Rock Street, San Francisco

Introduction

The ground
movement, caused by the grinding of tectonic plates, which eventually goes on
creating disasters such as failure of structure. This natural phenomenon is
known as the earthquake.

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The energy
that is discharged from those seismic activities makes waves. These waves cause
ground movement, which in turn is transmitted to the structure via the
foundation. Depending on the intensity of these vibrations, cracks and
settlement is caused to the structure.  Force is induced in structure; because of this
the damage to the building increases with ground motion.

Many
engineers introduce ductility to the
building (pliable but not losing its
toughness, meaning a building could be deformed during an earthquake but would
not collapse upon the people inside it. Imagine an elastic band. Though able to
withstand great force, it will eventually break; in the context of the
building, the maximum point it can deform before breaking, called the Elastic
limit. Upon reaching the Elasticity limit, cracks form in the building). However,
elastic materials add much more to the overall construction price which is a
increasing the strength, giving a disadvantage to the components of the building
with less strength. Though the reinforcements are strong, all the force of the
earthquake is directly transferred to the building.

Base
isolation is a frequently adopted earthquake-resistant system. Isolation being
the separation of a building from its foundations, The basic principle of base
isolation is to differentiate the building from its foundation, so during
seismic action, the building stays unaffected from the ground motion. Separating
the structure from the ground by introducing a flexible isolation system
between the foundation and the structure is the solution to this problem.

Base
isolation reduces the displacement of the structure during seismic event.

Utilizing bearing pads which is put between the foundation and the building
(the substructure and the superstructure). Bearings are built to be strong
vertically (meaning when force is applied, it will not bounce) but flexible
horizontally (therefore, in the event of an earthquake, the superstructure
would move parallel to the direction of the earthquake.)

Figure 1: A Conventional Fixed-Base
Building in Contrast to Base-Isolated Buildings.

Note in
figure 1 the comparison between a fixed-based building and a base-isolated
building in seismic activity. The ground beneath each building, therefore, is
moving towards the left (see the arrow beside Ground motion). However, the
fixed-based building is displaced towards the right. Its plane of movement is
the same as ground motion though its direction is different due to inertia. In
most seismic actions, inertia forces are what most affects and deforms the
buildings. Inertial forces inducted to the building is proportional to the
speed of the ground motion (acceleration).

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