As science stands now, predicting the precise location, time and magnitude of earthquakes is not possible. However, the regions that are more vulnerable to quakes are well known. Much like the subduction zone off Sumatra, the Himalayan belt, in particular, is a seismically active region. As recently witnessed in Nepal and parts of India, the 7.8 magnitude earthquake ended up killing thousands of people. The culprit was unsafe buildings. After all, earthquakes do not kill, unsafe buildings do.
The extent of damage to buildings depends not only on the magnitude of the earthquake, but also on the type of construction practice followed in a particular region or country. For example, an earthquake in Tokyo or Los Angeles may result in damage of only a few buildings because strict construction regulations are adopted. But a similar earthquake may be catastrophic in Mumbai or Delhi, in terms of buildings damaged and lives lost, because the building design and construction practices are not adequately regulated.
There is much we can do to protect our buildings and thus save lives. It is possible today to assess the vulnerability of any building to earthquake, and, wherever possible, to undertake retrofit measures to make it safe.
When an earthquake strikes, the ground shakes violently, depending on several factors like the magnitude, the depth of the focus and the nature of soil. In some rare cases involving sandy soils in the presence of ground water, the soil can suddenly behave like quicksandcausing buildings to sink or tilt and collapse. In such regions, buildings should be either supported on pile foundations resting on hard strata or should be constructed after suitable ground improvement measures are undertaken.
In hilly terrains (as in the Himalayan regions), landslides are likely to be triggered, bringing down buildings located on the unstable slopes. Slope stabilising measures can help to some extent to arrest the damage.
Even if the soil and foundations in a structure are safe, collapse of a building can occur if it does not have adequate strength to resist the horizontal forces that are generated during an earthquake. Also, there should be adequate ductility, which is the ability of the structure to deform without collapsing during the earthquake. For this, it is important to ensure that the connections at the various interfaces of building components remain intact during the shaking.
The seismic forces generated increase with the mass and the height of the building. Therefore, it is desirable to adopt light-weight materials and low-rise structures in highly earthquake-prone regions, unless they are properly designed, detailed and constructed, based on the prevailing standards.
Most of the construction in India are ‘non-engineered’ and built in masonry. Often, the connections between the roof and the walls, and between cross-walls, are weak, rendering such buildings vulnerable to collapse.
There are recommendations available in our national codes on providing seismic-resistant features in such buildings — such as providing small reinforced concrete bands in the walls at the plinth, lintel (above the windows and doors) and roof levels, and various other measures to ‘tie’ the components of the building together. These measures ensure that integrity is preserved during ground shaking. The concrete bands should be horizontally continuous throughout the walls to help in tying the components of the building together. It is also important to ensure that the materials used (such as brick and mortar) as well as construction practices, should be of good quality.
Retrofitting old buildings that do not have such concrete bands is also possible. Ferrocement bands and embedded metal strips that run across the walls (horizontally and vertically) can preserve the integrity of the buildings to a large extent.
In the case of modern buildings, which are ‘framed’ (comprising a skeleton of columns and beams, typically made of reinforced concrete) or having shear walls, it is possible to ensure safety against collapse through proper structural design and detailing, to achieve the desired strength and ductility. There should be adequate number of frames in the two perpendicular directions in plan. The frames should be more-or-less symmetrically distributed to minimise twisting of the building.
Unlike buildings that have a basement, those built on stilts — with no walls in the ground storey — are more likely to collapse, as evidenced during the 2001 Gujarat earthquake. The structural instability is triggered by yielding in the ground storey columns, causing the upper storeys to come crashing down. In this case, the vertical walls of the building do not reach the ground; they suddenly end at the first floor of the building.
As a result, a sudden discontinuity in mass and stiffness of the walls arises leading to a concentration of stresses in the ground floor columns. The embedded steel in the column yields at the beam-column junction in the ground storey causing the whole building to collapse — technically called a pancake collapse.
Buildings on stilts need to be specially designed. Columns in the stilts should be stronger and stiffer than those in the rest of the building to take the additional stress introduced by the stilt construction.
Thus, we can safeguard the lives of our people by adopting proper building design and construction practices.
AMLAN K. SENGUPTA and DEVDAS MENON
Professors, Department of Civil Engineering, IIT Madras
