31 guideline _for _cyclone _resistant_ construction _of _building.pdf

8/14/2019 31 Guideline _for _Cyclone _Resistant_ Construction _of _Building.pdf

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G U I D E L I N E S F O R

C Y C L O N E R E S I S T A N T C O N S T R U C T I O N

O F R U I L O I N G S I N G U J A R A T

GSDMA

Gujarat State Disaster Management Authority

Government of Gujarat

December - 2001


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GUIDEL INES FOR

CYC LONE RE S I S T A N T CONS TRUC T I O N

OF BU I L D I NGS I N GU J A RA T

Gujarat State Disaster Management Authority

Government ofGujarat

Decembe r - 2 001


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Foreword

The State ofGu j a r a t ,being a coasta l s tate w ith m ajor r i vers pass ing through it , is prone to both

f l oods any cyc lones . In addit i on it is a lso sub jec t to se ismic r i sk . Thus the area faces theposs ibil ity of three m ajor natural ha zards. Fi ftee n dis tr ic ts in the state have been ident i fied as

being prone to mu l t i -hazard r i sk , w here cyc lones and ear thquakes can af fec t a dist r ic t . These

dis t r ic ts and 87 ta lukas prone to cyc lone are l is ted in these g uide l i nes . Tho ugh it wi ll be

unusual to expec t that these hazards can s t r i ke s imul taneou s ly , it impl ies in any case that the

bu i ld ings and s t ruc tures have to be des igned to have adequat e res is tance agains t both the

hazards . Fur therm ore, s torm surges of substant ial he ight are expec ted to accompany a cy-

c lone, part i cu larl y in the 1 0-15 km w ide coas tal be lt . Th is cons t i tutes a ser ious hazard in

add it i on to h igh w ind ve loc i t i es .

Gujara t coas t hav ing ac t i ve por ts , set t l ements and indus t r ies (som e of them in the Surat -

Baroda belt and along the Saurasht ra and Kachc hh coas ts deal ing wi th hazardous chem ica ls)

the State has to prep are itselftominimize disasters in the region more par t icular ly s ince chemical

l eaks can add another d imens ion to a s i tuat ion of natura l d isas ter .

In regard to the ear thquake r isk in the State , GSDM A has al ready brought out three Guide-

l ines for ach iev ing ear thquake res is tance in a ll bu i ld ing c ons t ruc t i ons .

Now in order to safeguard the new as w ell as ex is t ing bui ld ings f rom the fu ry of cyc lon ic w inds

and the ac com pany ing s torm surges , these guide l i nes on Cyc lone Res is tant Cons t ruc t i on of

Bui ld ings have been prepared in w hich s imple m ethods of re t rof it t i ng the ex ist i ng bui ld ings

agains t h igh w inds have also been inc luded. It is hoped that fu ll use of these g uide li nes wi ll be

made in the new as w ell as ex is t ing bui ld ings in the State so as to min im ize the ex tent of

damage in the fu ture cyc lone events .

Dr. P K Mishra,

Decem ber , 2001 Chie f Execut i ve Of f i cer ,

Gujara t Sta te Disaster Managem ent Author ity

Gandh i nagar


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To The Reader,

A severe cyclonic storm had hit the Gujarat coast about 25 km north of Porbandar Port , on

June 09, 1998. The projected peak w ind-speed at landfall w as reported to be betweenQ40 -45

m /s (160 to 170 km ph). The cyclone proceeded almost in a straight l ine bear ing 40 N to-

w ards the Jam nagar port , on the northern coast of the Saurashtra peninsula. The exit of

cyclone from Jam nagar to the Gulf of Kachchh w as roughly at w ind speed less than 42 m/s

(150 km ph). After crossing the Gulf of K achchh, the cyclone passed to the east of the major

port of Kandla. The w ind speeds reported at landfal l near Kandia port , w ere betw een 33 and

39 m /s (120 and 140 km ph). The c yclone then proceeded through the Rapar region of Kachchh

onw ards into Rajasthan at less than 30 m /s (110 km ph). This cyclone is est imated to have

traveled over land for a total of 250 km in Saurashtra and Kac hchh. Torrent ia l rainfall of

varying intensity accom panied the cyclone.

Comm unicat ions in the ent ire region w ere badly disrupted. A storm surge accompanied the

cyc lone in m any of the st re tches a long the c oast . The damage caused by the storm surge w assevere in the ports of Kandla and Navlakhi on the Gulf of Kachc hh. The shelv ing of the coast

l ine and t iming of the c yclone w ith the high t ide aggravated the impact of the surge at Kandla.

The surge in this event w as about 2.2 m r id ing over the high t ide of 3.4m rose to 5.6m above

the mean sea level . Th is was a major cause of the dam age and death in th is area, s ince

impact of the cyc lone w ind on both human l ife and bui ld ings w as much less in com par ison w ith

the dam age caused by the surge.

Financial loss due to this c yclone is est imated at Rs.1200 Crores out of w hich about Rs.500

Crores w as the est imate for the por ts of Kandla and Navlakh i . The c yc lone w as also respon-

sible for the loss of about 1200 human l ives, apart from the m any persons missing.

Quest ion ar ises - What can we do ?

Cyclones have occurred before and w ill cont inue to occur in future. Their occurrence is a

natural ph enom enon on w hich m an has no control. Beside a cyclone br ings the much needed

benefic ial rains too. What we need to do is to construct all our bui ld ings and structures to let

the w inds pass w ith m inimum obstruct ion ( in streamlinef low )and build them so as to be able

to resist their pressures and suct ion effects w ithout dam age. The best approach for protect ion

from the storm surge w ater f low and inundat ion w ill be to build on higher g round. This Guide-

l ines aims to a ddress the issues of safer c onstruct ions against cyclone w inds and protect ion

f rom the storm surge.

Prof. A S Arya,

Decem ber, 2001 Seismic Advisor ,

Gujarat State Disaster Management Author ity.

Gandhinagar


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-4-25'

68 69 70 71 73 7 75 76

GujaratWindand Cyclone HazardMap _.

0 20 40 60 80 100 km

Scalcl M M N H H

I 20

68 69 70 7T n 73 74-f -f - -I-

BMTPC:Vulnerabil ity Atlas Consultant: Dr. Anand S. Arya Map is based on : 1:2.5 M Map, 1990, S.O.I., G.O.I. Basic Wind Speed Map, I.S.:875(3)-1987 Cyclone Data, IMD, G.O.I.

24 H-

2S -t-

22 +

District Town

Very Higti Damage Risk Zone - B (50m s)

Moderate Damage Risk Zone (47m/s)

Moderate Damage Risk Zone - A (44m/s)

Low Damage Risk Zone B (39m/s )

Probable Max. Surge Height (m)

Severe Cyclonic Storm only

Cyclonic Storm crossing Lat.

Risk Factor High

Risk Factor Low

2t"

20 +

r r n

Fig. 1


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GUIDELINES FOR CYCLONE RESISTANT CONSTRUCTION

OF BUILDINGS IN GUJARAT

/. INTRODUCTION

The coastal areas of Gujarat have rece ived a num ber of c yc lon ic w ind storm s causing

devasta t ion over large areas due to ( i ) h igh-speed w inds, which destroy trad it iona l

homes and uproot trees and elec tr ic l ine supports ( i i) local f loods, caused by heavy

ra ins , and ( i ii ) s torm surge w aters , f i rst f low ing tow ards the land then receding back

tow ards the sea, drow ning people , destroy ing hom es, agr icu l ture , trees etc . , w hatever

comes in the path of the f lowing w aters . High speed w ind storms on main land also

many t imes c ause severe dam age to bui ld ings, part icu larly l ight weight roofs , free

stand ing boundary w al ls , etc . Hort icul tural crops suf fer badly in both c ases at sea

coast and inland under high-speed w inds.

Al though the main destruc t ion dur ing cyc lones or h igh w inds occurs in the trad it iona l

non-engineere d bui ld ings buil t using local clay, stone, Adobe or agro based mat eria ls,

the engineered bui ld ing hav ing high sheeted roofs also suf fer huge damage unless

appropria te precaut ions are taken in des ign as w ell as construc t ion. Even in heavy

construc t ions, substant ial non-s truc tura l damage occurs to doo rs , w indows, c ladd ing

wall panels , g lass panes etc .

The aim of t hese guide lines isf i rs t ly ,to br ie f ly exp la in the act ion of w ind on build ings

and state the general principles of planning and design; sec ondly, to bring out details

to prevent the non-s truc tura l damage in the var ious bui ld ings;th ird ly ,to deal w ith the

safety aspects of tradit ional non-engineered bui ld ings; andf inal ly ,to suggestretrofit t ing

detai ls w hich c ould be adopted in ex is t ing bui ld ings to m in im ize the damages under

h igh w inds.

2. SCOPE

These guide lines deal w ith the construc t ion and retro fi t t ing for ach iev ing w ind/cyc lone

resistant bui ld ings. Both engineered and tradit ional bui ld ings are considered. Safety

of non-struct ural c om ponents is also included.

Wind zoning m ap of Ind ia is g ive in IS: 875 (Part -3) - 1987. The sam e has been used

here (See Fig .1) as drawn for Gujarat in Vulnerab ly At las of Ind ia (199 7). It is seen that

a land w idth of 45 to 65 km from the sea coast is subjec ted to very h igh cyc lon ic w inds

up to 50m /s (180 km /h). Th e Distr ic ts and Talukas situated in this belt are l is ted in

Table - 1. It m ay also be seen that the rem ain ing parts of K achchh dis t r ic t , the w hole

of Banaskantha dis t ric t and the N-W 50% of M ahesanadistr ictfal l in that part of cyclone

prone zone w hich could be subjec ted to w inds of 47 m /s (169 km /h) speed.

7


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Table 1

List of Talukas situated in the Cyclone prone zone near the sea coast of Gujarat

00

Sr. No. Disirici Taluka Area in cycl one zone

1. Bhuj Lakhpat, 1 0 0%

Abadasa, 1 0 0%

Nakhtrana, 50 % (South -West )

mandv i , 1 0 0%

Mundana, 1 0 0%

Bhu j , 40 % (South )Bhachau, 60 % (South )

An ja r , 1 0 0%

2. Rajkot Maliya, 8 0% (Wes t )

Morvi, 100 %

Tankara, 60 % (Nor th )

3. Jamnagar Jodiya, 1 0 0%

Jamnagar , 1 0 0%

kalavad. 5 0 % ( N -W )

Lai pur 1 0 0%

Khumbha i l i ya , 1 0 0%

Kalyanpur, 1 0 0%

Bhahvad, 1 0 0%

Jamjodhpur , 1 0 0%

Dawarka, 9 0 %

Dhrol 1 0 0%

4. Porbandar Porbandar, 1 0 0%

Ranavav, 1 0 0%

Kuntiyana 1 0 0%

5. Junagadh Manavadar 90 % (South )

Vanthl i 40 % (S-W)

Keshod 1 0 0%

Mendrada 80 % (S -W)

Mangrol 1 0 0%

Malia 10% (N )

Visavadar 20 % (South )

Talala 1 0 0%

Kodinar 1 0 0%

Una 100 %

Patan-veraval 1 0 0%

Sutrapada 1 0 0%

6. Amr eli Dhari, 20 % (South )

Kahmbha 80 % (South)

Jafarabad 1 0 0%

Savarkundia 40 % (South)

Rajula 1 0 0%

Sr.No. District Taluka Area in cycl one Zone

7. Bhavnagar Mahuva

TalajaPali tanaSihor

GhoghaBhavnagar

UmrallaValabhipurBotad

1 0 0%

1 0 0%60 % (S -W)80 % (East )1 00%100%

50 (East)1 00%50 % (East )

8. Ah med ab ad DhandhankaBarvalaRanpur

1 0 0%1 0 0%80 % (East )

9. Surendranagar LimdiChudaBaviaDholka

60 % (S -W)50 % (East )80 % (South )90 % (South )

10. Kheda MatarTarapur

PetladKhambhatBorsad

40 % (S -W)1 0 0%30 % (West )1 0 0%20 % (West )

11. Vadodara Padra 15% (West )

12. Bharuch JambusarAmod

VagraBharuchAnkleshwarValiaHansot

1 0 0%80 % (West )1 0 0%75 % (West )1 0 0%20 % (West )1 0 0%

13. Sural MangrolOlpad

KamrejBardoliValodSurat

Palasana

60 % (West )1 0 0%1 0 0%70 % (West )15% (West )1 0 0%1 0 0%

14. Navsari Navsari

Jabalpur

GandeviChikhl i

Bansada

1 0 0%1 0 0%1 00 %

1 0 0%25 % (West )

15. Valsad ValsadDharampurUmargaonPardiKaprad

1 0 0%60 % (West )1 0 0%1 0 0%80 % (West )


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3. WIND PRESSURES ON BUILDINGS AND STORM SURGE HEIGHTS

3.1 Basic Wind Speed Zones

The m acro-level w ind speed zones of India have been form ulated and published in IS:

875 (part s) - 1987 t it led " Indian Standard Code of Pract ice for Design Loads (other

than ear thquakes) for Bui ld ing and Structures, Part 3, Wind Loads". There are six

basic w ind speeds 'VO' c onsidered for zoning, namely 55, 50, 47, 44, 39 and 33 m /s.

From wind damage view point , these could be descr ibed as follow s:

55 m /s (198 km /h) - Very High Damage Risk Zone - A

50 m /s (180 km /h) - Very High Damage Risk Zone - B

47 m /s (169.2 km /h) - High Damage Risk Zone

44 m /s (158.4 km /h) - Moderate Damage Risk Zone - A

39 m /s (140.4 km /h) - Moderate Damage Risk Zone - B

33 m /s (118.8 km /h) - Low Damage Risk Zone

The w ind speed zoning map of Gujarat as per Vulnerabil i ty At las of India (1997) is

shown in Fig. 1. The cyclone af fected coasta l areas of Gujarat are class if ied in zones

of 50 m /s, and 44 m /s. The basic w ind speeds are applicable to 10 m height above

mean ground level in an openterrain w itha return period of 50years .At higher elevat ions

and longer return periods, the values w ill be higher.

3.2 Design Wind Speed and Pressures

The basic w ind speed is reduced or enhanced for design of bui ld ings and st ructures

due to fo l low ing factors:

( i) Th e risk level of the structure m easured in terms of adopted return period

and li fe of struct ures (5,25,50 or 100 yea rs),

( i i ) Terra in roughness determined by the surrounding build ings or t rees and,

height and size of the structure.

Local topograph y like hil ls, valleys, c l i f fs, or r idges, etc.

Thus general basic w ind speed being the same in a given zone, st ructures in di fferent

s ite cond i t ions cou ld have apprec iab le m odi f ica t ion and must be cons idered in

determ ining design w ind velocity as per IS: 875 (Part 3) - 1987.

The design w ind pressure at height z above ground level on a sur face normal to the

wind st ream is given by

2Pz = 0.0006 Vz (1)

Whe r e ,

Vz = design w ind veloc i ty , m /s

Pz = design w ind pressure, kN/m


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Cornerareasof severesuction

The va l u e of w i nd

pressure actuallytobe

considered on various

e lements depends on

( I) ae rod yna m i c s of

f low around bui ld ings,

( i i ) t h e w indwa rd

ve r t i c a l fac es being

subjec ted to pressure,

(Hi) the leew ard and

la te ra l faces ge t t i ng

suctioneffect s,and(iv)

t h e s l o p i n g roo f s

ge t t i ng p ressu res or

s u c t i o n e f f e c t s

d e p e n d i n g on th e

slope. The project ing

w indow shades , roof

p r o j e c t i o n s at e a v e

levels are subjected to

u p l i f t p r e s s u r e s

s e v e r a l t im e s t he

intensity of pz. These

factors play an im portant role in determ ining the vulnerabi li ty of given bui ld ing types in

g iven w ind speed zones. For exam ple, Fig . 2 shows the var ious c ladd ing areas of a

bui ld ing, which w ill have di f ferent pressure coef f ic ients .

F igures 3(a) and (b) show typ ica l ef fec ts of openings in the w alls for a given angle of

attack of w ind as indicated:

(a) Only one large opening in a w all w ill cause very large internal p ressure say

0.7pz.w hich com bined w ith external pressures/suct ions wi ll m odi fy the w ind

ef fec ts on c ladd ing and thei r c onnect ions im mensely .

(b) A bui ld ing w ith all w indow s and doors locked w ill have zero or very small internal

suct ion or pressure, 0.2 Pz. If a room has openings distr ibuted in all w alls or at

least in opposite w alls and the overal l porosity is less than 5%, the passage of

airw illcause only low internal pressure say only 0.2 P^.Effect sof wind upl i ft on

roof project ions can also be seen in Fig. 3 (a), (b). For a design speed of 50 m/

s, the basic pressure w ill be 1.5 kN /m ^ and the design pressure could be obtained

I Areas of higher suction / pressura.

H General areas of norma / pressure P^

Fig. 2 Extenal wind pressu re areas on build ing faces

10


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1 11.8 0.6 1.3

t1 t

\it0.7

57

0.7

D.r

0.7

;

i i

aas

I 0 2-*B-


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w ind speeds (by as much as 40 to 55%). But for des ign of s truc tures (except those

considered v ery important ) the above macro-level zoning stated in 3.1 is considered

as suff ic ient .

The frequency of occurrence of cyc lones on the di f ferent port ions of the coast hasbeen di f ferent . (See Fig .1) . Even for the same design w ind spee d, the r isk of damage

per year wi ll be higher in areas subjec ted to more frequent ly occ urring cyc lones.

3.4 Storm Surge

Besides t he very high velocity w inds, the coastal areas suffer from the onslaught of

seawater over the coast due to storm surge generated by cyc lones. A storm surge is

the sudden abnorm al r ise in sea level caused by the cyc lone. The surge is generated

due to interact ion ofair ,sea and land. The seaw aterf low sacross the coast as w ell as

inland and t hen recedes back to the sea. Huge loss of l ife and property takes place in

the process. The height of the storm surge is even higher during the period of high

tides. The estimated com bined heights of high tide and the surge due to cyclonic w indcould be as high as indicated in Table 2.

Since during cyclones, maximum loss of lives occurs by drowning and washing away in

storm surge waters as in 1998 cyclone in Kandia, the coastal plains should be sun/eyed

to prepare 0.5m interval contour plans and the area zoned for various habitat purposes

to minimize storm surge losses to the communities.

Table 2

Estimated Highest Water Level above Mean Sea Level under Cyclones on Gujarat Coast

No. Location Normal High

Tide above

IVISL (m)

Estimated

Maximum

Storm Surge

(m)

Combined

Maximum

Water Level

above MSL

(m)

Remarks

1 Okha0 0

(22 28', 69 05')

Porbandar0 0

(21 38', 69 37')

Kandia0 0

(23 o r , 70 13 ')

Veraval0 0

(20 54', 70 22')

Navlakhi0 0

(22 58', 70 27')

Pipavav Bandar0 0

(20 57', 71 32')

Bhavnagar0 0

(21 45', 72 14')

Coast of Gulf of

Cambay

1.94 2.50 4.44

2

Okha0 0

(22 28', 69 05')

Porbandar0 0

(21 38', 69 37')

Kandia0 0

(23 o r , 70 13 ')

Veraval0 0

(20 54', 70 22')

Navlakhi0 0

(22 58', 70 27')

Pipavav Bandar0 0

(20 57', 71 32')

Bhavnagar0 0

(21 45', 72 14')

Coast of Gulf of

Cambay

1.30 2.60 3.90

3

Okha0 0

(22 28', 69 05')

Porbandar0 0

(21 38', 69 37')

Kandia0 0

(23 o r , 70 13 ')

Veraval0 0

(20 54', 70 22')

Navlakhi0 0

(22 58', 70 27')

Pipavav Bandar0 0

(20 57', 71 32')

Bhavnagar0 0

(21 45', 72 14')

Coast of Gulf of

Cambay

3.43 2.50 5.93

4

5

Okha0 0

(22 28', 69 05')

Porbandar0 0

(21 38', 69 37')

Kandia0 0

(23 o r , 70 13 ')

Veraval0 0

(20 54', 70 22')

Navlakhi0 0

(22 58', 70 27')

Pipavav Bandar0 0

(20 57', 71 32')

Bhavnagar0 0

(21 45', 72 14')

Coast of Gulf of

Cambay

1.10

3.68

2.80

2.80

3.90

6.48

6

Okha0 0

(22 28', 69 05')

Porbandar0 0

(21 38', 69 37')

Kandia0 0

(23 o r , 70 13 ')

Veraval0 0

(20 54', 70 22')

Navlakhi0 0

(22 58', 70 27')

Pipavav Bandar0 0

(20 57', 71 32')

Bhavnagar0 0

(21 45', 72 14')

Coast of Gulf of

Cambay

1.83 4.00 5.83

7

Okha0 0

(22 28', 69 05')

Porbandar0 0

(21 38', 69 37')

Kandia0 0

(23 o r , 70 13 ')

Veraval0 0

(20 54', 70 22')

Navlakhi0 0

(22 58', 70 27')

Pipavav Bandar0 0

(20 57', 71 32')

Bhavnagar0 0

(21 45', 72 14')

Coast of Gulf of

Cambay

5.31 4.80 10.11.

8

Okha0 0

(22 28', 69 05')

Porbandar0 0

(21 38', 69 37')

Kandia0 0

(23 o r , 70 13 ')

Veraval0 0

(20 54', 70 22')

Navlakhi0 0

(22 58', 70 27')

Pipavav Bandar0 0

(20 57', 71 32')

Bhavnagar0 0

(21 45', 72 14')

Coast of Gulf of

Cambay

5.31 say 4.80 10.11 Like in Bhavnagar.Storm

surge near Valsad=5.0 m

12


12/29

4. TYPES OF DAMAGE DURING CYCLONES

The w ind pressures and suct ion ef fects on f lat objects could besuffic ienttoliftthem off

andflyawayfromtheir place of rest unless adequately t ied dow n to substant ial supports.

Table 3 shows the aerofo il ef fects of some cyclonic w ind speeds.

Table 3: Aerofoil Effect of Wind

Wind Speed, m/sec. Typical Possible Movement30-35.1 Roof sheets f ixed to bat tens f ly

35-40 Smal l aircraf ts take off speed

40-45 Roof t i les nailed to battens f ly

45-50 Garden w alls blow over

50-55 Unreinforced brick w al ls fa il

55-60 Major damage f rom f ly ing debris

60-65 75 mm thick concre te s labs f ly

As a c onsequence of the w ind pressures/suct ions act ing on elements obst ruct ing the

passage of w ind the fo l low ing types of damage are comm only seen to occur during

high w ind speeds:

i. Uproot ing of trees, w hich disrupt rai l and road transportat ion, hence. Relief

Supply M iss ions.

i i. Failures of many cant i lever structures such as sign posts, electr ic poles,

and t ransmiss ion l ine tow ers;

i ii . Damage to improper ly at tached w indow s or w indow frame s;

iv. Damage to roof project ions, chhajjas and sunshades.

V. Failure of imp roperly attached or constructed parapet s,

v i . Over turn ing fai lures of com pound wal ls of var ious types;

vi i . Failure of w eakly buil tw al lsand c onsequentfai lureofroofsand roofcovering;

vi i i . Failure of roof ing elements and w alls along the gable end par t icu larly due to

high internal pressures;

ix. Failure of large industr ial buildings w ith l ight w eight roof co ver ings and long/

tall w alls due to c ombinat ion of internal and external pressures;

X. Brit t le fai lure of asbestos - cement (AC ) sheet ing of the roofs of Industrial

sheds; fa i lure of A C sheets is general ly along eaves, r idges, and gable ends;

xi. Punching and blow ing off of corrugated iron roof ing sheets attached to steel

t russes;

x i i . Thoughathatch roof c omm only employed in rural construct ion lacksdurabil i ty,

it prov ides greater permeabi li ty and at t racts less forces of w ind compared to

an im permeab le m embrane.

13


13/29

5.PLANNING ASPECTS

5.1 Site Selection

i. T h o u g h c y c l o n ic s t o rms

a lw ay s app roach f r om the

direc t ion of the sea tow ards

the coast , the w ind veloc i ty

and d i rec t i on re la t i ve to a

Fig. 4 Shielding of house by hillock

SsSdfeigfrom high windaut oi!rws o


14/29

5.2 Plan forms & Orientation

III.

For individual b ui ld ings, a ci rcu lar or polygonal plan shape is preferred over

rectangular or squ are plans, but f ronn the view point of funct ional ef f ic ienc y,

the rec tangu la r p lan is commonly used.

A symmet r i ca l bu i ld ing wi th

a compac t p lan-fo rm is m ore

s table than an asymm et r ica l

bui ld ing w ith a zig-zag plan,

hav ing em pty pocke ts , as the

la tter is more prone to w ind /

cyc lone re la ted dam age (see

Fig. 7) .

i1

1

wSymmetri c building withmp!y pockets are morevulnsrabloto damage

Symmetric Ixiiidings aremore stable

Fig. 7 Desirable orientation and plan form for

reducing wind danage.

In case of c onst ruct ion of grou p of bui ld ings, a cluster arrangem ent (see

F ig . 8) can be fo llowed in p re fe rence to row t ype .

I lRow planning creates wind zig-zag planning avoids wmr t

Fig. 8 Group Planning of buildings

5.3 Roof Architecture

The overa l l e f fec t of

w i nd on a p it c h e d

roof bui ld ing and the

crit ical locat ions were

show n in F igs . 2 & 3 .

Some damage t ypes

. are show n in Fig. 9.

It i s seen tha t roof

p r o je c t i o n s be y ond

the w a ll s expe r i ence

highupl i f t .The re fo re ,

t he roo f p ro jec t i ons

should be kept to a

m i n im um , s a y n o t

e x ceed i ng 500 m m,

o r e l s e , t he l a r ge r

p r o j e c t i o n s be tie d

down adequate ly (Fig . 10).

Hg aFlooting sheets lift

'a

fig c

Reeperlifts fromtherafter

figbRoofingsheetsliftat the gable er)d Holding dowr] of rafter

to wall inadequate

Fig. 9 Types of roof damage due to wind

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^^iJiiiiiim

Tia

J:

I Rough^ finish

Avoid large overtiangs / use t ies / openings

In overtiangs, rough finished walls desirable

Note. For rain protection, aminimum roof projection of 500mmisdesirable. Tying down willbe very advantageous.

l^ rg e overtiangs get lifted up and broken,

Smootti finish on walls undesiraWe

Fig. 10 Over han gs

i i. For the purpose of reduc ing w ind

fo rces on the roo f , a h ipped o r

pyram ida l roof is preferable to the

gable t ype roof (see Fig. 11)

Hi. In areas of h igh w ind or those

located in regions of high cyclonicac t i v i t y , l igh t w e igh t (G l o r AC

shee t )low pitchroofsshould either

be avoided or strongly held dow n

topurl ins.Pitchgdro^^jfs w ithslopes

in the range 22 -30 , that is , p itch

of 1/5 to 1/3.5 of sp an, w ill not only

reduce suct ion on roofs but would

alsofaci l i tatequick drainage of rain

water .

5.4 Wall Openings

Openings in general are areas of w eakness

a n d s t r e s s c o n c e n t r a t i o n , b u t n e e d e d

essent ia lly for l ight ing and vent i la t ion. The

fo llow ing norms are recomm ended in respect

of ope nings.

high upliti

Wind

Gable ondeci root got high uplift

Hip endputhed down

lowupUft

push

Wind

Wir>d

Hip root get lower uplift

Pyramidal roof get lowest uplift

Fig. 11

Effects of roof archi tecture on uplift forces

Openings in load bear ing w alls should not be w i thin a dis tance of h/6 from

inner corner forthepurpose of p roviding lateral support to crossw al ls ,v,/here

'h ' is the storey height up to eave level.

Open ing jus t be low roof leve l be

avoided except that two smal l vents

w ithout shutter should be provided

in o p p o s i t e w a l ls t o p r e ve n t

suf focat ion incase room gets f il led

w ith w ater and peop le may t ry to

c l imb up on lof ts or pegs.

S ince the fa i l u re of any door o r

w indow on w ind-w ard s ide may lead

to ad verse up li ft p ressures under

roof , F ig . 3 (b) , the openings shouldhave s t rong ho ld fas ts as we ll as

c los ing/lock ing arrangement (F ig-12 ).

J2.

Fig. 12

A d eq u at e an c h o r ag e of door an d

window frames with Holders

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5.5 Glass Panelling

a. One of the m ost dam aging ef fec ts of s trong w inds or cyc lones is the extensive

breakage of g lass panes caused by high local w ind pressure or impact off ly ing

objec ts in a ir . The large s ize glass panes may shat ter because they are too th into resist the loca l w ind pressures [See Fig . 13 (a)] . A broken glass pane on

w indw ard s ide opening increases interna l pressures ab norm al ly , F ig . 3(b), and

may lead to a chain of events inc lud ing a roof fa i lu re .

(a)Large and thin unprotected

glass area in w indows

(b) glass paotection by (c)

adhesive tapes

Fig.13 Protect ion of Glass Panes

small and thick / w ired glass

protected w ith guard bars1

tapes / w ooden bat tens

b. The w ay to reduce th is prob lem is to prov ide well des igned th icker g lass panes.

c . Further, recourse may be taken to reduce the panel s ize to sm aller d im ens ions.

Also glass panes can be strengthened by past ing ih in p last icfilmor paperstr ips

[F ig . 13(b) ]. This w ill he lp in hold ing the debr is of g lass panes from f ly ing in

case of breakage. It wi ll a lso introduce some damping in the glass panels and

reduce the ir v ibra t ions.

d . Further, to prevent dam age to the glass panels fromf ly ingw ind borne m issi les ,

a m eta ll ic fabr ic /m esh be prov ided outs ide the large panels [See Fig . 13 (c)] .

6. FOUNDATIONS

Bui ld ings usually have shallow foundat ion on stiff sandy soil and deep foundat ions in

l iquef iab le or e xpans ive c layey so i ls . It is des irab le that in format ion about soil t ype be

obtained and est imates of safe bear ing capac i ty made from the avai lab le records of

past construct ions in the area or by proper soi l investigat ion. In addit ion the fol low ing

param eters need to be proper ly accounted for in the des ign of foundat ion.

i. Effect of Surge or Flooding - Invariab ly a cyc lon ic storm is accompanied by

torrentia l rain and tidal surge (in coastal areas) result ing into flooding of the

low -ly ing areas. The t ida l surge ef fec t d im in ishes as it t rave ls on shore,

w hich can extend even upto 10 to 15 km . Flood ing c auses saturat ion of so il

and thus s ign i ficant ly affec ts the safe bear ing capac ity of the soi l. In f lood

prone areas, the safe bear ing c apac ity should be taken as half of that for the

dry ground. Also the l ike lihood of any scour due to receding t idal surge

needs to be taken in to account w hile dec id ing on the depth of foundat ion

and the protec t ion w orks around a ra ised ground used for locat ing cyc loneshelters or other bui ld ings.

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a. Building on Stilts- Wherea bui ld ing

isconstructed on stiltsit isnecessary

that st il ts are p roperly braced in both

the pr inc ipa l d irec t ions . Th is w i ll

p rov ide s tab il it y t o the com p lete

building under lateral loads. Knee

braces w i ll be p re fe rab le to fu ll

d i a g ona l b r a c i n g s o as not to

obs t ruc t t he passage of f l oa t ing

debr is dur ing storm surge (Fig. 14).

7.0 MASONRY WALLS

7.1 External Walls

Fig.14 Buil ding on sti lts

All external w alls or w all panels must be designed to res is t the out of planew ind p ressures ad equately. The lateral load due to wind is f inal ly resisted

either by all w alls lying parallel to the lateral forc e direct ion (by shear wall

act ion) or by RC fram es to w hich the panel w al ls m ust be f ixed using

appropr iate reinforcem ent such as 'seismic' bands at w indow sil l and lintel

level.

7.2 Strengthening of Walls against High Winds/Cyclones.

For high w inds in cyclone prone areas it is found nec essary toreinforc ethe

w al ls by means of reinforced concrete bands and ver tical re inforc ing bars

as for earthquak e resistance. ForGuja ra t ,

equivalence as in Table - 4 maybe used.

Table-4

Reinforcing of Masonry Buildings in Cyclone Prone Areas of Gujarat

Cyclone Prone Zone in District Reinforcement in walls as

in Seismic Zone

1 Bh uj , Rajkot , Jam nagar , Probandar , As for Seismic ZoneV

Juna gadh, Am rel i , Bhavnagar ,

Surendranagar , Ahmedabad.

2 Kh eda, Vadoda ra, Bharuch, As for Seismic Zone IV

Surat , Navsa r i , Valsad

(1) SeeTable- 1 for theTalukasof the districts lying in Cyclone Prone Zones

(2) Forreinforcing detailsofseismic bandsand vertical steel, refer toGSDMA "Guidelines forReconstruction

and New Construction of Houses in Kachchh Earthquake Affected Areas ofGujarat", November,

2001.

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8 FRAMED BUILDINGS

As an alternat ive to vert ical load bearing w alls, reinforced conc rete, steel or t imber

f raming can be used. In R C const ruct ions, the f rame c om pr ises of r ig id ly connected

beams and c olumns or posts. In steel and t imber c onstruct ions, complete structuralf ram ing should be adequate ly braced both in the vert ical and the horizontal planes.

St ipulat ions for cyclonic regions as m ade in 7.0 for m asonry w alls are appl icable to the

cladding v\/all panel- also. The guidelines recommended for the design of fram es are

as fol low s:-

a. Loading - The di f ferent loads and load com binat ions to be considered for the

design are as per IS: 875 (parts1to 4) . The dead loads, super im posed loads,

w ind and snow loadstobe c onsidered are given in parts 1 ,2 ,3 and 4 respect ively.

b. Cladding - For enclos ing the space it is necessary that c ladding be provided,

f i rmlysecuredtoc olumns orposts,on all the externalfacesand w here part i t ioning

is required. It is usual to have m asonry w all panels as cladding in bui ld ings with

R Cf raming .Th e d esign of panel w all shall be c arr ied out for out of plane local

w ind pressu res as per IS: 1905 - 1987.

In Indust rial bui ld ings corrugated galvanized iron/asbestos cement (CGI /AC)

sheet c ladding may be used for s ide co ver ing. The design should be carried

out for /oc a/w ind pressures. Proper attent ion be paid to connect ions specially

near corners and roof edge w here local pressures/suct ions are very high.

Alternat ively, t imber planks, if av ailable, may be used for panell ing part icularlyw ith t imber posts. The planks and their c onnect ions w ith end posts shal l be

designed as per IS: 883-1970.

c. Bracing - Adequate diagonal brac ings with st rong end connect ions shall be

provided in steel/t im ber fram ing in both the horizontal and vert ical planes to

improve their lateral load resistance. In industr ial buildings emp loying steel

f ram ing, at least the two end bays shall be braced in the vert ica l and hor izonta l

plane as per Fig. 15.

root truss

ratterbracingvertical braeina

j go t tru ss

eaves level bracing

knee bradng

a.Bracing in planesofrafters

b.Eaves level knee bracing

Fig. 15 Typical roof bracings for industr ial buildi ngs

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d.

In t imber f ram ing it is normal prac t ice to

b race each bay as w ell as to p rov ide

brac ing in hor izonta l p lane as show n in

F ig -16 so that com p lete s t ruc tu re is

in tegra ted.

Anchoring - The fr ame co lumns and

shear w all w here used sha ll be proper ly

anchored in to the founda t ion aga ins t

up li ft forces , as found nec essary . For R

C fram es, usua lly a m onol ith ic foot ing is

p rov ided w h ich p rov ides due s tabi li t y

aga ins t up l if t . In case of steel f ram ing

too, co lumn posts are proper ly t ied to

horizontal plane

Fig. 16 Wind bracings in timber frames

wooden post

nailswooden

crosspieces

Fig. 17 Ancho ring of wooden post using

cross pieces

Steel/c o n c re t e f loo r ing t h rough anc horbo l t s . For t imber pos ts usua l ly c ross

pieces are nai led at bottom end of the post

and bur ied into the ground to p rov ide

necessary anc horage (Fig . 17).

The fo l low ing gu ide lines shou ld be used

in bu i ld ing f ram ed houses:

a . The m a in f raming should be made w ith

t imber posts , bam boos or hol low p ipes ,

and, ensur ing proper c onnect ions of post

w ith eaves leve l beam and ra f te rs. (See

Fig. 18)

b . F r am es shou l d be

p r o pe r l y b r a c ed in

both hor izonta l and

vert ical planes using

knee braces or us ing

cross t ies (See Fig .

15, 16).

c . Cladd ing par ie ls may

b e mad e u s i n g

m eshes of bamboo

st r ips , caneor ikraor

o ther su i tab le f ib rous

a g r o w a s t e , an d

plaster ing In Situ.

cross bj^adngs

Fig. 18 Wind bracing of frame

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d. Openings in the w alls should be small and located aw ayfromthe edges and not

imm ediate ly below theroof.In case openings are non-closa ble (w ithoutshutters)

these should be less than 5 % of w all area each and in pairs on the opposi te

w alls to prevent high suct ion on the roof f rom ins ide.

Foundation

a.

b.

c.

9.

Th e d r a i n a g e a r o u n d t h e

bu i l d i n g be i T i p r o v ed to

prevent w ater c ollect ion forthe

du r ab i l i t y o f wa l l s and

foundat ions.

A ll posts be proper ly anchored

into the ground orinreinforced

cement foot ing. Al ternat ively ,

the pos ts w ith cross m embers

connected at the low er end be

embedded in ground (See Fig.

19) by a m inimum depth of 750

mm.

| 4P C

!

timber or concrete pad

Softerthe ground,deepertheposts should betow ithstand windforce

Fig. 19 Proper foot ings for timber post

Walls be raised f rom a w ell com pacted lean c oncrete bed or well compacted

ground f rom a min imum depth of 450 mm below the ground leve l.

Floors

Floors may c onsis t of the follow ing:

( i ) R C slabs

( i i ) Wood en or R C joists , or invertedT-i rons ,placed closely spaced and

ca rry ing br ick t i les, stone slabs or reeds w ith clay to pping, and

( i i i ) Prefabricated R C elements of var ious designs placed side by side

Wherea s R C s labs are

rigidintheirow nplanes,

t h e o t h e r t y p e s w i ll

require their integrat iont h r o ugh d i a gona l

brac ing or topping R C

screed (structural deck

conc re te ) . S t ruc t u ra l

deck concrete of grade

not l eane r t han M20

should be provided over

pre-cast com ponents to

a c t mono l i t h i c w i t h

them. (Fig. 20)

WlbeajTl

dia 6 M.S. dowel bars0150 c'c

oroiectedout ot tie beam

d 6 M J ^ r s 0 ISOc/cbolhways

concreteBoorfinish

wall/beam

Fig. 20 Provision of reinfor cement in struc tural deck conc rete

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Wherev er , deck concrete is to be prov ided, the top sur face of the c omponents should

be fin ished rough. Cement slur ry with 0.5 kg of cement per sq.m of the sur face areas

should be appl ied over the com ponents imm ediate ly before lay ing the deck concrete

and the c oncrete should be ful ly compacted. The minimum thick ness of deck concrete

shall be 35 to 40 mm reinforced w ith 6 mm dia bars @ 150 mm apart both w ays andanchored into the roof band or t ie beam placed all round.

Floors usual ly c arry no w ind loads unless the bui ld ing is const ructed on st ilts (in a

cyclonic surge prone area) . The design is carried out for vert ica l loads only. For a

bui ld ing on st il ts, flow of w ind underneath the f loor is possib le, thereby causing wind

forces (both upl if t and suct ion t ype ) . The forces as calculated using design w ind

pressures should be considered.

10. CONSIDERATIONS FOR ROOFS

Depending upon the c onst ruct ion mater ia l used and the geometr ical aspects,

the roofs can be broadly classi fied into two main types:

a. Flat roofs of var ious types

b. Pi tched roofs w ith var ious cover ing mater ia ls

10.1 Flat Roofs

The f lat roofs usual ly c onsis t of the same types of const ruct ion as given for floors in

Para 9.

In view of large upl if t forces , par t icu larly if w ind s p ^ d could exceed 55 m /s, the tota l

roof w eight should preferably be kept about 375 kg/m . Lighterroofsshould be designedfor net hogging forces and proper ly held down to suppor t ing beam s/w al ls , etc.

10.2 Use of Ferro-Cement (FC) as Roofing Material

Ferro-Cem ent (FC) has the advantages of reduced dead w eight compared to an R C

roof , as well as bet ter c orros ion resis tance. This new mater ia l could be used forflator

s loping roofs provided that the fer ro-cem ent sheets are adequately anchored to the

support ing w al ls /beam s against the w ind-upl i ft forces.

10.3 Pitched Roofs

a. The main load bear ing st ructura l mem bers are t imber or steel t russes, pur l ins,and bracings. The cladding may be of Gl or AC sheet ing, t i les, t imber planks,

thatch or prefabr icated concrete elements. It wi ll be safer to use sheet ing or

tileswith adequate fixtures.

b. The different design requirements for pitched roofs are as follows:

Ana lysis and design of pitched roof is carr ied out as per provisions of relevant

codes of pract ice i.e. IS: 800 -198 4 for steel trusses and IS: 883 for w ooden

trusses. Under high velocity w ind along the r idge of pitched roofs, the suct ion

forces m ay exceed the dead load of the roof appreciably , causing com pression

in the bot tom chord and st ress reversal in all t russ m embers in gene ral . Buckl ingconsiderat ion in all m em bers of roof t russes w hich are normal ly under tension,

22


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therefore, assumes signif icance. There fore, the m ain t ies of roof t russes also

require lateral bracing and strut t ing against their buck l ing in lateral direct ion.

c. Roof Covering -

As stated before (See Fig. 2 & 3.) the c orners and roof edges are zones ofhigher local w ind suct ions and the connect ions of c ladding/sheet ing to the t russ

needtobe designedfor theincreasedforces.Failure at any one of these locat ions

could lead progressively to complete rooffai lure.The fol lowing precaut ions are

recommended.

(i) Sheeted roofs: A reduced spac ing of bolts, 3/4 of that adm issible as per

IS : 800, is recommended. For normal connect ions, J bolts may be used

but for cyclone resis tant connect ions U-bol ts are recomm ended as shown

in Fig. 21(a) . Al ternat ively a st rap may be used at least along the edges

to f ixthe c ladding w ith the pur l ins as shown in Fig.21 (b)to avoid punc hing

through the sheet . Proper ly connected M.S.flatcan be used as reinforcing

band in high suct ion zones as shown in Fig. 21(c) .

Nut

30x30x6 gauge

M.S washer

Sea l ing w asher

neoprene or

bi tum inous lel t

//

wa1

J bolt - cyclone connection lor roof

cladding to purlins

(M30x30x6 gauge

Sea l ing washer

neoprane or

b i t um inous le l t /

a2

U bolt cyclone connection for roof

cladding to purlins

Cor ruga ted sheet ing

txjits flattenned arxd

nailed to purlin with lap

Fixing of corrugated sheeting to Purlins with bolts

M.S.f la t

^ h igh w ind suct ion

Using reinforcing bands in high suction zones

Fig. 21 Cyclo ne resis tant con nect ion details

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(ii) Clay tile roofs: Because of low er dead w eight , these may be unable to

res is t the upli ft ing force and thus exper ience heavy dam age, part icu larly

dur ing cyc lones . Anc hor ing of roof t i les in to R.C. s t rap beams is

recomm ended for improved cyc lone resistance (Fig . 22). As alternat iveto the bands, a cement mortarscreed,reinforced w ith galvanised chicken

m esh, m ay be laid over the entire t iled roof.

epneane stripe

concrete strips

lOdia HSDtjarcoocrpte strips

tite

ratter

30X24 gauge

-QiiVKjrtr^

Cb)Connection of concrete strips to rafter

(a) Conerete str ips

Fig. 22 Securing tiles against cycl ones

(Hi) Thatch roof: Tha tc hed roo f

should beproper lytied down to

woodenf ramingunderneath by

us ing organic or ny lon ropes in

d iagonal pat tern as show n in

Fig .22(a) . The spac ing of rope

should be kept 450 mm or less

so as to hold dow n the thatch

leng th . For connec t i ng thew ooden m embers , use of non

cor rod ib le f ix tu res shou ld be

m ade. If non-m etal l ic elements

a re used , t h e se may need

frequent rep lacement [See Fig-

23 (a)]. Afteracyc lone w arn ing

is rec eived, all the l ighter roofs

should preferably be held dow n

by a rope net and p rope r l y

anchored to ground [See Fig .

22(b) ] .

Ihalch root

ropes

GaWetype roof house

Fig. 23 Rope tie-back s for weak s tru ctu res

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d. Anchoring of roof framing to wall/posts -The

connect ion of roof fram ing to the vert ical load

resist ing elennents i.e., w all orpost ,by providing

proper ly designed anchor bolts and base plates

is equally im portant for overal l stabil ity of the

roof . Typical connect ion of wooden f raming to

w ooden postisshown in Fig. 24 through cyclone

bolt or metal st raps. The anc hor ing of roof

f r am i n g to ma son r y w a ll s hou ld be

accom pl ished t hrough anchor bolts embedded

into concrete cores. The w eight of part ic ipat ing

m asonry at an ang le of half hor izonta l to 1

vert ica l as show n in Fig. 25 should be more

than the total upli ft at the suppor t . In case of

RatrCfliiinn jast

Gaivanaed strap connecting. nxt Mming.to framing

Bolting

' Bvel wastw

roo) truss

wet??!. gtouji^ in coiweiepart ic i fx j t ingm i i sonry

tan'''(1/2)

Fig. 25 Anch orin g of roof framing in masonry

e. Bracing- Adequate diagonal or

knee bracing should be provided

both at the rafter level and the

eaves level in a pitched roof (See

Fig. 26) . The pur l ins should be

proper ly anchored at the gableend. It is desirable that at least

two bays , one at each end, be

braced both in hor izonta l and

v e r t ic a l p l a ne t o p r o v i d e

a d e q u a t e w i n d r e s i s t a n c e .

Where number of bays is more

than 5, use addit ional brac ing in

every fourth bay.

f . Flutter.- In order to reduce w ind

induced f lu t t e r /v ib ra t ion of theroof in cyc lon ic reg ions , it is

Connecting mofIramet owall IrameFig. 24 Connecti on of roof

framing to wall traming

largeupli ft forces,the anchor ing

bars can be taken down to the

foundat ion level w ithastructural

l ayou t t ha t cou ld ensu re the

part ic ipat ion of f i l ler and c ross

w alls in res is t ing the upl i ft .

Fig. 26 Bracing the raftered roofs

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recomm ended that all m embers of the t russ and the bracings be connected at

the ends by at leasttw orivets/bolts orw elds .Furtherthecross bracing mem bers

be w elded/connected at the crossings to reduce vibrat ions.

11. RETROFITTING OF EXISTING BUILDINGSRetrofi t t ing m easures are advocated to reduce the risk of damage or fa i lure for all the

exis t ing st ructures not having adequate c yclone resistance. Some m easures along

with approxim ate cost as a propor t ion of the cost of the bui ld ing are given in Table5 for

preliminary guidance. These measures are based on the lessons learntfromthe post-

cyc lone dam age surveys conducted in the past . Whi le the recomm endat ions would

depend upon building typolo gy, construct ion mater ial and pract ices prevalent in the

region, it is how ever , recomm ended that for cyclone af fected zones of thecountry ,the

ret rof it m easures be evolved through a detailed study based upon bui ld ing typology.

11.1 Engineered Constructions

In engineered const ruct ions, the m aximum w ind forces should be evaluated as per the

w ind code and var ious elements checked for the w orst com binat ion of dead and l ive

loads to ident ify the points of w eakness requir ing retrofi t t ing. Some points for special

at tent ion are indicated in fol low ing paras:

11.1.1 Roof-

a. In case of l ight roofs (AC or CGI shee t ing) connect ions near the edges should

be strengthened by providing addit ional U bolts. Mild Steel f lat t ies m ay be

provided to hold down the ro of. J-bolts if used earl ier m ay be replaces by U-

bolts.

b. All project ions in roofs be properly checked for strength against uplif t and t ied

dow n if found nec essary, part icular ly, if longer than 500 mm , (See Fig. 10 )

c. All metal lic connec tors for dif ferent com ponents of roof should preferably be

of non-corros ive m ater ial , or else must be galvanized or painted and c hecked

before each c yclone season and doubt ful ones be replaced.

d. There m ust be proper bracings ( i ) in the plane of raf ters, in plan at eaves

level, and, in the vert ical plane of columns along both axes of the building in

suff ic ient number of pane ls determined by calculat ion (See Fig. 25).

e. Flat roofs may be integrated to behave as hor izonta l diaphragm s and either

w eighted down by dead w eights or held down against upli ft forces.

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Table: 3

Retrofitting Measures for Buildings and Structures to Increase Cyclonic Resistance.

Sr.

No.

Type

Measures

Retrofit/Maintenance

of cost of building

Appr oximate cost as a propor tion

1 Non Engineered

Building Thatched

House

Provision s of metal straps

and nails at joints

Holding down coir ropes.

Replacem ent of w orn out

f ibre ropes

Retrof it - 4.5%

Maintenance - 1%

2 Tiled Building Concret e strips

Holding down rods

Metal straps for connection

to trusses

Provision of eaves holding

down angle/metal strap Maintenance replacem ent

of broken t i les, w orn out

bolts, metal st raps, etc.

R.C.C. holding down rafters

Retrof it - 8%

Maintenance - 1 %

3 Compound wall Checking the avai lable capacity

and detai l ing retrof it measures

consisting of vertical and

horizontal reinforced concrete

bands to obtain the required

strength

Add i t i ona l cos t va r i es in

the range of 25 to 60% of new

constructionsat isfy ingthe design

requirements. Retrofitt ing

cost + ex is t ing s t ruc t ure cost

approximatelye quals the cost of

new construct ion.

4 Lamp Masts Provision of a foundat ion

block and extending it upto

certain height above a ground

level to ensure natural

f requency is greater than

1.5 Hz.

Undergrou nd cables to

reduce load on lamp

mast/fai lure of masts by

fal ling branches of trees.

Cost of i nd i v i dua l l amp m ast

with foundat ion will be increased

by 40 to 50%.

5 Water Tanks (Ferro

cement /Other

Lightweight Tanks)

Provision of holding

dow n/prevent ing sl iding etc.

Marginal

11.1.2 Framed Buildings

a. In case of a framed structure, the total system requires to be properly braced. If

exist ing lateral strength or bracing is inadequate, braces be provided to improve

the overall stabil ity.

All roof trusses be properly connected to posts w ith the help of anchor bolts or

metall ic straps.

Undesirable openings in the w alls specially nearthecorners or edges be c losedpermanent ly to improve the c ross w alls.

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b.

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11.1.3 Load Bearing Walls

a. Buttresses be provided to improve the lateral load resistance of long w alls,

achiev ing cross w all spacing to less than 5m, thus reducing the unsuppor ted

lengths.

b. If the hor izonta l bands w ere not prov ided dur ing const ruct ion, the exter iorper im eter m ay be belted all round by using ferro-cem ent plat ing in the spandrel

w all port ion between lintel and eave/roof levels.

11.1.4 Glass Panelling

a. The size of large glass panes be reduced by adding bat tens at appropriate

spacing. Large glass panes be st rengthened by f ix ing adhesive tapes, along

and parallel to diagona ls, at 100-150mmspac ings priortoeach c yclone season.

Alternat ively, thin plast ic f i lm be pasted on both fac es of the panes to prevent

shat ter ing.

b. Protect ive coverintheformof mesh orirongrill be providedtoprevent breakage

of glass panels byf ly ingm issiles.

11.1.5 Foundations

a. Proper d rainage around the building should be provided to prevent pooling of

w ater in its vic inity.

b. The plinth should be protected against erosion by using pitching of suitable

type.

11.2 Non Engineered Constructions

a. In case of thatched roof it should be proper ly t ied to t imber f raming on

underside . Use of m etal lic/synthet ic connec tors is desirable. Use of w ater

proof mud plaster may be m ade to make it leak proof .

b. In case of tiledroofs,the over lapsbejointed through use of cement m ortarto

provide m ore stabili ty.o o

c. Whi le relay ing of roofs, its s lope be changed to about 20 to 30 to reduce

the w ind suct ion on roof and thus reducing the dam age potent ia l. At the

same t ime, eave level w ooden band should be int roduced on top of w alls.

d. The w ooden f ram e w here used for the st ructure of the bui ld ing should be

properly braced in both horizontal and vertical planes by instal ling knee braces

or c ross t ies.

e. For greater durability ofamudw all ,against rain and w ateretc .,external faceof w all upto 1.0 to 1.5 m height above plinth level should be covered with

burnt clay t i les laid in cement mortar of 1:6 m ix.

f . The roof raf ters be properly t ied to posts using m etall ic strap connect ors.

g. All openings very closetow all edges be c losed. All asymm etr ic non-closable

openings befi lled up toeliminate any u nfavourable roof pressurefromw ithin.

Two small vents in opposite w alls c lose to the roof may be left open.

h. If the foundationsoftheposts are not made heavy enoughtoprevent uprooting

of the building, it is advisable that before the c yclone, season, a protect ive

net be provided on the roof and securely t ied to the ground to preventf ly ing

away of roof/bui ld ing.

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Annexure - A

DESIGN PROCEDURE FOR WIND RESISTANT BUILDINGS

The followingprocedure may be follow ed to design a bui lding thatw illbe resistant

to damages dur ing high w inds/cyc lones.

A.l Fix the Design Data

a. Ident ity the nat ional w ind zone in which the bui lding is situated. This can be

seen from w ind code (IS: 875 Part 3-198 7) or the Vulnerabil ity Atlas of India

(1997).

b. Corresponding to the zone, fix the basic des ign w ind speed , Vb w hich can be

treated as constant upto the height of 10m.c. Choose the risk c o-eff ic ient or the importance factor k -|, for the bui lding, as for

exam ple given below :

Building type Coeffic ient ki

i . Ordin ary residential building 1.0

ii. Important building (e.g. hospital; 1.08

police stat ion; t elecommunicat ion,

sc hool, community and religious buildings,

cyc lone shelters, etc.

d. Choose appropriate value of k2 c orresponding to bui lding height, type ofterrain

and size of bui lding structure, as per IS: 875 (part 3), 1987. For bui ldings upto

10m height and category -A ,w hich w ill c overthe major ityof housing, the values

are:

Terrain Coeffic ient kg

i . Flat sea-coas tal area 1.05

ii. Level open ground 1.00

i i i. Bui l t -up suburban area 0.91

iv. Built-up city area 0.80

e. The factor k^ , depends upon the topography of the area and its location above

sea level . It acc ounts for the accelerat ion of w ind near crest of c l i ffs or along

ridge l ines and decelerat ion in val leys etc.

A.2 Determine the wind forces

a. Determine the des ign w ind veloc ity V z and normal des ign pressure P^

V z = V b k i k2 k32 2

Pz = 0.0006 Vz , Pz w ill be in kN/m for Vz in m/s

b. Corresponding to the bui lding dim ensions (length, height, w idth), the shape in

plan and elevat ion, the roof typei and its slopes as w ell as project ions beyond

the w alls, determine thec oeff icientsfor loads on all w al ls, roofs and project ions,

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taking into c onsiderat ion the internal pressures based on s ize and locat ion of

openings. Hence calculate the w ind loads on the var ious elements nornnal to

their surfac e.

c. Decide on the lines of resistance w hich will indicate the bracing requirements in

the planes of roof slopes, at eave level in horizontal plane, and in the plane of

w al ls . Then , determine the loads generated on the fol low ing connect ions:

Roof cladding to Purlins

Purlins to rafte rs/trus ses

Rafte rs/tru sse s to wall elements

Between long and cross w alls

Wa lls to foot ings.

A.3 Design the elements and their connect ions

a. Load effects shall be determined consider ing all c r it ical c ombinat ions of dead

load, live load and w ind load. In the design of elem ents, stress reversal under

w ind suct ions should be given due considerat ion. Members or f langes which

are usuallyintension under dead and live loads may be subjectedtocom pression

under dead load and w ind, requir ing considerat ion of buck l ing resistance in

their design.

b. Even thin reinforced concrete slabs, say 75mm thick, may be subjected to upli ft

under w ind speeds of 55 m /s and larger, requiring holding dow n by anchors at

the edges, and reinforcement on top face! As a guide, there should be ext ra

dead load (l ike insulat ion, w eather ing c ourse, etc) on suchroofsto increase the

ef fect ive w eight to about 375 kg/m .

d. Resistance to corrosion is a def inite requirement in cyclone prone sea coastal

areas. Paint ing of steel structures by corrosion-resistant paints must be adopted.

In reinforced concrete c onstruct ion, a mix of M20 grade w ith increased cover to

the reinforcement has to be adopted. Low w ater cement rat io w ith densi ficat ion

by m eans of vibratos w ill m inimise corrosion.

e. All dyn am ically sensit ive structures such as chimney stacks, specially shaped

w ater ta nks, transm ission line tow ers, etc. should be designed fol low ing the

dynamic design procedures given in var ious IS codes.

f . The m inim um dim ensions of electr ical poles and theirfoundat ionscan be c hosen

to achieve their fundamental f requency above 1.25 Hz so as to avoid large

amplitude vibrat ions, and consequent structural fai lure.

It may be em phasised that good quali ty of design and construct ion is the single factor

ensur ing safety as w ell as durabil ity in the cyclone hazard prone areas. Hence ail

bui lding m ater ials and building techniques must fol low the applicable Indian Standard

31 guideline _for _cyclone _resistant_ construction _of _building.pdf

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