All about retrofitting structures
Retrofitting becomes essential after detailed evaluation of weak structures. Existing structures need to be checked to see if they conform to Bureau of Indian Standards code
The Delhi Development Authority ( DDA) has extended its deadline to submit forms for its ongoing DDA housing scheme – 2014 till October 15. The reason for this extension is said to be lastminute rush due to a number of holidays just before the closing date.
The scheme - with 25,034 flats on offer - was launched on September 1 and was to close on October 9. The draw of lots has been tentatively fixed for November 5, 2014. According to reports, DDA sold 16 lakh applications for the scheme. However, only three lakh prospective homebuyers have filled the forms till now.
Anticipating a last- minute rush after a tepid response, DDA has extended its deadline to submit forms for its largest ever housing scheme till 5pm on October 15.
The draw of lots is tentatively fixed for November 5, supervised by a committee headed by a retired high court judge and two IT professionals, one from IIT-Delhi and the other from the National Informatics Centre.
DDA sold 16 lakh applications for the scheme but got back just three lakh, partly due to a fortnight-long Shraddh period, considered ‘inauspicious’, followed by the festive season.
However, DDA officials said the real reason for the subdued response was a new lock- in clause under which allottees cannot sell their apartments for five years. Also, with very few two and three-bedroom flats on offer, the middle class seems to have kept away from the scheme.
DDA had received some 7.4 lakh applications for a much smaller housing scheme (16,000 flats) in 2010.
The decision to extend the deadline was taken last weekat a meeting with representatives of empanelled banks. “They insisted there was a huge rush of applicants for the scheme at the last minute and the last date should be extended. Also, only about three lakh forms have been received so far and people need more time,” DDA vice-chairman Balvinder Kumar was quoted as saying.
When is retrofitting essential? To comply with the Bureau of Indian Standards code, buildings need to be retrofitted if found weak. What this means is that existing structures need to be checked to see if they conform to standard practices and code specifications and if they are efficient to sustain an earthquake.
Retrofitting becomes essential after a detailed evaluation of existing structures. For this, a structural engineer will have to identify deficiencies, formulate a strategy for seismic strengthening and give a detailed analysis of how adequate the strengthening procedure suggested by him is.
Preliminary evaluation is a procedure to establish actual structural layout and assess characteristics that are likely to effect its seismic vulnerability. Also, it is usually the bottom part of the structure that needs to be stregthened as forces are maximum at this end.
How is retrofitting done?
Just as shock absorbers are used in vehicles, dampers are used to add strength to a building to ensure that the impact of an earthquake is lessened.
Most new buildings and constructions use these and hence may not require retrofitting, but existing buildings that have open stilt floors may need to be strengthened through this method.
Using carbon fibre reinforced polymer (CFRP) is another popular method to retrofit a building. This is used for both commercial and residential buildings but is slightly more expensive as it is not manufactured in India but imported from Switzerland and Japan, says Dr Vasant Matsagar, associate professor at IIT-D and a retrofitting expert.
There are also some conventional ways such as jacketing of structures that include adding plates etc. There are several types of jacketing practices. These include reinforced concrete jacketing, fibre jacketing, addition of reinforced concrete shear wall, addition of steel bracing, addition of steel bracing, addition of supplementary damping and base isolation.
Jacketing is done after deficient frames and joints are identified by structural engineers after detailed evaluation of building.
This method involves making existing columns wear a jacket of sorts, adding a layer of steel and concrete around the frames and joints. Shear walls and steel bracing are added as new elements to increase the strength and stiffness of the structure. Reinforced concrete jacketing helps improve column strength and ductility.
In case of fibre jacketing, columns of buildings are wrapped with carbon fibre and in some cases a shear wall.
As for base isolation method, the foundation itself is isolated and rubber pads are installed to the super structure so that the foundation of the structure moves independent of it in case of an earthquake. What this means is that the rubber pads are stretched in case of an earthquake so that the super structure above moves very little, explains Sandeep Donald Shah, a structural engineer from Miyamoto India. The typical cost of retrofitting a building is 30 to 40% of reconstructing the entire structure. If the cost of retrofitting is found to be 50% of reconstructing the building, it may not be viable. The option then is to go in for new construction. The cost of retrofitting a structure is generally around ₹ 300 to ₹ 500 per sq ft, says Shah.
Time taken for retrofitting
If dampers are available off the shelf a retrofit can take less than a week and if CFRPs are used, it is not likely to take more than two weeks.
Most buildings are retrofitted by using dampers and jacketing