While many earthquakes are moderate in size and cause minimal damage, a powerful quake sometimes strikes a community that is not sufficiently prepared for the potential destruction. Some of these severe events affect thousands of lives and cost billions in repairs. One of the most effective ways to prevent significant damage from an earthquake disaster is by following building codes with optimal construction and seismic design standards.
Engineers have been taking measures to guard against earthquakes for decades, and they have made significant progress in understanding the impacts earthquakes have on buildings. Besides constructing new buildings that are more resistant to damage, construction companies can use these innovations to renovate older structures, making them more stable in the event of a disaster.
Earthquakes happen when groups of rocks in the Earth's crust slip and collide. This movement occurs along a fault line, a fracture in a body of rock that can extend for many miles. When the rocks slip, they release enormous amounts of energy that spread through the crust as seismic waves in all directions, causing the ground to vibrate and shake at the surface.
Ground shaking is the primary reason why buildings sustain damage during an earthquake. Multiple factors can influence the strength of the shaking, including the earthquake's magnitude, the soil type and the building's proximity to the fault. Although many traditional building designs can handle vertical forces of gravity and weight, some cannot withstand the horizontal forces caused by seismic waves.
Before beginning a major construction project, engineers evaluate a building site's seismic activity with the help of national seismic hazard maps prepared by the United States Geological Survey. Specific building codes indicate seismic design provisions based on these maps. In these instances, builders must adhere to stricter standards to handle potential destruction in high-hazard areas.
Some specific building techniques that can help prevent a structure from sustaining damage during an earthquake include:
Base isolation requires creating a flexible foundation to “float” the building using isolating springs, bearings or cushioned cylinders. If the foundation moves during an earthquake, these isolators vibrate while the rest of the structure remains steady.
One of the most popular isolation designs utilizes lead-rubber bearings containing a solid lead center sheathed in layers of steel and rubber. The lead centers make these bearings strong and stiff in vertical alignment, while the steel and rubber layers offer flexibility on the horizontal plane.
Active mass damping involves placing a heavy weight at the top of a building connected to dampers that absorb shock. When the building shakes, this mass acts like a pendulum and moves in the opposite direction to provide stability. This method works well for skyscrapers or in buildings with multiple stories.
Other damping methods involve vibrational control devices on multiple levels throughout a building. Dampers containing piston heads inside lubricated cylinders fit between the building's columns and beams. The building transfers an earthquake's vibrational energy into the pistons, which push against the silicone oil inside the cylinders and transform the energy into heat.
Another method of redistributing forces that travel through a building during an earthquake is reinforcing a building's structure with shear walls, diaphragms and moment-resisting frames.
Although these reinforcing processes are a significant help with shock and vibration absorption, selecting the most effective building materials is vital for ensuring adequate protection. Structures made from materials with low ductility, like brick and concrete, absorb little energy and are more likely to crack. In contrast, buildings consisting of structural steel or steel-reinforced concrete are more resilient because they offer higher shock absorption levels.
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