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है”ह”ह

IS 875-4 (1987): Code of Practice For Design Loads (OtherThan Earthquake) For Buildings And Structures, Part 4: SnowLoads [CED 37: Structural Safety]

IS : 875 ( Part 4 ) - 1987( Reaffirmed 1997 )

Indian Standard

CODE OF PRACTICE,FORDESIGN LOADS ( OTHER THAN EARTHQUAKE )

FOR BUILDINGS AND STRUCTURESPART 4 SNOW LOADS

(Second Revision).

Fourtll Rcprjnt OCTOBER 1997

UDC 624.042-42 : 006.7

@ Copyright 1988

B U R E A U O F I N D I A N S T A N D A R D SMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

Gr 4 October 1988

fndian Standard

CODEOFPRACTICE

IS:875(Bart4)-1987

FORDESIGNLOADS(OTHERTHANEARTHQUAKE)

FORBUILDINGSAND STRUCTURES r.PART 4 SNOW LOADS

(Second Revision)

0. F O R E W O R D

0.1 This Indian Standard ( Part4 ) ( SecondRevision ) was adopted by the Bureau of IndianStandards on 9 November 1987, after the draftfinalized by the Structural Safety SectionalCommittee had been approved by the CivilEngineering Division Council.

0.2 A building has to perform many functionssatisfactorily. Amongst these functions are theutility of the building for the intended use andoccupancy. structural safety, fire safety; andcompliance with hygienic, sanitation, ventilationand daylight standards. The design of the build-ing is dependent upon the minimum requirementsprescribed for each of the above functions. Theminimum requirements pertaining to the structuralsafety of buildings are being covered in this Codeby way of laying down minimum design loads whichhave to be assumed for dead loads, imposed loads,wind loads, snow loads and other external loads,the structure would be required to bear. Strictconformity to loading standards recommended inthis Code, it is hoped, will not only ensure thestructural safety of the buildings which are beingdesigned and constructed in the country andthereby reduce the hazards to life and propertycaused by unsafe structures, but also eliminate thewastage caused by assuming unnecessarily heavyloadings. Notwithstanding what is stated regardingthe structural safety of buildings, the application ofthe provisions should be carried out by compe-tent and responsible structural designer who wouldsatisfy himself that the structure designed inaccordance with this code meets the desiredperformance requirements when the same iscarried out according to specifications.

0.3 This Code was first published in 1957 for theguidance of civil engineers, designers and archi-tects associated with the planning and design ofbuildings. It included the provisions for thebasic design loads ( dead loads, live loads, windloads and seismic loads ) to be assumed in thedesign of buildings. In its first revision in 1964,the wind pressure provisions were modified onthe basis of studies of wind phenomenon and itseffects on structures undertaken by the special

committee in consultation with the Indian Meteo-rological Department. In addition to this, newclauses on wind loads for butterfly type structureswere included; wind pressure coefficients forsheeted roofs, both curved and sloping, weremodified; seismic load provisions were deleted( separate code having been prepared ) and metricsystem of weights and measurements was adopted.

0.3.1 With the increased adoption of the Code,a number of comments were received on the pro-visions on live load values adopted for differentoccupancies. Simultaneously live loads surveyshave been carried out in America, Canada andother countries to arrive at realistic live loadsbased on actual determination of loading( mov-able and immovable ) in different occupancies.Keeping this in view and other developments inthe field of wind engineering, the Sectional Com-mittee responsible for the preparation of thisstandard has decided to prepare the secondrevision in the following five parts:

Part 1 Dead Loads

Part 2 Imposed Loads

Part 3 Wind Loads

Part 4 Snow Loads

Part 5 Special Loads and Load Combinations

Earthquake load is covered in IS : 1893-1984*which should be considered along with the aboveloads.

0.3.2 This part ( Part 4 ) deals with snow loadson roofs of buildings.

The committee responsible for the prepara-tion of the code while reviewing the availablesnow-fall data, felt the paucity of data on whichto make specific recommendations on the depthof ground snow load for different regions effectedby snow-fall, In due course the characteristic

*Criteria for earthquake resistant designing of strue-trues (fourth revision ).

1

IS:875(Part4)-1987

snow load on ground for different regions will ‘Basis for design of structures - Determinationbe included based on studies. of snow loads on roofs’, issued by the Interna-

0.4 This part is based on IS0 4355-198 1 ( E )tional Organization for Standardization.

1. SCOPE

1.1 This standard (Part 4) deals with snow loadson roofs of buildings. Roofs should be designedfor the actual load due to snow or for the &posedloads specified in Part 2 Imposed loads, whicheveris more severe.

NOTB - Mountainous regions in northern parts ofIndia are subjected to snow-fall.

In India, parts of Jammu and Kashmir ( BaramulahDistrict, Srinagar District, Anantnag District andLadakh District ); Punjab, Himachal Pradesh( Chamba, Kulu, Kinnaur District, Mahasu District,Mandi District, Sirmur District and Simla District );and Uttar Pradesh ( Dehra Dun District, Tehri GarhwalDistrict, Almora District and Nainital District ) experi-ence snow-fall of varying depths two to three times ina year.

2. NOTATIONS

3.

3.1

p ( Dimensionless) - Nominal values of theshape coefficients, tak-ing into account snowdrifts, sliding snow,etc, with subscripts, ifnecessary.

Ij ( in metres ) - Horizontal dimensionswith numerical sub-scripts, if necessary.

hj ( in metres ) - Vertical dimensionswith numerical sub-

. scripts, if necessary.

fii (in degrees) - Roof slope.

so (in pascals ) - Snow load on ground.

SI ( in pascals ) - Snow load on roofs.

SNOW LOAD IN ROOF (S)

The minimum design snow load on a roofarea or any other area above ground which issubjected to snow accumulation is obtained bymultiplying the snow load on ground, s, by theshape coefficient CL, as applicable to the particularroof area considered.

S=c(S0

where

s = design snow load in Pa on plan areaof roof,

p = shape coefficient ( see 4), and

so = ground snow load in Pa( 1 Pa = lN/ma ).

NOTE - Ground snow load at any place depends onthe critical combinati.m of the maximum depth of un-disturbed aggregate cumulative snow-fall and itsaverage density. In due course the characteristic snowload on ground for different regions will be includedbased on studies. Till such time the users of thisstandard are advised to contanct either Snow andAvalanches Study Establishment ( Defence Researchand Development Organization ) Manali ( HP) orIndian Meteorological Department ( IMD ), Pune inthe absence of any specific information for anylocation.

4. SHAPE COEFFICIENTS

4.1 General Principles

In perfectly calm weather, falling snow wouldcover roofs and the ground with a uniform blanketof snow and the design snow load could be consi-derd as .a uniformly distributed load. Truly uni-form loading conditions, however, are rare andhave usually only been observed in areas that aresheltered on all sides by high trees, buildings, etc.In such a case, the shape coefficient would beequal to untiy.

In most regions, snow falls are accompaniedor followed by winds. The winds will redistributethe snow and on some roofs, especially multi-level roofs, the accumulated drift load may reacha multiple of the ground load. Roofs which aresheltered by other buildings, vegetation, etc, maycollect more snow load than the ground level.The phenomenon is of the same nature as thatillustrated for multilevel roofs in 4.2.4.

So far sufficient data are not available to deter-mine the shape coefficient in a statistical basis.Therefore, a nominal value is given. A representa-tive sample of rcof is shown in 4.2. However, inspecial cases such as strip loading, cleaning of theroof periodically by deliberate heating of the roof,etc, have to be treated separately.

The distribution of snow in the directionparallel to the eaves is assumed to be uniform.

2

4.2 Shape Coefficients for Selected Types of Roofs

4.2.1 Simple Flat andMonopitch Roofs

Simple Pitched Roofs(Positive Roof Slope)*

p, = 0.8

4.2.2 Simple or Multiple Pitched Roofs(Negative Roof Slope)

Two-Span or MultispanRoofs

o*<p<3lE3&#<6

49>60*

t+.= p, =O.%

t'2~=0.8+04(~)

jL, =0*8

Pl**

pp1-6m-0

l For.asymmetrical simple Pitched roofs, each side of the roof shall be treated as me half of correspondingsymmetwal roofs.

3

Is:875(Partl)-1987

4.2.3 Simple Curved Roofs

The following cases 1 and 2 must be examined:

CASE 2

Restriction:

h<F3

a-OifB>60’

4

Is:875(Part4)-1987

4.2.4 Multilevel Roofs*

91 = 0’8

Bs = Ps + Pa

where

A - due to sliding

pw - due to wind

1, = 2ht but is restricted as follows:

SmCls<lSm

11 + f, < khPW=T -SO

with the restriction 0.8 < pw ( 4’0

where

h is in metres

so is in kilopascals ( kilonewtons per square metre )k =2kN/m8

p > 19” : ps is determined from an additional load amounting to SO percent of the maximum total load on theadjacent slope of the upper roofs, and is distributed linearly as shown on the figure.

B < 15” : ps = 0

*A more extensive formula for pw is described in Appendix A.tlf 1~ < I,. the coe5cient p is determined by interpolation between JJ, and ps.SThe load on the upper roof is calculated according to 4.2.1 or 4.2.2.

5

4.25 Complex Multilevel Roofs

c

1, - 2h1: h - 2h,: p1 - 0’8

Restriction:

Sm< I,< Urn;

Sm<b<lSm;

11 and BW ( ccl + @W ), are calculated according to 4.2.1,4.2.2 and 4.2.4.

6

I.S:875(Part4)-1987

4.2.6 Roofs with Local Projections and Obstructions

where/I is in metressO is in kilopascals (kilonewtons per square metre)k I= 2 kN/ma/I1 = 0.81=2/l

Kestrictions:0’8 < /Ia < 2-OSm41615m

4.3 Shape Coefficients in Areas Exposed to Wind

The shape coefficients given in 4.2 and Appen-dix A may be reduced by 25 percent provided thedesigner has demonstrated that the following con-ditions are fulfilled:

4

b)

The building is located in an exposedlocation such as open level terrain withonly scattered buildings, trees or otherobstructions so that the roof is exposedto the winds on all sides and is ndtlikely to become shielded in the futureby obstructions higher than the roofwithin a distance from the building equalto ten times the height of the obstructionabove the roof level;The roof does not have any significantprojections such as parapet walls whichmay prevent snow from being blown offthe roof.

NOTE - In some areas, winter climate may not beof such a nature as to produce a significant reductionof roof loads from the snow load on the ground. Theseareas are:

a) Winter calm valleys in the mountains where some-times layer after layer of snow accumulates onroofs without any appreciablewind; and

removal of snow by

b) Areas (that is, high temperature) where the maxi-mum snow load may be the result of single snow-storm,removal.

occasionally without appreciable wind

In such areas, the determination of the shape coeffi-cients shall be based on local experience with dueregard to the likelihood of wind drifting and sliding.

5. ICE LOAD ON WIRES5.1 Ice loads are required to be taken into accountin the design of overhead electrical-transmissionand communication lines, over-head contact linesfor electric traction, aerial masts and similarstructures in zones subjected to ice formation.The thickness of ice deposit alround may be takento be between 3 and 10 mm depending upon thelocation of the structure. The mass density ofice may be assumed to be equal to O-9 g/cm”.While considering the wind force on wires andcables, the increase in diameter due to ice forma-tion shall be taken into consideration.

7

IS:875(Part4)-1987

A P P E N D I X A

( Clauses 42.4 and 4.3 )

SHAPE COEFFICIENTS FOR MULTILEVEL ROOFS

A more comprehensive formula for the shape coefficient for multilevel roofs than thatgiven in 4.2.4 is as follows:

-- WIN0OIRECTIONS

c

Pr -1+ + ( ml iI + mI 1, )( 1, - 2 h )

Cl = 0’8

i,=hh

fh and I being in metres)

Restriction :

whereso is in kilopascals (kilonewtons per square metre)k is in newtons per cubic metreI,< ISmValues of m, ( mr ) for the higher ( lower ) roof depend on its profile and are taken as equal to:

0.5 for plane roofs with slopes @ < 20’ and vaulted roofs with f< +-

0’3 for plane roofs with slopes p > 20” and vaulted roofs with f >$

The coefficients m, and ma may be adjusted to take into account conditions for transfer of snow on the roofsurface ( that is, wind, temperature, etc. ).

NOTE - The other condition of loading also shall be tried.

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