick destructive forces of earthquakes. The systems greatly increase the chances that a structure will remain functional after a seismic event.

How is it achieved?

There are two types of base isolation systems used today:

1. Elastomeric Bearings (Lead-Rubber Bearings or High-dampening Rubber Bearings)
2. Friction Pendulum Systems (FPS)

The first type, elastomeric bearings, is comprised of high-density rubber and steel plates and could include a lead core depending on its type. These bearings support the weight of a structure and absorb the acceleration of an earthquakes’ forces. The rubber isolators are similar to motor mounts on a car.  A motor mount is ridged enough to hold the engine in place but allow the engine to flex with the movements and vibrations from the motor.

The architects for the deYoung Museum, located less than five miles from the San Andreas Fault, trusted CS’s XLS Seismic Joint Covers.

The second type is the friction pendulum system or sliding system. This type of system consists of a stainless steel concave bowl and a self-lubricating slider. The slider is a post that supports the weight of the building and sits within the steel bowl. When seismic movement occurs the building is free to move in the dish and return to the center once the movement is complete. In this design the buildings weight is used to dampen the seismic loads by transferring the horizontal movement to a vertical movement. Think of a skateboarder in the middle of a half pipe – if you move the half pipe rapidly, he will roll up the sides and return to the center once the movement has stopped. The only difference is the FPS can move 360 degrees.

Friction pendulum systems containing CS Expansion Joint Covers include: Cathedral of Christ the Light, UCSF-IRM, San Francisco International Airport, San Francisco General Hospital.

What problems must be overcome?

Both base isolation systems allow buildings to safely move a significant amount in all directions. Because of this potential movement, a gap must be left all the way around the building so it can move freely. This gap is referred to as the seismic moat. This area can be covered with cantilevered slabs or metal plates. Another popular solution is pan-style moat covers. The covers are attached to the building structure and are recessed into the slab adjacent to the building. The pans are then filled with the adjacent finish materials to help disguise the moat covers. The joint system remains flush until the seismic event occurs and then the pans are designed to pop up and allow the building to move freely. After the event, the building and pan-style joint system will return to its nominal position to allow egress in and out of the building. Depending on the building design, provisions may need to be made to allow sections of the moat cover to displace laterally within the system. These provisions usually occur at transitions to vertical covers or at horizontal miters.

The items below are important things to consider when faced with base isolation and seismic moat designs.

  • How much movement is anticipated?
  • Will there be any vertical movement from the isolation system?
  • Do I have any transitions from horizontal to vertical?
  • Does the site slope?
  • Does the seismic moat need to handle heavy traffic (transport or emergency vehicles)?
  • Does the system need to be waterproofed or fire rated?
  • Are there any provisions needed for utilities to handle the seismic movement?

For additional information on base isolation systems, refer to the following links: