Earthquake-Safe Structures - The way a building is built its construction is a large factor in what happens during an earthquake. Building construction is the reason many more people died in the 1988 Armenia earthquake than the 1989 Loma Prieta, California earthquake. Although the Armenian earthquake was only slightly lower in magnitude, the mud houses that are found throughout the area collapsed. Most buildings in California’s earthquake country are designed to be earthquake safe. However even earthquake safe buildings can be damaged by a large earthquake.
Engineers who design earthquake safe buildings must understand seismic waves and how they affect different types of ground. Skyscrapers and other large structures built on soft ground must be anchored to bedrock, even if it is lies hundreds of meters below the ground surface.
The materials used to construct a structure affect its ability to weather an earthquake. The type of material that is best depends on the size of the building. Small structures, like houses, do better if they are constructed of materials that bend and sway such as wood and steel rather than brick, stone, and adobe, which are brittle and will break. Brittle materials are less likely to break if they are reinforced by steel or wood. Larger buildings must sway, but not so much that they touch nearby buildings. Counterweights and diagonal steel beams are used to hold down sway.
A completely different approach for large buildings is to place them on rollers so that they move with the ground but do not collapse. Buildings may also be placed on layers of steel and rubber, which absorb the shock of the passing seismic waves. Structures that fail usually do so because they are weak at the connections, such as where the walls meet the foundation. Earthquake safe buildings are well connected. In a multi-story building, the first story must be supported or the structure may collapse.
Older structures can be retrofitted to be more earthquake safe. Retrofitting includes adding steel or wood to reinforce a buildings structure and its connections. Elevated freeways and bridges can also be retrofitted so that they do not collapse. The goal of retrofitting is different depending on the type of structure being altered. Most structures are retrofit only to a strength that protects human life. More important structures, like bridges, are made to survive intact, but may need extensive repair after the earthquake.
Structures that need to be used in an emergency, like hospitals, are retrofit to higher standards so that they will need only superficial repairs after an earthquake. The highest level of protection is a retrofit that will allow a building to survive unaffected. This is very expensive and is only done for buildings that are of great historical or cultural significance.
Of course, one of the biggest problems stemming from earthquakes is fire. Fires start because earthquakes rupture gas and electrical lines. Breaks in water mains compound the problem by making it difficult to fight those fires. One effective way of dealing with this is to zigzag pipes so that they bend and flex when the ground shakes. Straight pipes will break in aquake. In San Francisco, water and gas pipelines are separated by valves so that areas can be isolated if one segment breaks.
Since engineers know what sorts of structures do best in earthquakes, why aren’t all structures in earthquakes zones constructed for maximum safety? Of course, the reason is cost. More sturdy structures are much more expensive to build. Since no one knows which structures will be exposed to a large earthquake during their effective lifetimes, communities must decide how safe to make their buildings.
They must weigh how great the hazard is, what different building strategies will cost, and how much risk they are willing to take. In poor communities, the choice may come down to spending money on earthquake-safe buildings or funding other priorities, such as a water sanitation project. The choice often comes down to protecting against a known risk versus unknown one; for example, many people in developing nations die each year from drinking and bathing in unclean water.
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