Earthquake-Proofing Techniques
Earthquakes occur in many parts of the world, sometimes with great regularity. The magnitude of an earthquake is measured by the Richter seismic scale, while the severity of a earthquake is measured in terms of damage and lives lost. The damage and casualties are related to more than the strength of the tremor, but also to population density and the quality of building construction.
In the San Francisco earthquake of 1906, a majority of the buildings withstood the tremors but were destroyed by the fire that followed. It not only spawned demands for fireproofing techniques, but also brought about earthquake-proof primary and secondary water supply systems for fighting fires.
During later earthquakes, many overpasses of the California interstate highway system collapsed or were damaged because of their inflexible design. In the 1950s, the concept of ductility, or pliancy, was formulated. It called for the use of energy-absorbing features and reinforced building materials.
The major thrust of earthquake-proofing by architects is to prevent the collapse of buildings. The ability of a building to withstand the stress of an earthquake depends upon its type of construction, shape, mass distribution, and rigidity. Various combinations of techniques are used. Square, rectangular, or shell-shaped buildings, and buildings with few stories, can better resist vibrations than L-shaped structures or skyscrapers. To reduce stress, a building's ground floor can be supported by very rigid, hollow columns, while the rest of the building is supported by flexible columns located inside the hollow columns. Another method is to use rollers or rubber pads to separate the foundation columns from the ground, allowing the columns to shake horizontally during an earthquake.
To help prevent collapse, roofs should be made of light-weight materials. Exterior walls can be made more durable by fortifying them with steel or wooden beams, or with reinforced concrete. Interior walls can bolster exterior walls, and a continuous collar can cap a rectangular shaped structure, aiding its stability. If nonstructural walls (not used for support) are attached only to the floor or only to the ceiling, they can move sideways as the building sways. Flexible window frames can hold windows in place without breaking during tremors.
Some architectural ideas are theoretical and, even after thorough laboratory testing, are not proven until an earthquake occurs. The San Fernando earthquake of 1971 taught engineers a valuable lesson, as the so-called soft story concept failed completely. It was thought that the upper stories of a high-rise building would suffer less damage if the first story was allowed to flex, having windows and facades instead of rigid walls and columns. However, many of these buildings collapsed.
The collapse of the Nimitz Freeway in Oakland, California during the San Francisco earthquake of 1989 made it clear that despite extensive research and building codes for resistant construction, not enough had been done to prevent damage. The success of architects in dealing with the destructive force of earthquakes will no doubt take many years of trial and error.
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