This presentation is intended for year-2 BEng/MEng Civil and Structural Engineering Students. The main purpose is to present how characterise wind loading on simple building structures according to Eurocode 1

1. Wind ac(ons according to
Eurocode I
Dr Alessandro Palmeri
<A.Palmeri@lboro.ac.uk>

2. Mo(va(ons
• Structural engineers need to consider the effects
of wind forces on civil engineering structures
2
Extensive damage to an office
building’s curtain wall and interior
during the 2000 Fort Worth tornado
(Texas)
A roof of a home has been ripped off aNer a
tornado struck in Alfreton, Derbyshire (October
2014)

3. Learning Outcomes
• When we have completed this unit (2 lectures
+ 1 tutorial), you should be able to:
– Iden(fy the key parameters influencing wind
loads on structures
– Apply Eurocode 1 to evaluate wind loads on a
simple civil engineering structure
3

4. Introduc(on
• Wind is flowing air
• Structures in the wind are
subjected to forces which
vary with (me and space
– The level of turbulence of the
wind can be measured by the
turbulence intensity Iv, defined
as the dimensionless
coefficient of varia(on of the
wind velocity, i.e. standard
devia(on divided by mean
value, i.e.
!!
Iv
=
σv
vm
vm
2Iv
4

5. Introduc(on
• Flexible structures(e.g.
tall buildings and long-
span bridges) are
par(cularly sensi(ve
to dynamic interac(on
phenomena
– Theory of
aeroelas,city must be
embedded in the
structural design
Burj Khalifa, Dubai [828 m tall]
Akashi-Kaikyo Bridge, Kobe
[central span of 1,991 m]

6. Introduc(on
• The lesson from the Tacoma Narrows Bridge
– Probably the most spectacular collapse in the whole history of
structural engineering (7th Nov 1940)
6

7. Introduc(on
• For ordinary buildings,
… with conven(onal structural systems,
… with “simple” shape and “small” extension,
… designers are allowed to use equivalent
sta,c forces to represent the actual effects of
the turbulent wind
• This is what you have to do for the Structural
Design coursework
7

8. Introduc(on
• You will need to become
familiar with two documents:
– BS EN 1991-1-4:2005
• Eurocode 1: Ac(ons on
structures, Part 1-4: General
ac(ons, Wind ac(ons
– NA to BS EN 1991-1-4:2005
• Corresponding UK Na(onal
Annex
– Both documents can be
downloaded by using your
account in the MyAthens
website
8

9. Flowcharts for wind load calcula(ons
(Figs. A.NA.1 and A.NA.2 in the UK NA)
Sites in country Sites in town
9

10. Where to start?
vb,0= Fundamental value of the basic wind velocity
• Is the 10-minute mean wind velocity
... at 10 m above the ground
... of a terrain with low vegeta(on,
... having an annual probability of exceedance p= 0.02
(i.e. a return period R= 1/p= 50 years)
• This can be evaluated as:
10
!!
vb,0
= vb,map
⋅calt

11. Map values of the basic wind velocity [m/s]
NA to BS EN 1991-1-4:2005, Figure NA.1
11
vb,map
= 22m/s

12. calt= Al(tude factor
• It depends on:
– Al(tude of the site above the mean sea level, A [m]
– Reference height of the structure, zs [m]
– It is acceptable to use the value of calt for zs= 10 m for all
cases, as this is a conserva(ve assump(on
12
calt
=
1+ 0.001⋅A , zs
≤10m
1+ 0.001⋅A⋅
10
zs
⎛
⎝
⎜
⎞
⎠
⎟
0.2
, zs
>10m
⎧
⎨
⎪⎪
⎩
⎪
⎪

13. Defini(on of the reference height
(BS EN 1991-1-4:2005, Figure 6.1)
Depending on the terrain category, zmin can vary from 1 to 10 m

14. vb= Basic wind velocity
• For par(cular design situa(ons, is it possible (not
recommended) to reduce the fundamental value of
the basic wind velocity, vb,0, taking into account:
– Direc(onal factor, cdir
– Seasonal factor, cseason
14
!!
vb
= cdir
⋅cseason
⋅vb,0
≤vb,0

15. cprob= Probability factor
• When required, this mul(plier
allows adjus(ng the annual risk of
being exceeded:
where R= 1/p is the return period,
while K and n are two parameters
defining the probabilis(c
distribu(on of extreme winds
– Suggested values: K= 0.2 and
n= 0.5
15
!!
cprob
=
1−K ⋅ln −ln 1− p( )( )
1−K ⋅ln −ln 1−0.02( )( )
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
n
10 20 50 100 200 500 1000
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
R !years"cprob R"[years]"

16. qb= Basic wind pressure
• The dynamic pressure due to the wind is the kine(c
energy per unit volume of the flowing air
– The density of the air is assumed to be ρair= 1.226 kg/m3.
• This is only a “nominal” or “reference” value of the wind
pressure, as the above equa(on assumes a uniform,
laminar flow of the air, and does not account for the
effects of the turbulence and varia(on with the height
16
qb
=
1
2
ρair
⋅vb
2

17. qp= Peak wind pressure
• The “basic” value of wind pressure qb can be transformed into the
“peak” value qp(z), varying with the height z, taking into account:
– The roughness of the surrounding terrain;
– The site’s proximity to the coast;
– How far the site is located within a town (if applicable)
• This is done with two coefficients:
– “Proximity to shoreline” exposure coefficient ce(z-hdis);
– “Site within town” exposure modifier ce,T(z-hdis),
• both depending on the height of nearby structures through the
“displacement height” hdis
17
!!
qp
(z)= ce
(z−hdis
)⋅ce,T
(z−hdis
)⋅qb

18. Typical logarithmic profiles of qp(z)
18
qp
(z)
qb
• According to the general Eurocode, not the UK NA!

19. hdis= Displacement height
• In the UK NA, this quan(ty effec(vely accounts for the
reduc(on in the wind velocity due to the presence of
closely spaced buildings and other obstruc(ons
19
hdis
=
min 0.6h,0.8have{ }, 0 ≤ x ≤ 2have
min 0.6h,1.2have
−0.2x{ }, 2have
< x < 6have
0 , x ≥ 6have
⎧
⎨
⎪⎪
⎩
⎪
⎪

20. Upwind distance to shoreline
20
The larger the distance to the shoreline, the larger the effects of the
“roughness” of the terrain, the lesser the “expected” gradient in the
profile of the mean wind velocity, the higher the “expected” level of
turbulence

21. ce(z-hdis)= Exposure coefficient
21
Values of ce are
read by
interpola(ng
between the
contour of the
chart given as
Figure NA.7 in
the NA to EN
11.0 – 8.0
14.5-8.0
1.62
2.05
Roughness
factors at 11 and
14.5 m of height,
assuming
distance upwind
to shoreline=
90 km and
displacement
height= 8 m

22. ce,T(z-hdis)= Exposure modifier
22
Values of ce,T are
read by
interpola(ng
between the
contour of the
chart given as
Figure NA.8 in
the NA to EN
11.0 – 8.0
14.5-8.0
0.76
0.87
Roughness
correc(on factors
at 11 and 14.5 m
of height,
assuming
distance inside
town terrain=
1 km and
displacement
height= 8 m

23. Is orography significant?
23
The orography factor
needs to be considered
only if the structure falls
within one of the
shadowed areas on the
right, otherwise co(z)= 1
Furthermore, the peak wind pressure qp needs to be calculated combining the effects
of mean wind velocity and turbulence, e.g.:
qp
(z) =
1
2
1+3.0 Iv
(z)⎡
⎣
⎤
⎦
2
ρair
vm
2
(z) , z > 50m

24. Is z > 50 m significant?
24
Up to four charts (Figs. NA.3 to NA.6) are required to calculate qp in this case
!!
vm
(z)= cr
(z)cr,t
(z)co
(z)vb
Iv
(z) =
Iv,flat
(z)kI,T
(z)
co
(z)cr
(z)
!!
cr,T
(z)
!!
Iv,flat
(z)
kI,T
(z)

25. External and internal wind pressure
• External (cpe) and internal (cpi) pressure
coefficients are required to get the actual
pressures on different external (we(ze)) and
internal (wi(zi)) areas from the peak velocity
pressure
– ze and zi being the per(nent reference
height
• In general, both cpe and cpi contribute to the
total wind load on the envelope of the
building
25
we
(ze
) = cpe
⋅qp
(ze
)
wi
(zi
) = cpi
⋅qp
(zi
)
⎧
⎨
⎪
⎩⎪

26. External and internal wind pressure
• Pressure coefficient can be
posi(ve (wind “pushing”) or
nega(ve (wind “pulling”, i.e.
“suc(on”)
26
• Presence, loca(on and size of any opening play a crucial role
in the defini(on of the pressure coefficients

27. Internal wind pressure / Dominant face
27
• A face of a building should be regarded as “dominant” when
the area of openings at that face, Aopen,dom, is at least twice
the area of openings and leakages in the remaining faces of
the building considered, Aopen,non-dom
• In this case the internal pressure should be taken as a frac(on
of the external pressure at the openings of the dominant face,
that is:
– cpi = 0,75 cpe , for Aopen,dom= 2 Aopen,nondom ;
– cpi = 0,90 cpe , for Aopen,dom≥ 3 Aopen,nondom ;
– Interpola(on for 2 < Aopen,dom/Aopen,nondom < 3

28. Internal wind pressure / No dominant face
28
h
d

29. External wind pressure
29
• External pressure coefficients depends on the size of the loaded
area A= Aref, along with the posi(on (zone) in the building
• Two limi(ng values are provided by the Eurocode, which can be
used for: small elements, with an area of 1 m2 or less, cpe,1; and
large elements, with an area of 10 m2 or more, cpe,10
• For intermediate situa(ons, i.e. 1 < A < 10 m2, a logarithmic
interpola(on is suggested:
!!
cpe
= cpe,1
−(cpe,1
−cpe,10
)⋅log10
(A)
Why must cpe,10 be always < cpe,1 ?

30. External wind pressure / Example
30
windward' leeward'
side'zones'

31. External wind pressure / Example
31
The eternal pressure should
assumed to be uniform over
each single horizontal strip
considered in the analysis
The shape of the pressure
profile changes with the
width/height ra(o

32. Wind forces
32
• External and internal pressures allow compu(ng the wind
forces on external (Fw,e) and internal (Fw,i) surfaces
where cs and cd are “size factor” and “dynamic factor”,
respec(vely, while Aref is the reference area for each surface
considered in the summa(on
– The product cscd accounts for the effects of: i) the non-
simultaneous occurrence of peak wind pressures on the surface
(cs) and ii) the vibra(ons of the structure due to turbulence (cd)
!!
Fw,e
= cs
⋅cd
⋅ we
(ze
)⋅Aref
surfaces
∑
Fw,i
= wi
(zi
)⋅Aref
surfaces
∑
⎧
⎨
⎪
⎩
⎪

33. Size factor cs (UK NA)
33
h"
b"
wind%

34. Dynamic factor cd (UK NA)
34
h"
b"
wind%

35. Structural Analysis
35
• Bracing systems, shear
walls or rigid beam-to-
column joints are
alterna(ve methods to
ensure the lateral
stability a building
structure under the
design wind forces
• For steel frames, the
laxer are combined with
the effects on ini(al
imperfec(ons in the
construc(on,
represented by
equivalent horizontal
forces (EHFs), which are
taken by the lateral
stability system