BS NA EN 1998-2: UK National Annex to Eurocode 8. Design ...
Presentation Wind and Eurocode UK
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Transcript of Presentation Wind and Eurocode UK
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
3
� 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
4
� 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
5
� 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
6
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
7
� 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
8
� 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
9
� 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
10
� 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
11
� 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
12
� 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
13
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
14
� 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
15
� 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
16
� 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
17
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
18
• 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
19
� 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
20
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
21
Roughness factor
Roughness Correction factor
for sites in Town terrain.
Limitation for Hdisp=3 m?
OROGRAPHY SIGNIFICANT: YES & Z>50 M
22
� 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
23
Turbulence Intensity in flat terrain
Turbulence Correction factor
for sites in Town terrain.
PEAK VELOCITY PRESSURE: QP
24
� Calculation of peak velocity
pressure with this equation
WIND PRESSURE (W ) AND WIND FORCES (F)
25
� 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
26
• 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:
27
� 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:
28
� 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
29
�VOS QUESTIONS S.V.P?