In countries where earthquakes are of frequent occurrence, important buildings are now designed to withstand all but the most severe shocks. Structures founded on hard rocks are generally less damaged than those on soft grounds and it is found that properly designed steel-framed or ferro-concrete structures posses the highest degree of immunity from damage. Thus a rigid frame carried on a strong foundation would undergo as a whole the same movement as the ground. In soft ground a concrete raft foundation should be used.
Bridges present special problems; after the Bihar earthquake of 1934, it was found that bridges with screw pile foundations had stood up the best to the shock. Brick arches were easily broken; and the girder bridges supported on stone work piers also failed. The heights of large buildings should not exceed 100 ft and heavy loads near the top should be avoided.
The acceleration of the ground when the shock begins is an important factor which should be taken into consideration while designing a frame structure rigid enough to withstand the force. In big earthquakes the acceleration may amount to nearly half that of gravity but it is generally not greater than 0.2 g. The following are the different techniques used in designing earthquake resisting structures:
1. Cross Bracing
If two diagonals are used in the form of cross-bracing they only need to resist the tension. This is because one brace is in tension for sideways force in one direction on the frame while the other brace is in tension when the force is reversed.
Steel cables can be used for cross-bracing as they can be stretched but not squashed.
2. Rubber Bearings
Rubber bearings are made from layers of rubber with thin plates between them, and a thick steel plate on the top and bottom.
The bearings are placed between the bottom of a building and its foundation. The bearings are designed to be very stiff and strong for vertical load, so that they can carry the weight of the building. However they are designed to be much weaker for horizontal loads, so that they can move sideways during an earthquake.
Rubber bearings have been used to protect the Museum of New Zealand from large earthquakes.
3. Friction Pendulum Bearings
Friction pendulum bearings are made from two horizontal steel plates that can slide over each other because of their shape and an additional articulated slider.
The bearings are placed between the bottom of a building and its foundation. They are designed to be very stiff and strong for vertical load, so that they can carry the weight of the building. However the fact that they slide means that earthquake movements will occur mainly in the bearings.
Friction pendulum bearings have been used in the San Francisco Airport International Terminal. The building has been designed to resist a magnitude 8 earthquake occurring on the San Andreas fault.
4. Viscous Fluid Dampers
Viscous fluid dampers are similar to shock absorbers in a car. They consist of a closed cylinder containing a viscous fluid like oil. A piston rod is connected to a piston head with small holes in it. The piston can move in and out of the cylinder. As it does this, the oil is forced to flow through holes in the piston head causing friction.
When the damper is installed in a building, the friction converts some of the earthquake energy going into the moving building into heat energy.
The damper is usually installed as part of a building’s bracing system using single diagonals. As the building sways to and fro, the piston is forced in and out of the cylinder.