Why do buildings fall during the earthquake?
Earthquake is an unplanned, sudden, unpredictable, seismic event that causes the shaking of the surface of the Earth due to the passage of seismic waves through the tectonic plates. It is a natural disaster that often leads to massive destruction, loss of lives and property.
Most of the seismologists say,” Earthquake don’t kill people, buildings kill people”. It is indeed true to some extent as in most of the cases people are killed due to the collapse of buildings and other structures during the earthquake and not merely due to the shake itself. Earthquake is a terrifying phenomenon and the terror has increased even more these days with the growing cities as there is increased risk of collapse of more buildings thereby posing the greater risk. Most of us think that buildings fall solely due to the shaking of the ground but in actual from an engineering point of view, this is not the case.
The main causes behind the falling of buildings during an earthquake can be summarized as follows:
1. Failure of Soil:
Earthquake causes shaking of the ground simultaneously particularly due to the passage of the seismic wave. The lateral forces imposed are so strong that it can easily turn the soft soil into the loose mass of sand-like particles completely abandoning its ability to bear weight. Such sand-like mass can transform sloppy sides into mudslides posing the risk of landslide. Thus, buildings that have been constructed in such soft soil or sloppy areas have a greater threat of collapse during the earthquake. During the motion of ground in the event of an earthquake, the building also moves back and forth. After the shaking stops, the buildings may sometime slump into the ground.
The buildings that are well-built and intact may remain erect but will topple in case of unstable soil on account of the failure of the soil. According to the reports, during the Mexico earthquake,1985; about four billion damage was caused due to the collapse of buildings owing to the failure of soil.
Figure 1: Cracks in Soil
2. Failure of Foundation:
One of the main cause behind the collapse of buildings during an earthquake is the failure of the foundation. When the foundation is not able to withstand the seismic stresses imposed, it fails thereby causing falling of building. Among the various factors that determine the ability of a foundation to resist the stresses is the mass of a building. The mass of a building can resist normal lateral forces like force exerted by the wind. However, in most of the cases, the buildings are not designed to resist the intensive multi-directional lateral forces. The foundations of the buildings in particular may shake from its original position and may not be able to hold the structure above.
Figure 2: Pile foundation during an earthquake
3. Failure of Soft Floors:
Soft floors are those floors that consist of large open spaces, minimal shear walls on the interior side and additional floor-to-floor height. In many cases, it has been observed that often the upper floors remain intact but the lower floors are either crushed or crumbled. This is because the seismic forces are maximum at the ground floor where most of the soft floors are located. The soft floors are also less intact than the building structure as a whole and thus are more prone to failure.
One such example of the collapse of building due to failure of soft soil was the Leaning Tower collapse during the Taiwan earthquake as shown in figure 3.
Figure 3: Collapse of building due to soft floor failure
4. Failure of Building Itself:
The strength of the building depends upon the materials that have been used during its construction. In general practice, buildings made up of wooden materials are less susceptible to collapse than the concrete buildings. This is because concrete buildings lose rigidity. Most of the buildings collapsed in the villages of Nepal during April 23, 2015 earthquake as shown in the figure below.