Base Isolation, What is it?

Almost everyone in the architecture and engineering professions has heard about it. Some manufacturers claim it is equivalent to what penicillin was to the medical industry. But, just what is base isolation? Base isolation refers to a method of either retrofitting or designing structures to better withstand earthquake vibrations. In a typical structure, the structure is attached to the foundations through columns or other supporting members. Most of us design the connections between the columns and the foundations as solid rigid connections. Usually for winds and other environmental loads, we have always found that a solid connection between the foundation and the supporting structure was the best performing connection. This same philosophy was also used for many years for resisting seismic loads.

When an earthquake occurs, the surface of the earth fractures and moves. This makes the soil around the fracture move and vibrate. This radiates from the fracture zone in all directions. As these vibrations reach a structure, the vibrations shake the foundations which in turn shake the structure attached to them. Now, just what would happen if we were to separate the foundation from the structure so that the vibrations could not be transmitted to the structure. This is the idea behind base isolation. We insert some sort of device between the foundation and the structure so that earthquake vibrations cannot be transmitted to the structure.
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But, how does base isolation affect how a structure behaves during an earthquake. First, lets look at a conventional structure that is attached to it foundations. The seismic vibrations are transmitted through the foundation of the structure to the supporting members of the structure. These vibrations can be sharp jarring movements. This causes large forces in the members of the structure and many times causes failures in the members and connections. But, unless the failure is catastrophic, the relative movements between the foundation and the structure are small.

Next, lets look at how a base isolated structure moves during an earthquake. First, it is virtually impossible to totally separate the foundation from the structure above it. Usually, some sort of bearing or other flexible device is used. Now, when we say flexible, this is relatively speaking. The actual devices can be very stiff. Therefore, these devices remove a significant amount of the attachment between the foundation to structure but they do not remove all of the attachment. So as the vibrations from the earthquake shake the foundation, only a portion of these vibrations are transmitted to the structure. Therefore, the structure does not move much. This decreases the forces generated in the structure. But, since the foundation is moving with the ground and the structure is not, there are large relative movements between the foundation and the structure above.

But, there is a conflict in how we want a structure to behave. First, we want the structure to feel solid for the support of vertical loads so that users feel comfortable on the structure. Second, we want the structure to not move appreciably during large winds and other lateral loads, so that the users of the structure feel comfortable. But, we want the structure to be flexible during an earthquake. This is why manufacturers have developed many different types of devices to accomplish this. Lets review some of the devices on the market and how they work.

These were first developed in New Zealand. The bearing is an elastomeric bearing with a lead core. The lead core adds stiffness to the bearing for small loads including wind load. The lead then deforms during an earthquake and separates the structure from the foundation. These systems have been installed on a number of structures.

Sliding bearings consist of either a flat or curved surface with a stainless steel surface mating against a PTFE (Teflon) surface. This has a sliding friction factor of three to four percent of the vertical load. These bearings depend on the friction factor to restrain small lateral loads and once large lateral loads are applied they slide. The curved surface bearings also depend on lateral loads trying to lift the structure up.

There have been a number of bearing and damping device combinations to try to make the bearings work as desired. These include elastomeric or sliding bearings combined with large shock absorbers or dampers. Just like a shock absorber on a car, the dampers have a higher resistance to fast moving motions compared to slow moving motions. This type of combination has been used for a number of years in the airplane and automotive industry.

There are a number of other devices on the market. The best way to specify a base isolation device is to determine what characteristics that you want the device to have. Then let the different manufacturers tailor their product to your needs. You can then select the product that works the best for you.