Conventional Building Code Philosophy

During a major earthquake, a large amount of kinetic energy is pumped into the building. The manner in which this energy is consumed in a structure determines the level of damage. All building codes recognize that it is economically not feasible to reconcile this energy within the elastic capacity of structure. The conventional building code philosophy is to design structures to resist moderate earthquakes without significant damage and to avoid collapse during a major earthquake. In general, reliance for survival is placed on the ductility of the structure to dissipate seismic energy while undergoing large inelastic deformations causing bending, twisting and cracking. This assumes permanent damage, repair costs of which could be significant.

Lessons Learnt from Recent Earthquakes

• Conventional construction, even in technologically advanced countries, including the U.S. and Japan, is not immune to destruction. In modern buildings, avoidance of structural collapse alone is not enough.
• The cost of non-structural components (70-80%) is much higher than the cost of the structure itself and must be protected.
• Buildings of post-disaster importance such as hospitals, police stations, telecommunications, educational institutions, etc. must remain operation.

Solution

• Establish performance based design criteria rather than minimum code requirements.
• Place less demand on ductility. Ductility involves permanent damage.
• Dissipate seismic energy mechanically, independent of primary structure.

In a typical undamped structure, the inherent damping is merely 1-5% of critical. With the introduction of supplemental damping of 20-30% of critical, the forces and deformations on the structure can be significantly reduced.

Putting Brakes to Earthquakes

Of all the methods so far available to extract kinetic energy from a moving body, the most widely adopted is undoubtedly the friction brake. Mechanical engineers have successfully used this concept for centuries to stop the motion of equipment, automobiles, railway trains, airplanes, etc. No other method has replaced friction brake. Reason! It is the most effective, reliable and economical mean to dissipate kinetic energy. Similar to automobiles, the motion of vibrating building can be slowed down by dissipating seismic energy in friction.