Concerns about safety of buildings following the recent earthquake of 7.9 magnitude in Nepal and a few regions of India have intensified over the last week. As the Capital falls under zone IV, where quakes of severe intensity can strike, Delhiites have started exploring options that can guarantee the safety of their dwellings – which is where retrofitting comes in.
While Noida and Gurgaon have a large number of multi-storeyed structures that have been newly built and comply with modern construction codes, Delhi has several non-engineered or poorly engineered low rise structures (up to five floors) that are vulnerable and require immediate retrofitting. This option, however, is not viable for many structures in which the masonry is so weak that reconstruction is the only option.
Retrofitting is required when there is change in the use of a building, additional floors have been added due to increase in the floor area ratio, additions or alterations have been made or some damage has occurred due to natural calamities such as earthquakes or manmade reasons such as blasts etc, explains Abhay Gupta, director, Skeleton Consultants Pvt Ltd, structural design advisors.
Old buildings requiring enhancement or structures with weak foundations or in which poor quality construction material has been used should also be retrofitted, he adds. This helps prevent further damage to the structure.
Retrofitting is also required if the following codes have not been adhered to while constructing a building: Indian standard codes IS 1893-1 (2002): Criteria for Earthquake Resistant Design of Structures, Part 1: General Provisions and Buildings [CED 39: Earthquake Engineering], IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd revision) and IS 13920, 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces.
A detailed structural audit is required for a building to be retrofitted. This starts with a visual and photographic survey and data collection exercise, based on which a structural engineer prepares an analysis programme to identify the deficiencies. In-situ strength of concrete, strength of bricks, rusting or condition of steel bars are assessed by non-destructive testing. Complete mapping is done through a diagnostic process called forensic civil engineering, before the retrofitting methodology is worked out, says Gupta.
Aware homebuyers have always been the first ones to go in for safe buildings. An eight-year-old house in Noida with a basement, ground and four floors had to be retrofitted after it was put up for sale as the new buyer demanded a structural safety certificate because two more floors had been added to the building after change in floor area ratio norms. “We carried out a complete analysis of the house and deficiencies were identified in columns and foundations. Some beams were also inadequate. These were retrofitted by using the fibre wrapping technique (columns are wrapped in glass and carbon fibre using synthetic resin). The cost? About `2crore over an area of about 60,000 sq ft,” Gupta explains.
Work was also done in buildings in the township of a public sector undertaking which was constructed in 2001. Cracks had developed in the structures due to poor quality of concrete and steel bars in the columns in the stilt parking area were rusted.
These were retrofitted using concrete jacketing to avoid any further damage due to earthquakes or weathering, he says.
Many low and mid-rise buildings such as builder floors in south Delhi have compromised on earthquake-resistant design norms. In the NCR, one of the major issues with high-rises is the soil below, the foundation and the foundation type. Before retrofitting the structure, homebuyers have to get detailed soil investigation done, says Gupta.
A 30-year-old project with three storeys in Delhi was also strengthened after its load bearing masonry, walls and slabs were retrofitted. However, since it was an old building, it could not be upgraded to full earthquake compliance.
According to Mahesh Tandon, managing director, Tandon Consultants Pvt Ltd and president, Indian Association of structural engineers, an earthquake is different from all other loads applied to a building. The vertical load-carrying capacity of a building has nothing to do with earthquake resistance. An earthquake imposes a horizontal load on a building so its structure has to be resilient and flexible. What this means is that during an earthquake deflection should take place in a building without impairing its load-bearing capacity. This is why structural engineers should be part of the design process.
For getting a structure retrofitted, the time of construction has to be factored in for being updated on codes of seismic design which are constantly being upgraded. Coordinates of the structural engineer who worked on a project should also be available with parties getting buildings retrofitted. Strength of the stilts on which many buildings stand should be checked as any weakness would be considered dangerous. The overall shape of the building should also be checked and any sign of stiffness checked. The structure should be symmetrical as far as possible. L shaped and T shaped buildings should be avoided as far as possible. If a building has stood for vertical loads for 50 years, there is no reason why it should stand for the next earthquake - the return period of an earthquake generally being 100 years.
Areas close to the Yamuna riverbed, particularly the left bank, are considered particularly vulnerable because of ‘liquefaction’ - poorly graded sand with nil bearing capacity, the stiffness of which can be reduced because of shaking in a quake. It’s advisable to have the dwellings checked in the area to see if the foundations are deep enough, if it has been built using piles, and how deep the piles go. Most buildings in Nepal tilted after the earthquake because the foundation was not deep enough. A geotechnical engineer can help reduce the problem of liquefaction if it exists in the area by using certain methods to take out the excess soil and then testing it in a laboratory to see if it is poorly graded sand.
It’s easier to build a new structure than to retrofit it, and the strengthening process is carried out for structures that are important. Hospitals, schools or lifeline structures as they are called have to function effectively in case of an earthquake.
To retrofit a structure, devices, called dampers, that can absorb the shock of an earthquake, are installed. As part of the ministry of earth sciences project (financed by the ministry) IIT Roorkee has developed a device in collaboration with a Noida-based firm called Resistoflex Dynamics Pvt Ltd. “We provided the technical knowhow. The device was developed last year. Its purpose is to hold the earthquake energy going into the structure, thereby reducing the damage that may be caused to the building,” says D K Paul, emeritus professor, IIT Roorkee
Buildings can also be strengthened by a combination of both dampers and conventional methods. Sensors can also be used in smart buildings to detect the vibrations of the structures. This data is then fed to a computer which decides how much neutralising force is to be applied to the structure. This force is applied through actuators which neutralise the impact of the vibrations on the structure, adds Paul.