Wind Engineering Fundamental Engineering Concepts

and Design

By

Moayyad Al Nasra, PhD, P.E.

WIND NATURE

and its Interaction with Structures

• Nature of Wind

• Wind/structure interaction

SNAPSHOT OF WIND

Dynamic nature

MEAN WIND SPEED AVERAGING TIME

VARIATION OF WIND SPEED WITH HEIGHT

LOG LAW

vz k

uz

zo

1

*ln

•Vz = wind speed at height z above ground •z = height above ground • zo = surface roughness length • u* = shear velocity of the flow •k = Von Karman’s constant; generally used as 0.4

POWER LAW

Vz = Vg

Vz = Vg z > zg

zz

1

0 z zg

g

• VZ = wind speed at any height above ground • Vg = gradient wind speed • z = height above ground • zg = gradient height • power law exponent • Values of zg and depend on terrain roughness

POWER LAW PROFILES

WIND TURBULENCE

Vmean

=19.7MPH

0

10

20

30

40

0 3 6 9 12 15

Time (Minutes)

Win

d S

pe

ed

(MP

H)

GUST FREQUENCY SPECTRUM

0.00

0.05

0.10

0.15

0.20

0.25

0.0001 0.001 0.01 0.1 1 10

Frequency (Hz, Cycles/Second)

n*S

u(n

)/

u2

0.1110100100010000

Period (Seconds)

TOPOGRAPHIC EFFECT

WIND-STRUCTURE INTERACTION

• Aerodynamics; Pressure and Force Coefficients

• Buffeting; Along-Wind Resonance

• Vortex Shedding

• Aeroelastic: Galloping, Flutter

AERODYNAMICS

AERODYNAMICS

AERODYNAMICS

AERODYNAMICS BERNOULLI’S EQN

AERODYNAMICS

PRESSURE COEFFICIENT Ppeak

Vpeak

Pmean

-1

0

1

2

3

0 3 6 9 12 15

Time (Minutes)

Win

d P

res

su

re

(PS

F)

Vmean

0

10

20

30

40

0 3 6 9 12 15

Time (Minutes)

Win

d S

pe

ed

(MP

H)

BUFFETING

VORTEX SHEDDING

AEROELASTIC • Flutter of stop sign

AEROELASTIC • Galloping of cable or bridge

WIND DAMAGE EXPERIENCE

• Wind Flow Damage

• Damage by Building Category

• Windborne Debris (Missiles)

• Some Damage Statistics

WINDBORNE DEBRIS (MISSILES)

• Windborne missiles range in size from roof gravel to large storage tanks and railroad cars

• Missiles cause damage by:

– Perforation

– Structural Collapse

Riot/Civil Disorder

1.0%

Explosion/Fire

4.5%

Earthquake

24.9%

Wind/Hail/Tornado

36.5%

Hurricane/Tropical Storm

32.7%

Other

0.4%

Wind Loads, ASCE 7-10 The design codes recognizes the following

• Design Elements

– Main Wind-Force Resisting System (MWFRS)

– Components and Cladding (C&C)

• Basic wind speed, V, = Three – second gust speed at 33 ft above the ground in exposure C.

• Buildings – Enclosed

– Open, building having each wall at least 80% open

– Partially enclosed

Wind Loads, continued

• Structures

– Rigid: in general – low rise that have a fundamental natural frequency > 1 Hertz, the gust effect factor may be taken as 0.85

– Flexible: slender structures that have a fundamental natural frequency < 1 Hertz, use formulae to calculate the gust-effect factor

Approximate natural Frequency

• The approximate formulae may be used if the building height < 300 ft and less than 4 times its effective length. – For structural steel moment-resisting-frame

• na = 22.2/h0.8

– For concrete-resisting-frame • na= 43.5/h0.9

– For structural steel and concrete • na= 75/h

– Where h= mean roof height (ft)

Wind Loads Parameters

• Basic Wind Speed, V.

• Wind Directionality Factor, Kd.

• Exposure Category

• Topographic Factor, Kzt

• Gust Effect Factor, G

• Enclosure Classifications

• Internal Pressure Coefficient, GCpi

Wind Loads may be determined by

• Directional procedure for buildings of all heights

• Envelop procedure for low rise buildings (main roof height < 60 ft, and does not exceed least horizontal dimension): derived from wind tunnel testing

• Directional procedure for buildings appurtenances (rooftop, signs, tanks, trussed towers, …)

• Wind tunnel procedure for any building or other structure

Steps to determine MWFRS wind loads Directional Procedure

• Determine risk category of the structure • Determine the basic wind speed, V • Determine wind parameters • Determine velocity pressure exposure coefficient,

Kz, or Kh

• Determine velocity pressure, qz, of qh(pressure calculated at h=mean roof height)

• Determine external pressure coefficient, Cp, or CN

• Calculate the wind pressure, p, on each building surface.

Velocity Pressure, Directional Procedure

• Velocity Pressure, qz, evaluated at height z shall be calculated by the following equation:

• qz= 0.00256KzKztKdV2 (lb/ft2)

• Where – Kd= wind directionality factor

– Kz = Velocity pressure exposure coefficient

– Kzt= Topographic factor

– V=Basic wind speed

– qh= velocity pressure calculated at mean roof height, h.

Wind Loads, Main Wind Force-Resisting System (MWFRS) ASCE/SEI 7-10, year 2010

• Enclosed and Partially Enclosed Rigid Building • p=qGCp-qi(GCpi) (lb/ft2)

• Enclosed and Partially Enclosed Flexible Buildings

• p=qGfCp-qi(GCpi) (lb/ft2)

• Open Buildings with Mono-slope, Pitched, or Troughed Free Roofs

• p=qhGCN

• Where: – q= qz for windward wall evaluated at height z above the ground – q= qh for leeward walls, side walls and roofs evaluated at height h. – qi= qh for windward walls side wall, leeward walls and roofs of

enclosed buildings and for negative internal pressure evaluation in partially enclosed buildings.

– qi=qz for positive internal pressure evaluation in partially enclosed buildings where height z is defined as the level of the highest opening in the building that could affect the positive internal pressure. For buildings sited in wind-borne debris region, glazing that is not impact resistance or protected with an impact resistance covering shall be treated as an opening. For positive internal pressure evaluation, qi may conservatively be evaluated at height h(qi=qh)

– G = gust effect factor – Cp= external pressure coefficient – (GCpi)= internal pressure coefficient – Gf= gust effect factor, flexible buildings – CN= net pressure coefficient , open buildings

Risk Category of Buildings and Other Structures for Flood, Wind, Snow, Earthquake, and Ice Loads

Use or Occupancy of Buildings and Structures Risk Category

Buildings and other structures that represent a low risk to human life in the event of failure

I

All buildings and other structures except those listed in Risk category I, III, and IV

II

Buildings and other structures, the failure of which could pose a substantial risk to human life.

III

Buildings and other structures designated as essential facilities IV

Basic wind speeds for occupancy category II buildings and other structures Values are nominal design 3-seconds gust wind (mph) at 33 ft above ground exposure C category Mountainous terrain, gorges, ocean promontories, and special wind regions shall be examined for unusual wind conditions Wind speed correspond to approximately a 7% probability of exceedance in 50 years

Kz

A B C D

0

100

200

300

500 H

eight,

ft

400

0.0 0.8 1.2 1.6 2.0 0.4

Velocity Pressure Exposure Coefficient, Kz

Velocity Pressure Exposure Coefficients, Kh, Kz

Height above ground level, z

Exposure

(ft) B C D

0-15 0.57 0.85 1.03

20 0.62 0.90 1.08

30 0.70 0.98 1.16

40 0.76 1.04 1.22

50 0.81 1.09 1.27

100 0.99 1.26 1.43

200 1.20 1.46 1.61

300 1.35 1.59 1,73

400 1.47 1.69 1.82

500 1,56 1.77 1.89

Topographic Factor, Kzt

• Kzt= (1+K1K2K3)2

• Kzt = topographic factor; greater or equal 1.0 • K1 = factor to account for shape of the

topographic feature and maximum speed-up effect;

• K2 = factor to account for reduction in speed-up with distance upwind or downwind of crest; and, K3 = factor to account for reduction in speed-up with height above local terrain.

Wind Directionality Factor, Kd

Structure Type Directionality Factor, Kd

Buildings MWFRS C &C

0.85 0.85

Arched Roofs 0.85

Chimney, tanks, and similar structures Square Hexagonal Round

0.9

0.95 0.95

Solid free standing walls and solid freestanding and attached signs

0.85

Open signs and lattice framework 0.85

Trussed towers Triangular, square, rectangular All other cross sections

0.85 0.95

Gust Effect Factor

• For rigid buildings / structures, the gust effect factor, G, shall be taken as 0.85 or calculated by the gust formulae provided by the code.

• For flexible or dynamically sensitive buildings / structures, the gust effect factor, Gf, can only be obtained by using the gust factor formulae provided by the code.

MWFRS, External Pressure Coefficient, Cp, all heights, Walls

Wall pressure Coefficient, Cp

Surface L/B Cp Use with

Windward Wall All values 0.8 qz

Leeward Wall 0-1 -.5 qh

2 -.3

>4 -.2

Side Wall All values -.7 qh

Roof Cp’s Windward

Angle, degrees

Leeward

Angle, degrees)

Wind direction

h/L 10 15 20 25 30 35 45 60#

10 15 20

Normal to

0.25 -0.7 -0.5 0.0*

-0.3 0.2

-0.2 0.3

-0.2 0.3

0.0*0.4

0.4 0.01 -0.3 -0.5 -0.6

ridge for

10o

0.5 -0.9 -0.7 -0.4 0.0*

-0.3 0.2

-0.2 0.2

-0.2 0.3

0.0* 0.4

0.01 -0.5 -0.5 -0.6

1.0 -1.3** -1.0 -0.7 -0.5 0.0*

-0.3 0.2

-0.2 0.2

0.0* 0.3

0.01 -0.7 -0.6 -0.6

Normal to

ridge for

10o

and Parallel

0.5 Horiz distance from windward edge 0 to h/2 -0.9 h/2 to h -0.9 h to 2h -0.5 >2h -0.3

*Value is provided for interpolation purposes. **Value can be reduced linearly with area over which it is applicable as follows:

to ridge

for all

1.0 0 to h/2 -1.3** >h/2 -0.7

Area Reduction (sq ft) Factor

100 (9.29 sq m) 1.0 250 (23.23 sq m) 0.9

1000 (92.9 sq m) 0.8

Flat roofs

MWFRS and C&C Internal Pressure Coefficient (GCpi)

Enclosure Classification (GCpi)

Open Buildings 0.00

Partially Enclosed Buildings +0.55 -0.55

Enclosed Buildings +0.18 -0.18

Example 1. 100 ft X 200 ft X 160 ft Office Building

2. Office Building on Escarpment

3. Exposure B, Category II, Wind Speed = 130 mph Topography: Flat Terrain: Suburban Dimensions: 100 ft x 200 ft in plan Roof height of 157 ft with 3 ft parapet Flat roof Framing: Reinforced concrete rigid frame in both directions Floor slabs provide diaphragm action Mullions for glazing panels span 11’-0” between floor slabs Mullion spacing is 5’-0” Cladding: Wind-borne debris resistant glazing panels are 5’-0” wide X 5’-6” high (typical)

ztK

Example 1

160 FT

3' PARAPET

Velocity Pressures

psf)2Vd

Kzt

Kz

0.00256Kz

q ........(

where:

qz = velocity pressure at height z

Kz = velocity pressure exposure coefficient

evaluated at ht z

Kzt = topographic factor

Kd=directional factor

V = basic wind speed

z = height above ground

Topographic Factor,Kzt

K 1 K K K 1.0

Since the topography is flat (H 60 ft)

Use K 1.0

zt 1 2 3

2

zt

Velocity Pressures

2Vd

K zt

K z

K 0.00256z

q

for this building:

Kzt = 1.0

V = 130 mph

Kd=0.85

qz = 0.00256 Kz (1.0)(0.85) (130)2

qz = 36.9 Kz psf

Step 3: Velocity Pressures

Height, ft Kz qz , psf

0 - 15 0.57 21.0

30 0.70 25.8

50 0.81 29.9

80 0.93 34.3120 1.04 38.3

Parapet ht=160 1.13 41.7

qz = 36.9 Kz psf

Step 4: Design Pressures for the MWFRS

)( piip GCqqGCp Where:

p= pressure on surface

q= velocity pressure

G= gust effect factor, use G=0.80 in this case the code allows

G= 0.85 for rigid building or it can be calculated by the gust effect formula

Cp = external pressure coefficient

qh= velocity pressure at mean roof height

GCpi= product of gust effect factor and internal

pressure coefficient

Step 4: Design Cases For MWFRS

• Wind normal to 200 ft face

– Positive and negative internal pressure

• Wind parallel to 200 ft face

– Positive and negative internal pressure

Wall External Pressure Coefficients

Wall Cp’s

Surface WindDirection

L/B Cp

Windward Wall All All 0.80

Leeward Wall to 200 ft face 0.5 -0.50

to 200 ft face 2.0 -0.30

Side Wall All All -0.70

Roof Cp’s

h/L

Normalto

ridgefor

10o

andParallel

0.5 Horiz distance fromwindward edge0 to h/2 -0.9h/2 to h -0.9h to 2h -0.5 >2h -0.3

*Value is provided forinterpolation purposes.

**Value can be reduced linearlywith area over which it isapplicable as follows:

to ridge

for all 1.0

0 to h/2 -1.3** >h/2 -0.7

Area Reduction(sq ft) Factor

100 (9.29 sq m) 1.0250 (23.23 sq m) 0.9

1000 (92.9 sq m) 0.8

Roof Cp’s (Wind Normal 200 ft Face)

Distance fromWindward Edge

h / L Cp RF Cp

0 to h / 2 1.0 -1.3 0.80 -1.04

> h / 2 -0.7 - -0.7

h/L = 160 ft /100 ft = 1.6

windward

edge

wind

B =

200’

L = 100’

h

2 80'

A =

16

00

0 ft2

Roof Cp’s (Wind Parallel to 200 ft Face)

D is ta n ce from h / L

W ind ward Edge 0.5 0.80 1.0

0 to h / 2 -0.9 -0.98 -1.04*

h / 2 to h -0.9 -0.78 -0.7

h to 2 h -0.5 -0.62 -0.7

h/L = 160 ft / 200ft = 0.8

*Cp = -1.3 * 0.8 = -1.04

windward

edge

B =

100’

L =200’

80'

0 h / 2 80'

h / 2 h 40'

h 2h

wind

GCpi

• Assume the openings are evenly distributed in the walls and roof. Enclosed building

GCpi = 0.18

MWFRS Net Pressures

roofandsidewall,wall,leewardforqq

wallwindwardforqq

where

0.18)(q(0.80)Cp

GCqqGCp

h

z

p

pip

7.41

MWFRS Net Pressures (Wind Normal to 200 ft Face)

Surface z or l q Cp Ext.Pres.

Net Pressure (psf)With

ft psf psf (+GCpi) (-GCpi)

Windward 0-15 21.0 0.8 13.4 5.9 20.9Wall 30 25.8 0.8 16.5 9.0 24.0

50 29.9 0.8 19.1 11.6 26.680 34.3 0.8 21.9 14.4 29.4

120 38.3 0.8 24.5 17.0 32.0160 41.7 0.8 26.7 19.2 34.2

L’ward Wall All 41.7 -0.5 -16.7 -24.2 -9.2Side Walls All 41.7 -0.7 -23.3 -30.8 -15.8Roof 0 - 80 41.7 -1.04 -34.7 -42.2 -27.2

80 - 100 41.7 -0.7 -23.3 -30.8 -15.8

internal pressure = 7.5 psf

Pressures (Wind Normal to the 200 ft Face External Only)

100 ft

120 ft

50 ft

30 ft

15 ft

26.7 psf

24.5 psf

21.9 psf

19.1 psf

16.5 psf

13.4 psf

160 ft

16.7 psf 80 ft

34.7 psf 23.3 psf

Internal pressure

7.5 psf

MWFRS Net Pressures (Wind Parallel to 200 ft Face)

Surface z or l q Cp Ext.Pres.

Net Pressure (psf)with

ft psf psf (+GCpi) (-GCpi)

Windward 0-15 21.0 0.8 13.4 5.9 20.9Wall 30 25.8 0.8 16.5 9.0 24.0

50 29.9 0.8 19.1 11.6 26.680 34.3 0.8 21.9 14.4 29.4

120 38.3 0.8 24.5 17.0 32.0160 41.7 0.8 26.7 19.2 34.2

L’ward Wall All 41.7 -0.3 -10.0 -17.4 -2.5Side Walls All 41.7 -0.7 -23.3 -30.8 -15.8Roof 0 - 80 41.7 -0.98 -32.8 -40.2 -25.2

80 - 160 41.7 -0.78 -26.0 -33.5 -18.5160 - 200 41.7 -0.62 -20.7 -28.2 -13.2

internal pressure = 7.5

Pressures (Wind Parallel to the 200 ft Face External Only)

120 ft

50 ft

30 ft

15 ft

26.7 psf

24.5 psf

21.9 psf

19.1 psf

14.5 psf

13.4 psf

80 ft

160 ft

10.0 psf

32.8 psf 26.0 psf

20.7 psf

200 ft

Internal pressure

7.5 psf

Design Pressures for Components and Cladding

p q GC GCp pi

Where:

p = pressure on component

q = qz for positive pressures at height z

q = qh for negative pressures

GCp = product of gust effect factor and external

pressure coefficient

GCpi = product of gust effect factor and internal

pressure coefficient

Width for Edge Effects

2 2a

a 1 2

3

h

z

a

5

5

4

ROOF PLAN

WALL ELEVATION a = 0.10 (least dimension) 3 ft

a = 0.10 (100)

a = 10 ft

Wall GCp’s

Wall GCp’s

GCp

Comp. A, ft2

Zone 4 Zone 5 Zones 4&5

(-GCp) (-GCp) (+GCp)

Mullion 55 -0.84 -1.55 0.81

Panel 27.5 -0.88 -1.72 0.87

Compute Net Pressures

Mullion in Zone 4 (Negative Pressure)

p = 41.7[(-0.84) - ( 0.18)] psf

p = -42.4 psf with positive internal pressure

p = -27.4 psf with negative internal pressure

Mullion in Zone 5 (Negative pressure)

p = 41.7[(-1.55) - ( 0.18)] psf

p = -72.0 psf with positive internal pressure

p = -57.0 psf with negative internal pressure

Mullion in Zone 4 or 5 (Positive pressure, z = 25 ft)

p = 25.8[(0.81) - ( 0.18)] psf

p = 16.3 psf with positive internal pressure

p = 25.5 psf with negative internal pressure

Mullion Net Pressures

Design Pressures, psf

Comp. z Zone 4 Zone 5

ft Positive Negative Positive Negative

Mullion 0 - 15 20.8 -42.4 20.8 -72.0

15 - 30 25.5 -42.4 25.5 -72.0

30 - 50 29.6 -42.4 29.6 -72.0

50 - 80 34.0 -42.4 34.0 -72.0

80 - 120 37.9 -42.4 37.9 -72.0

120 - 160 41.3 -42.4 41.3 -72.0

Panel Net Pressures

Design Pressures, psf

Comp. z Zone 4 Zone 5

ft Positive Negative Positive Negative

Panel 0 - 15 25.8 -44.1 25.8 -79.0

15 - 30 29.9 -44.1 29.9 -79.0

30 - 50 33.4 -44.1 33.4 -79.0

50 - 80 37.3 -44.1 37.3 -79.0

80 - 120 40.8 -44.1 40.8 -79.0

120 - 160 43.7 -44.1 43.7 -79.0

Roof GCp

Figure 6-8

Roof External Pressure Coefficients

External Pressure Coefficient

Comp. At Zone 1 Zone 2 Zone 3

ft2 (-GCp) (-GCp) (-GCp)

Comp. 20 -1.31 -2.18 -3.04

100 -1.11 -1.89 -2.67

250 -0.99 -1.72 -2.46

400 -0.93 -1.64 -2.35

500 -0.90 -1.60 -2.30

Roof Net Pressures

Design Pressures, psf

Comp. A Negative

ft2

Zone 1 Zone 2 Zone 3

Comp. 20 -62.1 -98.1 -134.2

100 -53.6 -86.1 -118.7

250 -48.7 -79.3 -110.0

400 -46.2 -75.8 -105.4

500 -45.0 -74.1 -103.3

HURRICANES

AND

TORNADOES

METEOROLOGICAL DESCRIPTION OF HURRICANES

HURRICANE WIND FIELD

WIND DISTRIBUTIONS

WIND GUSTS

HURRICANE RELATED PHENOMENA

ATMOSPHERIC PRESSURE CHANGE

HURRICANE FORMATION Tropical Disturbance Slight surface circulation,

at most one closed isobar

Tropical Depression Wind 32 mph, one or more

closed isobars

Tropical Storm Wind 33-73 mph

Hurricane Wind 74 mph

WIND INTENSITY

CategoryCentral Pressure

mb (in. Hg)Winds(mph)

Surge(ft) Damage

1 980 (28.94) 74-95 4-5 Minimal

2 965-979 (28.50-28.91) 96-110 6-8 Moderate

3 945-964 (27.91-28.47) 111-130 9-12 Extensive

4 920-944 (27.17-27.88) 131-155 13-18 Extreme

5 <920 (27.17) > 155 > 18 Catastrophic

Storm Surge

Example of 15 ft storm surge

Hurricane Surge- Dennis 2005

RANK HURRICANE YEAR

1 TX (Galveston) 1900 4 8000 a

2 FL (SE/Lake Okeechobee) 1928 4 2500 b

3 KATRINA (SE LA/MS) 2005 3 1500

4 LA (Cheniere Caminanda) 1893 4 1100-1400 c

5 SC/GA (Sea Islands) 1893 3 1000-2000 d

6 GA/SC 1881 2 700

7 AUDREY (SW LA/N TX) 1957 4 416 h

8 FL (Keys) 1935 5 408

9 LA (Last Island) 1856 4 400 e

10 FL (Miami)/MS/AL/Pensacola 1926 4 372

11 LA (Grand Isle) 1909 3 350

12 FL (Keys)/S TX 1919 4 287 j

13 LA (New Orleans) 1915 4 275 e

13 TX (Galveston) 1915 4 275

15 New England 1938 3 256

15 CAMILLE (MS/SE LA/VA) 1969 5 256

17 DIANE (NE U.S.) 1955 1 184

18 GA, SC, NC 1898 4 179

19 TX 1875 3 176

20 SE FL 1906 3 164

21 TX (Indianola) 1886 4 150

22 MS/AL/Pensacola 1906 2 134

23 FL, GA, SC 1896 3 130

24 AGNES (FL/NE U.S.) 1972 1 122 f

25 HAZEL (SC/NC) 1954 4 95

26 BETSY (SE FL/SE LA) 1965 3 75

27 Northeast U.S. 1944 3 64 g

28 CAROL (NE U.S.) 1954 3 60

29 FLOYD (Mid Atlantic & NE U.S.) 1999 2 56

30 NC 1883 2 53

31 SE FL/SE LA/MS 1947 4 51

Mainland U.S. tropical cyclones causing 25 or greater deaths 1851-2006.

CATEGORY DEATHS

RANK HURRICANE YEAR CATEGORY DAMAGE (U.S.)

1 KATRINA (SE FL, SE LA, MS) 2005 3 $81,000,000,000

2 ANDREW (SE FL/SE LA) 1992 5 26,500,000,000

3 WILMA (S FL) 2005 3 20,600,000,000

4 CHARLEY (SW FL) 2004 4 15,000,000,000

5 IVAN (AL/NW FL) 2004 3 14,200,000,000

6 RITA (SW LA, N TX) 2005 3 11,300,000,000

7 FRANCES (FL) 2004 2 8,900,000,000

8 HUGO (SC) 1989 4 7,000,000,000

9 JEANNE (FL) 2004 3 6,900,000,000

10 ALLISON (N TX) 2001 TS @ 5,000,000,000

11 FLOYD (Mid-Atlantic & NE U.S.) 1999 2 4,500,000,000

12 ISABEL (Mid-Atlantic) 2003 2 3,370,000,000

13 FRAN (NC) 1996 3 3,200,000,000

14 OPAL (NW FL/AL) 1995 3 3,000,000,000

#REF! FREDERIC (AL/MS) 1979 3 2,300,000,000

16 DENNIS (NW FL) 2005 3 2,230,000,000

17 AGNES (FL/NE U.S.) 1972 1 2,100,000,000

18 ALICIA (N TX) 1983 3 2,000,000,000

19 BOB (NC, NE U.S) 1991 2 1,500,000,000

The costliest mainland United States tropical cyclones, 1900-2006, (not adjusted for inflation).

TORNADOES

FORMATION

The Fujita Scale

F-Scale

Number

Intensity

Phrase

Wind

Speed Type of Damage Done

F0 Gale

tornado

40-72

mph

Some damage to chimneys; breaks

branches off trees; pushes over

shallow-rooted trees; damages sign

boards.

F1 Moderate

tornado

73-112

mph

The lower limit is the beginning of

hurricane wind speed; peels

surface off roofs; mobile homes

pushed off foundations or

overturned; moving autos pushed

off the roads; attached garages

may be destroyed.

F2 Significant

tornado

113-

157

mph

Considerable damage. Roofs torn

off frame houses; mobile homes

demolished; boxcars pushed over;

large trees snapped or uprooted;

light object missiles generated.

F3 Severe

tornado

158-

206

mph

Roof and some walls torn off well

constructed houses; trains

overturned; most trees in fores

uprooted

F4 Devastating

tornado

207-

260

mph

Well-constructed houses leveled;

structures with weak foundations blown

off some distance; cars thrown and large

missiles generated.

F5 Incredible

tornado

261-

318

mph

Strong frame houses lifted off

foundations and carried considerable

distances to disintegrate; automobile

sized missiles fly through the air in

excess of 100 meters; trees debarked;

steel re-inforced concrete structures

badly damaged.

F6 Inconceivable

tornado

319-

379

mph

These winds are very unlikely. The

small area of damage they might

produce would probably not be

recognizable along with the mess

produced by F4 and F5 wind that would

surround the F6 winds. Missiles, such as

cars and refrigerators would do serious

secondary damage that could not be

directly identified as F6 damage. If this

level is ever achieved, evidence for it

might only be found in some manner of

ground swirl pattern, for it may never

be identifiable through engineering

studies

A

FUJITA SCALE FOR TORNADO INTENSITY

• F0 Light Damage (40-72 mph)

• F1 Moderate Damage (73-112 mph)

• F2 Considerable Damage (113-157 mph)

• F3 Severe Damage (158-206 mph)

• F4 Devastating Damage (207-260 mph)

• F5 Incredible Damage (261-318 mph)

FUJITA-PEARSON SCALES

• Fujita Scale (Intensity)

• Pearson Path Length

• Pearson Path Width

•For example, a tornado rated 4, 3, 2, is

• - F4 (207-260 mph)

• - PL3 (10-31 mi. long)

• - PW 2 (56-175 yds. wide)

TORNADO DATA BASES

• Storm Prediction Center (SPC)

• University of Chicago

UNIVERSITY OF CHICAGO DATABASE

• Records from 1916 to 1989

• Path length per F-scale

SPC DATABASE • 1950 to present

• Date and time

• State

• Latitude and longitude of path

• Actual path length and width

• FPP

• Deaths and injuries

• Some path characteristics

REPORTED TORNADOES PER YEAR (SPC)

0

200

400

600

800

1000

1200

1400

1600

Tornadoes Per Year

TORNADO FREQUENCIES AND F-SCALE CLASSIFICATIONS

FOR 1950-1994 (NSSFC, 1995)

F-Scale (WindSpeed Range)

Number ofTornadoes Percentage

CumulativePercentage

F0 (40-72 mph) 11,046 31.3 31.3

F1 (73-112 mph) 12,947 36.7 68.0

F2 (113-157 mph) 7,717 21.9 89.9

F3 (158-206 mph) 2,523 7.2 97.1

F4 (207-260 mph) 898 2.6 99.7

F5 (261-318 mph) 121 0.3 100.0

TOTAL 35,252 100.0

TORNADO FREQUENCIES BY F-SCALE

TORNADO OCCURRENCES

Number of Tornadoes per 10,000 sq. mi per yr.

MAXIMUM TORNADO WIND SPEEDS

• Cannot be measured

• Must use indirect methods

• Once thought to be 400-500 mph

• Most intense tornadoes observed have wind speed in the 250-300 mph range

ATMOSPHERIC PRESSURE CHANGE

• Rotating winds create low pressure near center of storm

• Difficult to measure

• Theoretical value can be calculated

• Maximum APC is less than 3 psi in the most intense tornado

MISSILES AND DEBRIS

• Tornado-generated missiles

• Roof gravel, tree limbs, sheet metal

• Timber planks, plastic pipes

• Steel pipes and wide-flange sections

• Storage tanks, automobiles, railroad cars

TORNADO RECORDS • Incomplete records present difficulties in

tornado hazard assessment

• Occurrence distribution must be determined

• Damage path area must be estimated

• Size of region and time period must be properly defined

• Model should compensate for classification errors, unreported and unrated tornadoes

FOUR CONCEPTS

• Definition of tornado hazard probability

• Classification errors

• Variation of intensity within the damage path

• Allocation of confidence intervals

SOURCES OF CLASSIFICATION ERRORS

• Unintentional bias of persons making judgment

• Lack of indicators in damage path

• Unknown quality of construction in path

TORNADO DAMAGE PATH

ALLOCATION OF CONFIDENCE INTERVALS

• Confidence of tornado hazard probability is first selected

• Confidence intervals of each component in the model are adjusted to achieve target confidence interval for model

ANNUAL FREQUENCY OF TORNADOES

MULTIPROTECTON DESIGN

Disasters are great or sudden

misfortunes that result in

•economic disruption

•social chaos

•deaths and injuries

Natural disasters are rarely

predictable over long periods of

time, but are inevitable in some

locations.

MULTIPROTECTION DESIGN

Is the execution of a building

design in which every design

parameter is studied for its

effect on the mitigation of

defined disasters

MPD SHOULD BE A COMMUNITY-WIDE PROGRAM

Requires cooperation between

• Architect and Engineering Community

• Code/Standard enforcing agencies

• Insurance industry

• Research and development agencies

EFFECTIVE MPD WILL YIELD

• Increased life safety

• Reduction in property damage

• Added monetary value

DESIGN PARAMETER / HAZARD RELATIONSHIPS

Earthquake

Extreme

Wind

Building

Design

Parameters

Floods

Fire

MPD is an integrated design,

rather than an assembled

design with various hazards

treated separately

MPD pursues a variety of approaches, including:

• measures to strengthen the built environment and

• measures that pay attention to site development schemes

MPD CONSIDERATIONS

Avoid

– Extreme height/depth ratio

– Extreme length/depth ratio

– Variation of stiffness in perimeter walls

– False symmetry

– Re-entrant corners

– Mass eccentrics

MPD CONSIDERATIONS

Pay Attention to:

– Building separations

– Shear wall design

– Diaphragm design (shape, openings)

MPD CONSIDERATIONS

Potential Vertical Layout problems:

– Weak column/strong beam

– Discontinuous shear walls

– Variation of column stiffness

– Soft story frames

UNCERTAINTIES IN BUILDING PERFORMANCE

Building Shapes

– Complex Shapes

– Cruciform Shapes

– U-shape plan

– L-shape plan

– T-shape plan

– Set backs

– Split level

– Multiple towers

UNCERTAINTIES IN BUILDING PERFORMANCE

Abrupt Discontinuities

– Changes in mass/stiffness ratios

– Changes in structural members

– Changes in vertical resisting members

– Combination shear walls and moment resisting frames

UNCERTAINTIES IN BUILDING PERFORMANCE

Unusual Structural Features

– Buildings on hillsides

– Staggered Truss System

– Shell structures

– Cable supported structures

– Earth covered structures