Abdurahman Ahmed

Wind & Eurocode

June 1st 2016

CONTENTS

2

� Introduction

� Scope of BS EN 1991-1-1-4 2005 +A1 2010 & UK National Annex (NA)

� Flow charts for obtaining peak velocity pressure

� Basic wind velocity pressure

• Correction factors

� Orography significant or Not?

� Peak wind velocity pressure

� Wind pressure & Wind Forces

� References

� Q&A

INTRODUCTION

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� What: Wind loading according to BS

EN1991-1-4- 2005 & UK National Annex

� Why: Gaining Knowledge and sharing it

� How: through presentation/ worked

examples

� After: you can calculate wind peak

pressure by hand, know about the

different coefficients, used terminology,

know the UK National determinate

Parameters (N.D.P)

SCOPE OF BS EN 1991-1-1-4 2005 WIND & NA

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� Give guidance on determination of wind

actions for building structures and civil

works.

� Applicable for buildings and civil

structures up to 200 meters.

� Applicable for bridges having no span

greater than 200 meters (with some

criteria bases on dynamic response).

� Some structures are excluded from this

Eurocode part 1-4.

FLOW CHARTS FOR PEAK VELOCITY PRESSURE

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� NA includes 2 useful flow

charts.

� fig. A. NA.1 (Country terrain)

� fig. A. NA.2 (Town terrain)

� Presentation layout will be

based on these charts with

some worked examples.

BASIC WIND VELOCITY PRESSURE: QB

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Basis wind velocity: Vb

� Fundamental basic wind

velocity: Vb,map

• Obtaining from fig. NA.1

• Value of the wind at 10 m

above ground in open

country terrain, in a period of

10 minutes

� Correction factors: C

CORRECTION FACTORS: CALT

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� adjust wind speed of site

above sea level: A factor

� adjust wind speed of site

above ground level: Z factor

� NA gives 2 equations for the

altitude factor: Calt

� Equation NA.2a may be used

for any building

(Conservative for tall

buildings).

� Fig. 2 from BRE publication:

Digest DG-436-1

CORRECTION FACTORS: CDIR

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� Factor for wind directions.

� NA consider 12 directions of

30 degree (time consuming

by hand calculation.)

� Considering 4 directions (like

fig. 3)

� Value of Cdir=1 for all

directions.

CORRECTION FACTORS: CSEASON

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� Factor for seasons, table

NA.2.7 from NA

� For temporary structures and

for structures in the

execution phase, season

factor may be used

� For transportable structures

C season = 1

� For building, it should be

taken eq. to 1

CORRECTION FACTORS: CPROB

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� Factor for probability

� Cprob = 1 for return period of

50 years.

� Cprob = 0.9 for return period

of 10 years.

� Cprob = 1.1 for return period

of about 250 years.

BASIC WIND VELOCITY PRESSURE: QB

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� Fundamental basic wind

velocity: Vb,map (see above)

� Factors (see above)

� Basis wind velocity: Vb

� Air density by NA = 1.226

kg/m3

� Basic wind velocity pressure

OROGRAPHY SIGNIFICANT? YES OR NO

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� Orography = Topography

� is the topography features

such as hills, cliffs ,

escarpmentsG

� Co = orography factor

� Used to account the speed

up of the mean wind speed

over hills, escarpments.

� Orography can increase the

peak pressure by over 70%

near the crest of a steep hill.

OROGRAPHY SIGNIFICANT? YES OR NO

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When is Topography

siginficant?

� The effets of orography

should be considered by the

shaded area in fig. NA. 2

� Out site the shaded area Co

may be taken as 1

OROGRAPHY SIGNIFICANT? NO

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� Out side shaded area Co = 1

� Use equation NA.3a of NA.3b

� Determine Exposure factor

Ce(z)

• Distance to shore?

• Effective height?

� Determine Exposure

correction factor Ce,T for sites

in Town terrain

• Distance to edge of Town?

WORKED EXAMPLE

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� Building out side of Leeds.

� 60x35x12= LxBxH, altitude = 50 m, distance to sea = 100 kM, at edge of Town

• Determine Vb,map fig NA.1 = 22.8 m/s

• Calt = 1+0.01A= 1+0.001x50=1.05

• Vb,0 = Vb,map x Calt = 22.8 x 1.05 = 23.9 m/s

• Vb = Cdir x Cseason x Vb,o = 23.9 m/s (Cdir, Cseason taken =1)

• Orography check?

• Ø=H/Lu=2/100=0.02 (halfway of Hill) < 0.05 : Orography notsignificant.

• Hdisp=0, Zef=12-0=12 m

• Qb=Ce(z)xCe,tx0.613xVb^2= 2.5x1x0.613x23.9^2= 2.5x 350 = 875 Pa

OROGRAPHY SIGNIFICANT? YES & Z<50 M

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� Orography is significant

� In side shaded area: Co > 1

� Determine Co factor

� Building height <50 m.

� Use equation NA.4a (z<50m)

OROGRAPHY SIGNIFICANT? YES & Z<50 M

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Co factor? Get from Annex A of

EN

� Determine ø (slope angle)

� Determine factor s

� Determine Co factor

� Ø=0.05 >> 2.8 degree angle

� Ø=0.3 >> 17 degree angle

WORKED EXAMPLE: CO FACTOR

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• Ø=H/Lu=10/100 =0.1; 0.05<Ø<0.3

then Le=Lu

• Orography is significant

• Site 10 m from the crest of a Cliff

• x/Lu= 10/100=0.1

• z/Le= 12/100=0.12

• S=0.6

• Co=1+2sØ=1+2x0.6x0.1=1.12

• Ce(z)=2.5 (See last example)

• Qp = 2.5x [{co+0.6}/1.6]^2x

0.613x23.9^2 =2.5x 1.16x 350

=1015 PA

OROGRAPHY SIGNIFICANT? YES & Z>50 M

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� Orography is significant

� In side shaded area: Co > 1

� Determine Co factor (see

above)

� Building height>50 m.

� Use equation NA.4b (z>50m)

� Building z>200 m: out side of

the scope of this NA (specific

studies required)

OROGRAPHY SIGNIFICANT: YES & Z>50 M

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Vm = mean wind velocity

� Orography factor: Co (see

above)

� Roughness factor: Cr(z)

� Correction of Cr,T(z) in Town

terrain)

Mean wind velocity: Vm

WORKED EXAMPLE: ROUGHNESS FACTOR

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Roughness factor

Roughness Correction factor

for sites in Town terrain.

Limitation for Hdisp=3 m?

OROGRAPHY SIGNIFICANT: YES & Z>50 M

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� Turbulence Intencity: Iv(z) in

flat terrain

� Correction of KI,T in Town

terrain

� Orography factor: Co (see

above)

Turbulence Intencity: Iv(z)

WORKED EXAMPLE: TURBULENCE

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Turbulence Intensity in flat terrain

Turbulence Correction factor

for sites in Town terrain.

PEAK VELOCITY PRESSURE: QP

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� Calculation of peak velocity

pressure with this equation

WIND PRESSURE (W ) AND WIND FORCES (F)

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� External or Internal Wind

pressure

� Cpe, Cpi: clause 7.2 for

buildings

� Cpe1: for area less than 1

m2

� Cpe 10: for area greater than

1 m2

� Cpi: open, close building.

� CsCd= Size and dynamic

factor: can be calculated

separately according to NA

� May have a favorable effect

WORKED EXAMPLE: CSCD FACTOR

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• Office building with plan of

60mx20m with a height of 10 m in

town terrain: CsCd?

• Option 1:

� Clause 6.2(1) of EN 1991-1-4:

CsCd is equal to 1 for building <

15 meters.

• Option 2: separately cal.

� h/b= 10/60=0.167 >> Cd=1

• table NA.3: z=10, B+H=60+10=70,

zone C>> Cs=0.77 , CsCd = 0.77

(23% reduction)

CONCLUSION:

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� Scope of BS EN 1991-1-1-4 2005

Wind & NA

� Flow chart for obtaining peak

velocity pressure

� Basic wind velocity pressure

• Correction factor: Calt, Cdir,

Cseason, Cprob

� When orography is significant

� Peak wind velocity pressure

• Roughness factor

• Orograpfy factor

• Turbulence factor

• Correction factor for sites in Town

terrain

� Internal en external factors

� Size and dynamic factors

� Illustration with worked examples

REFERENCES:

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� BS EN 1991-1-4 2005 +A1 2010

� NA to BS EN 1991-1-4 2005 +A1 2010

� PD 6688-1-4:2015 (Published Document):

Background information to the National

Annex to BS EN 1991-1-4 and additional

guidance.

� CCIP-Wind loading: chapter 2.6

� WSP worked example introduction to

1991-1-4 from 2009

� BRE publication in Digest DG-436-1, 2

� Additional online search

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�VOS QUESTIONS S.V.P?