As is often said - earthquakes don't destroy life and property, unsafe buildings do. The aftermath of countless earthquakes that wreaked havoc could have been substantially reduced if safety and durability were foremost in the minds of the architects and developers who built those buildings. Earthquake Engineering or Seismic Retrofitting is the construction or modification of buildings such that they are more resistant to seismic activity, soil failure due to earthquakes and ground motion.

Levels of modification

Seismic retrofit is primarily applied to achieve public safety, with various levels of structure and material survivability determined by economic considerations:

Public safety only: The goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passers by, and that the structure can be safely exited. Under severe seismic conditions the structure may be a total economic write-off, requiring tear-down and replacement.
Structure survivability: The goal is that the structure, while remaining safe for exit, may require extensive repair (but not replacement) before it is generally useful or considered safe for occupation. This is typically the lowest level of retrofit applied to bridges.
Primary structure undamaged and the structure is undiminished in utility for its primary application. A high level of retrofit, this ensures that any required repairs are only "cosmetic" - for example, minor cracks in plaster, drywall and stucco. This is the minimum acceptable level of retrofit for hospitals.

The most common structures requiring earthquake engineering are bridges, dams, road viaducts, towers, unreinforced masonry and insufficiently enforced concrete structures. Low rise buildings (around ten stories high) will be relatively stiff and light and so have a higher resonant frequency while higher buildings will usually incorporate ductile steel frames and by their height will have lower natural frequencies.

Success stories

It is said that no building is earthquake proof but by engineering a building upon the building codes designed on the basis of the seismic history of the region the extent of damage can be greatly reduced. Some of the prominent structures which suffered little or no damage in earthquakes have been based on earthquake engineering.

California falls in a highly seismic region in the west coast of the U.S. In October 1989, a high magnitude quake rocked the Santa Cruz Mountains in central California. The impact was felt in downtown San Francisco- l00 kilometers away - where occupants of the Transamerica Pyramid were unnerved. The 49-story office building shook for more than a minute. The U.S. Geological Survey (USGS) instruments, installed years earlier, showed that the top floor swayed more than one foot from side to side! However, no one was seriously injured, and the Transamerica Pyramid was not damaged. This famous San Francisco landmark had been designed to withstand even greater earthquake stresses, and that design worked as planned during the quake.

The 1994 Northridge earthquake cracked the surface pavement on the upstream slope of the Los Angeles Dam. Overall, the dam, designed to withstand severe shaking, suffered very little damage.

Future trends

Earthquake-resistant design of structures has grown into a true multi-disciplinary field of engineering wherein many exciting developments are possible in the near future. Most notable among these are:

A complete probabilistic analysis and design approach
Performance-based design codes;
Multiple annual probability hazard maps for response spectral accelerations and peak ground accelerations with better characterization of site soils, topography, near-field effects;
New structural systems and devices using non-traditional civil engineering materials and techniques; and
New refined analytical tools for reliable prediction of structural response, including nonlinearity, strength and stiffness degradation due to cyclic loads, geometry effects and more importantly, effects of soil-structure interaction.

Conclusion

Earthquakes, though tragic, also provide the momentum to the process of improvements in seismic design codes and construction practices. It is important to bear in mind that there is no such thing as an earthquake proof building. But a properly engineered tall building should be able to withstand the maximum credible earthquake for its area without collapse, and lesser seismic events without major structural damage.

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