Eurocode 1-Snow Information

7/23/2019 Eurocode 1-Snow Information

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Professor Haig GulvanessianProfessor Haig Gulvanessian

Professor Haig Gulvanessian CBECivil Engineering and Eurocodes Consultant,

Visiting Professor, Imperial College London

EN 1991-1-3: Eurocode 1:Actions on Structures: Part 1-3:

Snow Loads




Scope of Presentation

u Description of EN 1991-1-3 Eurocode 1: Part

1-3: Snow Loads

u Background research for snow maps for

Europe, Accidental (exceptional) loads etc.u Differences between EN 1991-1-3 and BS

6399: Part 3




EN 1991-1-3 provides guidance for the

determination of the snow load to be used for

the structural design of buildings and civil worksfor sites at altitudes under 1500m.

EN 1991-1-3 – Field of application




EN 1991-1-3 – Field of application

EN 1991-1-3 does not give guidance onspecialist aspects of snow loading, forexample:

u “impact snow loads” resulting from snow sliding off

or falling from a higher roof;u the additional wind loads which could result from

changes in shape or size of the building structuredue to the presence of snow or the accretion of ice;

u loads in areas where snow is present all the year;

u ice loading;u lateral loading due to snow (e.g. lateral loads exerted

by drifts).




EN 1991-1-3 - Contentsu Foreword

u Section 1: General

u Section 2: Classification of actions

u Section 3: Design situations

u Section 4: Snow load on the ground

u Section 5: Snow load on roofs

u Section 6: Local effects

u ANNEX A: Design situations and load arrangementsto be used for different locations

u ANNEX B: Snow load shape coefficients forexceptional snow drifts

u ANNEX C: European Ground Snow Load Mapsu ANNEX D: Adjustment of the ground snow load

according to return period

u ANNEX E: Bulk weight density of snow




EN 1991-1-3 – Determination of

Imposed roof snow loads

u Characteristic ground snow loads

• Ground snow load map

• Altitude function

u Coefficients• Shape coefficient – Roof shape

• Exposure coefficient – Topography

• Thermal coefficient – Thermal

transmittance of roofing material




EC funded European snow research

programme for development of EN 1991-1-3

Characteristic Snow Loads on the GroundDevelopment of a Ground Snow Load Map for Europe

u There were inconsistencies at borders between

existing national maps

u

The research developed a consistent approachu Produced regional maps. These are given in Annex

C of EN 1991-1-3

• Snow load with Altitude relationship

• Zone numbers & altitude function

• Geographical boundaries




Ground Snow load map European Climatic

Regions: Member States presently covered

Proposal being prepared to extend map to cover the

whole of Europe




Ground Snow load map –

United Kingdom and Ireland

0,50,54,54,5

0,30,333

0,20,222

0,040,0411

kN/mkN/m22

(A=0)(A=0)

ZoneZone

NN°°




Ground Snow Load Map – Revised Maps for

National Annexes

u Revised UK and Republic of Ireland map

• Investigated small zones

• Increased the total number of zones

• Applied a normalising datum• Software to produce map

u Adopted in the UK National Annex




UK and Republic of Ireland

Ground Snow Load Map

Determination of Sk

Sk = (0,15 + [0,1Z + 0,05]) + ((A

– 100)/525))

Sk = Characteristic ground

snow load (kN/m2)

Z = Zone number (obtained

from map) A = Site altitude (m)




EC funded European snow research

programme for development of EN 1991-1-3

Characteristic Snow Loads on the

Ground

Development of a Ground Snow Load Map

for Europe

u Identified areas with exceptional snow falls

u Coefficients for the combination of actions




ClassificationClassification ofof actionsactions

u For particular conditions snow loads may betreated as accidental actions: “act ion, usual ly of

sho rt durat ion bu t of sign i f icant magn i tude, that

is un l ikely to occur on a g iven st ruc tu re dur ing

the design working l i fe”

Exceptional

snow load on

the ground

Exceptional

snow drifts




Exceptional Ground Snow Loads

u In some regions, particularly southern Europe,

isolated and extremely infrequent very heavy

snow falls have occurred

u This has resulted in ground snow loads whichare significantly larger than those that

normally occur.




Exceptional Ground Snow LoadsExceptional Ground Snow Loads

S Ad = Cesl Sk

S Ad = Site exceptional ground snow load design value

(kN/m2

)Cesl = Coefficient (recommended value 2)

Sk = Site Characteristic ground snow load (kN/m2)

u Value for coefficient given in National Annex

u Treated as Accidental actions




Action ψ 0 ψ 1 ψ 2

Imposed loads in buildings, category (see

EN 1991-1-1)

Category A : domestic, residential areas

Category B : office areas

Category C : congregation areas

Category D : shopping areas

Category E : storage areas

0,7

0,7

0,7

0,7

1,0

0,5

0,5

0,7

0,7

0,9

0,3

0,3

0,6

0,6

0,8

Category F : traffic area,

vehicle weight≤ 30kN

Category G : traffic area,

30kN < vehicle weight≤ 160kN

Category H : roofs

0,7

0,7

0

0,7

0,5

0

0,6

0,3

0Snow loads on buildings (see EN 1991-1-3)*

– Finland, Iceland, Norway, Sweden 0,70 0,50 0,20

– Remainder of CEN Member States, for sites

located at altitude H > 1000 m a.s.l.

0,70 0,50 0,20

– Remainder of CEN Member States, for sites

located at altitude H ≤ 1000 m a.s.l.

0,50 0,20 0

Wind loads on buildings (see EN 1991-1-4) 0,6 0,2 0

Temperature (non-fire) in buildings (see EN

1991-1-5)

0,6 0,5 0

NOTE The ψ values may be set by the National annex.* For countries not mentioned below, see relevant local conditions.

EN 1990: Table A1.1 - Recommended values of

ψ factors for buildings




EN 1991-1-3: Combination Coefficients for

other representative values for actions

u Representative values osk, 1sk and 2sk are defined

in EN 1990 Basis of Structural Design

u Values determined from analysis of daily data

u Recommended values for EN 1990 (< 1000 masl & >

1000 masl):Combination value coefficient 0 – 0,5 & 0,7

Frequent value coefficient 1 – 0,2 & 0,5

Quasi-permanent value coefficient 2 – 0,0 & 0,2

u

Value for coefficients given in National Annex to EN1990: Basis of Structural Design




Determination of Snow Load on the Roof Determination of Snow Load on the Roof

Nature of the loadThe design shall recognise that snow can be deposited on a roof in

many different patterns.

Properties of a roof or other factors causing different

patterns can include:u the shape of the roof;

u its thermal properties;

u the roughness of its surface;

u the amount of heat generated under the roof;

u the proximity of nearby buildings;

u the surrounding terrain;

u the local meteorological climate, in particular its windiness,temperature variations, and likelihood of precipitation (either as rainor as snow).




Determination of Snow Load on the Roof

Load arrangements

The following two primary load arrangements

are taken into account in EN 1991-1-3:

• undrifted snow load on roofs

• drifted snow load on roofs




Determination of Snow Load on the RoofDetermination of Snow Load on the Roof – –

Definitions of undrifted and drifted snow on roof Definitions of undrifted and drifted snow on roof

undrifted snow load on the roof

load arrangement which describes the uniformlydistributed snow load on the roof, affected only bythe shape of the roof, before any redistribution of

snow due to other climatic actions.

drifted snow load on the roof

load arrangement which describes the snow load

distribution resulting from snow having been movedfrom one location to another location on a roof, e.g.by the action of the wind.




UndriftedSnowUndriftedSnow load on roofsload on roofs




Determination of Snow Load on the RoofDetermination of Snow Load on the Roof – –

Definitions of undrifted and drifted snow on roof Definitions of undrifted and drifted snow on roof

undrifted snow load on the roof

load arrangement which describes the uniformlydistributed snow load on the roof, affected only bythe shape of the roof, before any redistribution of

snow due to other climatic actions.

drifted snow load on the roof

load arrangement which describes the snow load

distribution resulting from snow having been movedfrom one location to another location on a roof, e.g.by the action of the wind.




Drifted Snow on Roof With wind speeds in the range of 4 to 5 m/s, much of

the snow is deposited in areas of ’aerod ynam ic shade ’

DRIFTED SNOW LOAD ARRANGEMENT

Model in wind tunnel

wind velocity of 4m/s

Aerodynamic

shade wind wind




Drifted Snow on Roof

For situations where the wind velocity increases above 4

to 5 m/s snow particles can be picked up from the snow

cover and re-deposited on the lee sides, or on lower roofsin the lee side, or behind obstructions on the roof.

DRIFTED SNOW LOAD ARRANGEMENT

Model in wind tunnel for multi - pitched roof wind velocity > 5 m/s

wind





Drifted snow load on roofs

The National Annex may specify the use alternative driftpatterns dependent on climatic variation (maritime or

continental) for particular roof shapes. The alternatives apply for

specific locations

u where all the snow usually melts and clears between the

individual weather systems and where moderate to high windspeeds occur during the individual weather system. ( Annex B of

EN 1991-1-3: maritime)

u Where the snow that fall is more persistent and where snow

falling in calm conditions may be followed by further snow,

carried by another weather system driven by wind and there

may several repetitions of these events before there is any

significant thawing (Main text of EN 1991-1-3: continental)





Snow loads on roofs for the persistent / transient design

situations are determined as follows:

s = µ i C e C t s k

where:µ i is the snow load shape coefficient

s k is the characteristic value of snow load on the

ground

C e is the exposure coefficient

C t is the thermal coefficient





Snow loads on roofs for the accidental design situationswhere exceptional snow load is treated the accidentalaction is determines as follows

s = µ i C e C t s Ad

where:

µ i is the snow load shape coefficient

s Ad is the characteristic value of exceptional snow loadon the ground for the given location

C e is the exposure coefficient

C t

is the thermal coefficient





Snow loads on for theSnow loads on for the accidental design situations whereaccidental design situations where

exceptional snow drift treated as the accidental actionexceptional snow drift treated as the accidental action andand

wherewhere Annex B Annex B (used in the UK) applies(used in the UK) applies

s =s = µ µ ii s s kk

Where:Where:

µ µ ii is the snow load shape coefficientis the snow load shape coefficient

s s kk is the characteristic value of snow load on theis the characteristic value of snow load on the

groundground




Determination of Snow Load on the Roof –

Exposure Coefficients

exposure coefficient

A coefficient (C e) defining the reduction or increase of snow

load on a roof of an unheated building, as a fraction of the

characteristic snow load on the ground.

The choice for C e should consider the future development

around the site. C e should be taken as 1,0 unless otherwise

specified for different topographies.

The National Annex may give the values of C e for differenttopographies.





Recommended Exposure Coefficients

Windswept topography, where (C e = 0,8 ) areflat unobstructed areas exposed on allsides without, or little shelter afforded byterrain, higher construction works ortrees.

Normal topography, where (C e = 1,0 ) areas

where there is no significant removal ofsnow by wind on construction work,because of terrain, other constructionworks or trees.

Sheltered topography, where (C e = 1,2 ) areasin which the construction work beingconsidered is considerably lower thanthe surrounding terrain or surroundedby high trees and/or surrounded byhigher construction works.





Recommended Exposure Coefficients

thermal coefficient A coefficient defining the reduction of snow load on roofsas a function of the heat flux through the roof, causingsnow melting.

The thermal coefficient C t is used to account for thereduction of snow loads on roofs with high thermaltransmittance (> 1 W/m2K), in particular for some glasscovered roofs, because of melting caused by heat loss.

For all other cases: C t = 1,0

Further guidance may be obtained from ISO 4355.




Exposure (C e) and Thermal (C t) coefficients

u Recommended values obtained from European research

– Monitored snow accumulation

– UK data only small part of data set

u BS National Annex for EN 1991-1-3 specifies values of 1for both C e and C t

Design Situations and load arrangements to be used




Design Situations and load arrangements to be used

for different locations (Table A1 of EN 1991-1-)

NOTE 1: Exceptional conditions are defined according to the National Annex.

NOTE 2: For cases B1 and B3 the National Annex may define design situations which apply for the particular local

effects described in Section 6 of EN 1991-1-3.

Persistent/transient

design

situation

[1] undrifted i C eC t sk

[2] drifted i C eC t sk(except for roof shapes

in Annex B)

Accidental design

situation (where snow

is the accidental

action)

[3] undrifted i C eC t

C esl sk

[4] drifted i sk (for roof

shapes in Annex B)


design

situation


[2] drifted i C eC t sk(except for roof shapes

in Annex B)

Accidental design


is the accidental

action)

[3] drifted i sk (for roof

shapes in Annex B)


design

situation


[2] drifted i C eC t sk Accidental design


is the accidental

action)

[3] undrifted i C eC tCesl sk

[4] drifted i C eC t C esl

sk


design

situation


[2] drifted i C eC t sk

Clause 3.3(3)Clause 3.3(2)Clause 3.3(1)Clause 3.2(1)

Exceptional fallsExceptional driftNo exceptional fallsExceptional driftExceptional fallsNo exceptional driftNo exceptional fallsNo exceptional drift

Case B3Case B2 (e.g. UK)Case B1Case A

Exceptional conditionsNormal





Shape coefficients – Continental climates

u Section 5 of EN 1991-1-3 gives shape coefficients for

• Undrifted and Drifted snow load cases

• For the following roof shapes

u Mono-pitched

u

Pitchedu Multi-span

u Cylindrical

• Local effects

• Persistant/transient situations





Shape coefficients – Maritime climates

Annex B of EN 1991-1-3 gives

u Drifted load cases for:

• Multi-span roofs

• Roofs abutting and close to taller structures

• Drifting at projections, obstructions and parapetsu Annex B is based on guidance in BS6399: Part 3

u Considered as exceptional drifts due to classification of

load cases in section 5 of EN 1991-1-3

u Treated as Accidental actions





Shape coefficients – Mono-pitch roofs

u Snow shape

coefficients for mono-

pitch roofs

u Snow shape

coefficients shown

diagrammatically

2.0

1.0

0° 15° 30° 45° 60°

0.8

1.6

1

2

α

1





Shape coefficients – Duo-pitch roofs

1

1( 1) 1( 2)

0,5µ 1( 1) 1( 2)

1( 1)

0,5µ 1( 2)

2

Case (i)

Case (ii)

Case (iii)

2.0

1.0

0° 15° 30° 45° 60°

0.8

1.6

1

2

α

Snow shape coefficients for pitch roofs – Continental

climates




Determination of Snow Load on the Roof –Drifted Snow Load Arrangement for maritime climates





Drifted Snow Load Shape coefficients

maritime climates

0,01,2 (60 –

! )/30

0,8 +

0,4 (! -

15)/15

0,8i

! i "

60º

30º < ! i <

60º

15º < ! i #

30º

0º # ! i #

15º

Angle of

pitch of

roof

(! i , i=1,2)





Drifted Snow Load Shape coefficients

maritime climates

0,01,2 (60 –

! )/30

0,8 +

0,4 (! -15)/15

0,8i

! i " 60º30º < ! i < 60º15º < ! i # 30º0º # ! i # 15º Angle of pitch

of roof

(! i, for i =1,2)





Shape coefficients – Multi-span roofs

2.0

1.0

0° 15° 30° 45° 60°

0.8

1.6

1

2

α

1

1( 1)

2 1 2

1( 1) 1( 1)

1( 2)

1( 2) 1( 2)

2( ) = ( 1+ 2)/2

Case (i)

Case (ii)

Snow shape coefficients for multi-span roofs




Determination of Snow Load on the RoofDetermination of Snow Load on the Roof – – ShapeShape

coefficientscoefficients – –MultiMulti--span roofsspan roofs -- driftingdrifting

µ1(α1) µ1(α2) µ2(α)

α = (α1+ α2)/2

α α2 α

α2

Drifted load case

(Section 5 – Continental climate)

Exceptional Drif ted load case

(Annex B – Maritime climate)

h

b2 b1 b3

ls2 ls1

µ1

Local EffectsLocal Effects




Local EffectsLocal EffectsDrifting at projections and obstructions

µ1 = 0,8 µ2 = γ h/sk

where

0,8 ≤ µ2 ≤ 2,0

γ = 2 kN/m3 (weight density of

snow)

l s = 2h 5 ≤ l s ≤ 15 m

Snow overhanging the edge of a roof (recommended for sites above 800 m a.s.l.)

se=k s2 / γ

where

γ = 3 kN/m3

γ k = 3 /d < d γ (National Annex)

d is in meters




EN 1991-1-3 Informative Annexes

u Annex C – European Ground snow load maps

– Majority produced during European Research project

u Annex D – Adjustment of ground snow load for return

period – Expression for data which follow a Gumbel probability

distribution

u Annex E – Densities of snow

– Indicative density values for snow on the ground

Differences between EN 1991 1




Differences between EN 1991-1-

3 and BS6399: Part 3

u Small Buildings clause not included in EN 1991-1-3

u Imposed loads due to maintenance given in EN 1991-1-1

u Maximum altitude greater in EN 1991-1-3 (1,500m) than in BS 6399

Part 3 (500m)

u Zoned map in EN 1991-1-3 compared with isopleths in BS 6399:

Part 3




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Eurocode 1-Snow Information