Strengthening structures

For many years, there was no classification of the earthquake zone in Chennai. Then, in 1977, it was classified under zone II. 

A further revision in 2001 led to it being classified under zone III. 

This meant that building designs had to be altered to resist the intensity of earthquakes, as indicated in zone III. Zone classification is done based on the intensity of past earthquakes.

Therefore, buildings constructed under the old zone classifications, now need attention. Most of the Reinforced Cement Concrete (RCC) frame buildings, especially multi–storeyed buildings, have a ground floor primarily left for parking and other open utility areas. 

Otherwise known as stilt floors or soft storey in earthquake terminology, these are flexible and weak, and cannot withstand even small quakes.

Stilt floor buildings or soft-storeyed buildings have mostly collapsed in earthquakes around the world. This was particularly evident in the 2001 Bhuj earthquake.

Research showed that such collapses were predominantly due to soft-storeyed buildings or partial infill masonry in RCC frames. 

Thereafter, the IS codes for seismic forces were revised to ensure stability of the new constructions during earthquakes. 

However, several old buildings constructed as per earlier IS codes still exist in earthquake-prone areas.

In stilt floors, the floors above displace themselves as a single block and transfer a large amount of horizontal force during an earthquake and make the columns in the stilt floor fail.

Another reason for the collapses during Bhuj 2001 was the partial infill of masonry in the RCC frame. During past earthquakes, reinforced concrete (RC) frame buildings with columns of different heights within one storey suffered more damage in the shorter columns.

This is due to captive column effect in earthquake terminology. Poor behaviour of captive columns is due to the fact that in an earthquake, a tall column and a short column of same cross-section move horizontally, equally. However, the captive column is stiffer as compared to the tall column, and it attracts larger earthquake force. .

In new buildings, captive column effect should be avoided to the extent possible during architectural design stage itself. 

When it is not possible to avoid captive columns, this effect must be addressed in structural design. Several researchers worldwide, including India, have investigated the behaviour of RC frames with partial infill walls under lateral loads.

This author has studied experimentally and proved that adding a small brickwork insert in the opening improves energy dissipation capacity by at least two times, and the cost of this insert is very marginal.

R. SURESH BABU

PTK Architects
Source:The Hindu,CBE,2nd May 2010