CAPACITY > DEMAND
We know that earthquakes happen and are uncontrollable. So, in that
sense, we have to accept the demand and make sure that the capacity
exceeds it. The earthquake causes inertia forces proportional to the
product of the building mass and the earthquake ground accelerations. As
the ground accelerations increases, the strength of the building, the
capacity, must be increased to avoid structural damage.
It is not practical to continue to increase the strength of the building
indefinitely. In high seismic zones the accelerations causing forces in
the building may exceed one or even two times the acceleration due to
gravity, g. It is easy to visualize the strength needed for this level
of load – strength to resist 1g means than the building could resist
gravity applied sideways, which means that the building could be tipped
on its side and held horizontal without damage. Base isolation takes the
opposite approach, it attempts to reduce the demand rather than increase
the capacity. We cannot control the earthquake itself but we can modify
the demand it makes on the structure by preventing the motions being
transmitted from the foundation into the structure above.
Over the years several types of base isolation system have been
developed by researchers. Not all of those are practically feasible.
Base isolation systems can be classified according to the following: