Make your build­ings safe

Retrofitting is the so­lu­tion for struc­tures which have not been made earth­quake-re­sis­tant in seis­mi­cally volatile Delhi NCR

HT Estates - - FRONT PAGE - Van­dana Ram­nani

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 mul­ti­storeyed struc­tures that have been newly built and com­ply with moder n con­struc­tion codes, Delhi has sev­eral nonengi­neered 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 c o n s t r u c t i n g a bu i l d i n g : In­dian stan­dard codes IS 1893- 1 ( 2002): Cri­te­ria f or 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, rust- ing 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 eightyear-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 s t r uc­ture, 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 i s t o be ap­plied to the struc­ture. This force is ap­plied through ac­tu­a­tors which neu­tralise the im­pact of the vi­bra­tions on the struc­ture, adds Paul.

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