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Bridges do not just connect two far-off places; they serve a greater purpose. They minimize distance and help humans win over natural obstructions, rivers in particular. Building earthquake resistant buildings is important for human safety, but if a bridge is destroyed, connected places can be completely isolated from each other.
The 1989 San Francisco Bay Area earthquake suspended traffic for more than twenty hours because of the damage done to a bridge deck. In 2011, Japan suffered a major earthquake resulting in many bridge collapses, but the losses were minimized because the Japanese implemented earthquake resistant design practices. There have been many incidents in the US and other countries where post-earthquake studies have reported that a seismic design would have minimized the losses- or even absorbed the earthquake jolts.
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The Design Considerations
While constructing bridges in earthquake prone areas, especially in the seismically active Zones III and IV of the Indian subcontinent, one thing we know in advance is that the chances of earthquakes occurring in such areas are very high. And that is the only thing we know because predicting earthquakes is not yet possible.
While designing bridges in earthquake prone areas following things must be considered.
Earthquake history of the region is a major factor that needs due attention from engineers. If a particular seismic zone experiences earthquakes of different magnitudes periodically, historical facts will help in predicting the most probable times of the year when an earthquake can happen. Obviously, you do not want to construct a bridge at the time when the chances of earthquake are high. Bridge building for earthquake areas needs extensive study of the earthquake history of the region.
Load Factors: Only dead and live loads are considered when designing bridges in non-earthquake zones. However, in quake prone areas, another force is of concern. This is called "seismic load," and it includes the forces on the bridge due to acceleration produced by earthquake jolts. How do we fit seismic load in our design? Historical facts help us to determine the seismic load factor in our designs. In case a region has no earthquake history, a minimum load is considered in design, which varies from zone to zone.
Seismic retrofitting is what modern technology has to offer. It not only makes old bridges resistant to earthquakes, but also helps in cost reduction. (Constructing a new bridge is always a time and resource consuming practice.) After a detailed seismic performance evaluation, retrofitting is carried out on old or weak bridges using methods such as friction damper systems, carbon fiber plastic reinforcements, and external prestressing, as well as improving soil properties to keep a check on ground motion. Seismic retrofitting is a relatively new technique but the Civil Engineering societies and associations across the world are promoting it because this is going to save money, time, and resources, without compromising the reliability and safety factor.
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Minimizing Risk - The Bottomline
Japan, the pioneer of earthquake engineering, lost many lives and suffered huge property loss in 2011. Mastering technology is something that we humans can do, but the bottom line is that we cannot beat nature. This does not mean we should be pessimistic or passive; it means that we need to put our best foot forward and try to minimize the risk. Earthquakes won't affect us as much if we are prepared, and the best preparation at this time is to implement earthquake resistant design practices and to make the best use of seismic retrofitting technology. An optimistic approach and staying updated with information related to seismic zones and seismic design considerations will definitely help to build better and more stable bridges.