WIND ISSUES IN THE DESIGN OF TALL BUILDINGS - PEER

WIND ISSUES
TALL BUILDIN

Peter A. Irwin
RWDI

Los Angeles Tall Building
May 7, 2010

IN THE DESIGN OF
NGS

g Structural Design Council

Wind Issues fo

Structural integrity un
Deflections under ser
Building motions and
Uncertainties in buildi
(stiffness, damping)
Uncertainties in wind
Uncertainties in wind
Codes and standards
Computational Fluid D

or Structural Design

nder ultimate loads
rvice loads
occupant comfort
ing structural properties

loading
climate
s
Dynamics

Relationship be
wind and heigh

Importance
of wind
loads

etween importance of
ht

Building height

Vortex sheddin

Shedding frequency N is
given by

N =SU
b

S = Strouhal number
U = wind speed
b = building width

Directions of
fluctuating
force

wind

ng

s

Peak response du

Magnitude of

peak response

∝ 1
density × damping

ue to vortex excitation

Vortex excitation
Mode 1

n on a tapered spire

Vortex excitation
Mode 2

n on a tapered spire

Shape strategie

Softened corners
Tapering and setbacks
Varying cross-section shape
Spoilers
Porosity or openings

es to reduce excitation

e

Taper effect -

Petronas towers

Taipei 101 – c

Original

REDUC

Modified IN BASE MOM

corner softening

25%
CTION
MENT

Shape effe
101 wind t

ect, Taipei
tunnel tests

Burj Khalifa –

Set backs, changin
Completed Building

828 m

ng cross-section, orientation

Early 1:500 scale wind tunnel tests

Shanghai Cen

Twisting and

nter – 632 m

d tapering

151 storey tow

Creation of bleed slots a
Full scale rendering

wer in Korea

at edges to suppress vortex shedding

1:500 wind tunnel model

Use of corner s
corners

Plan view
without slots

Crosswind
motion

Wind

slots to bleed air through
s on a tall building

Plan view with
slots

Base moments
reduced by
60%

Wind

With vortex Without
excitation vortex

10 yr excitation

(a)

With vortex
excitation

Without
vortex

excitation

(c)

700 yr
50 yr

(b)
50 yr 700 yr

(d)

Reliability conside

Expression for loa

λW = 1 eα 2β
Kw

Vw = Coefficie

β = Reliabili

Kw = Bias fac

α = Combin

wwhheerere

erations for flexible buildings

ad factor λ
w

βVw

ent of variation of wind load
ity index
ctor
nation factor

First order, secon
analysis for rigid

Code analysis metho

Vw = (Vq2i + Va2n )1/ 2 = (0.2

λW = 1 eα 2βVw =
Kw 1

Wind tunnel method

Vw = (0.22

λW =1
1.0

Both the coefficient of var
smaller in the wind tunne
at about the same.

nd moment reliability
buildings

od

22 + 0.32 )1/ 2 = 0.36
1 e(0.752×3.5×0.36) = 1.56
1.3

2 + 0.12 )1/2 = 0.22
e(0.752×3.5×0.22) = 1.54

0

riation and bias factor are
el method. Load factor ends up

Reliability of flex
monotonic respo

Wind load varies as po

W ∝U

Rigid building

Vw = (V

Flexible building

Vw = (Vq2i +

Vdyn ≈ nd2ampVς2 + (n −

Damping term Freque

xible buildings with
onse

ower law

Un

Vq2i + Va2n )1/ 2

+ Va2n + V2 )1/ 2
dyn

− 2)2V 2 + (n − 2)2VV2
f

ency term Velocity sensitivity term

Required load
buildings with

Rigid building

Vw = (Vq2i + Va2n )1/ 2 = (0.22

λW = e(

Typical flexible building

Vw = (Vq2i + Va2n + V2 )1 / 2 = (0
dyn

λ = e(
W

Highly flexible building

Vw = (Vq2i + Va2n + V2 )1/ 2 = (0
dyn

λW = e

Note:- Wind tunnel method is a

d factors for flexible
monotonic response

+ .12 )1/ 2 = 0.22
(0.752×3.5×0.22) = 1.54

0.22 + 0.12 + 0.112 )1/ 2 = 0.249
(0.752 ×3.5×0.249) = 1.63

g

0.22 + 0.12 + 0.222 )1/ 2 = 0.31
e(0.752×3.5×0.31) = 1.84

assumed in all cases.

Wind loads de
ultimate return
tunnel method

Building R
type n ndamp Lo

Rigid 20

Flexible 2.5 0.25

Very

Flexible 3 0.5

etermined directly at
n period by wind
d

Required Actual
oad Factor Load Factor Ratio

1.264n
1.54 1.60 1.04
1.63 1.80 1.10

1.84 2.02 1.10

Slightly conservative

Consequence of v
reliability assessm

Low ratio
Uncertainty in loads is
mostly dictated by
uncertainty in this peak

Need to carefully assess uncertainty
since the normal assumption that unc
longer valid. Damping and frequency

vortex shedding on
ment – Nakheel Tower

o of 1000 year to 50 year loads

in peak vortex shedding response
certainty in wind speed governs is no
y uncertainties become important.

Reynolds num

Originate from viscosity
Can cause changes in f
scale model relative to f
Not a concern on sharp
Can be a concern on cu
Is lessened by high turb
roughness

mber effects

of air
flow patterns on a small
full scale
edged structures
urved shape buildings
bulence and surface

Effect of Reynolds
coefficient around

1.0

0.5

0 20

Cp -0.5

Reynolds number

-1.0

Re = Ub -1.5
-2.0
ν

Kinematic -2.5
viscosity of
air

number on pressure
a circular cylinder

θ degrees U θb

60 100 140

Sub-critical Re < 2×105
Transcritical Re > 3×106

1:50 Scale Model of
High Reynolds Numb

Upper Portion of Burj Khalifa
mber Tests, Re ~ 2x106

Note:
Full scale Re~7x107