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

HT Estates - - FRONT PAGE - Van­dana Ram­nani

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.

THINKSTOCK

Most build­ings are retro­fit­ted by us­ing dampers and jack­et­ing

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