Photos. Typical Reinforced Masonry buildings in India The earth’s surface is under continuous movement, in other words is always vibrating. However, these vibrations are so low in magnitude that they are not felt by us humans at all; unless they are large in size, which happen during events such as earthquakes.
One of the methods which earthquake experts apply to understand earthquakes and these vibrations is by classifying these in the form of waves. These waves which are generated inside the earth’s crust during an earthquake have their movement in all directions. Known as seismic waves, these waves can be categorized in to two types- 1. Body, and 2. Surface. While the Body wave would move through the body of the earth; the surface wave would take a little longer route because it has to make its way to the nearest earth’s surface first, and thereafter travels as a surface wave. See some simple educational websites Website 1, Website 2 which provide simplified understanding of these earthquake waves.
The science of structural engineering is concerned how these waves travel and when they hit, have an impact on the building.
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In recent last one year history has seen two unfortunate earthquake events, with some significant aftershocks. New Zealand and Japan, faraway places were in news. Last year significant damage was noted in other places such as Chile as well. See what the Structural Engineers have to say about what they felt:
New Zealand (22nd Feb 2011)
Roberto T. Leon, president of the Structural Engineering Institute, ASCE has an interesting blog on being present in Christchurch, New Zealand when the 2nd earthquake had unexpectedly stuck there. The whole blog is an interesting read, where in a portion he summarizes a comparison with the Chilean event.
It was eerily different for me because my most recent recollections are from the large Chilean earthquakes that I experienced last year. The Chilean ones “build up” slowly and last a very long time. This one seemed to be three or four very sudden, violent jerks, with a strong vertical feel, and lasting probably no more than 15 seconds or so.
Japan (11th March 2011)
Balaji, a friend and a structural engineer, was in the fourth floor his ten storey office building in Tokyo when the earthquake stuck. Pretty much inland where the Tsunami could not reach, a few days later he told me over Skype [http://skype.com] how he first felt the body wave which came stinging in no time and took him off ground, and then he waited for the surface waves to arrive. While he prepared himself for a floor dance, for the surface wave would arrive in no time, next moment he saw his colleague flying away and his back curved hitting the wall. It was a nasty bump but thankfully both are safe. The buildings their colleagues designed ensured they are.
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Ten years back, I personally felt the Gujarat, India earthquake (26th Jan 2001), because it lasted for approximately two minutes. Halfway, it woke me up from my sleep; and the ground plus two building I was in, a typical Reinforced Cement Concrete (RCC) confined structure [See pictures] took the shaking well. One can contend such buildings, which are a typical design for institutional buildings across India, therefore take building movements and uncertainties into consideration when designing/constructing one. Otherwise, many buildings which are built keeping only enclosure in mind were not that lucky. Many had collapsed. A structural engineer colleague of mine, when we were students, in all this chaos, was wise in saying,
… either you design the building so stiff that nothing happens to it, or make it so flexible that you can bend or twist it endlessly anywhere, or you make the joint so ductile that no matter what the earthquake force or duration is, let it be endless, the joint shall never ever snap out.
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Website 1: http://www.matter.org.uk/schools/content/seismology/pandswaves.html
Website 2: http://earthquake.usgs.gov/learn/topics/?topicID=63
ASCE President blog: http://www.asce.org/PPLContent.aspx?id=12884904978
Skype: http://skype.com
Pictures of Typical Reinforced Cement Concrete (RCC) confined structure
