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Low-rise buildings

Wind loading and structural response

Lecture 18 Dr. J.D. Holmes

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Low-rise buildings

Low-rise buildings : enclosed structures less than 50 feet (15 metres) in height

Immersed within aerodynamic roughness - high turbulence, shelter

effects are important

Sustain most damage in severe wind storms

Extensive research on wind loads in 1970s, 1980s and 1990s - wind

tunnel and full scale

Wind loads on roofs are very important

Internal pressures are important - especially for dominant openings

Resonant effects are negligible

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Low-rise buildings

Full-scale studies

Small shed used by Jensen in Denmark in 1950s

110

slope

1600

15003050

Dimensions in mm :

h/zo=170

(Jensen Number )

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Low-rise buildings

Full scale studies

Aylesbury Experimental Building, U.K. 1970-5

Variable pitch roof (adjustable between 5 and 45 degrees)

Use for an international comparative wind tunnel experiment

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Low-rise buildings

Full scale studies

Texas Tech Field Experiment , U.S. 1987- now

Flat roof. Can be rotated on turntable.

High quality data on fluctuating local and area-averaged pressures

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Low-rise buildings

Wind-tunnel studies

Comparison of mean pressures on centerline by Jensen (1958)

h/zo=170 h/zo=4400h/zo=13 h/zo=

rougher terrain smoother terrain

need to match correct Jensen Number (h/zo) to get correct mean pressure coefficients

Cp=1.0

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Low-rise buildings

General flow characteristics (0o to wall):

(movie by Shimizu Corporation, Tokyo, Japan)

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Low-rise buildings

General flow characteristics (45o to wall):

(movie by Shimizu Corporation, Tokyo, Japan)

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Low-rise buildings

General flow characteristics :

Flow separates at leading edge of roof and at ridge for roof pitches greater than

about 10o

Distance to reattachment depends on turbulence (Jensen Number)

Separationbubble

StagnationPoint

Fluctuating re-attachment

point

Shear layer positions:High turbulenceLow turbulence

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Low-rise buildings

General flow characteristics :

Four values of pressure coefficients :

2

ha

0p

U2

1

ppC

2

ha

0p

U2

1

ppC

2

ha

0p

U2

1

ppC

2

ha

2

Cpp

U2

1

pC

Time

Cp (t)

Cp

Cp

C p

Cp

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Low-rise buildings

Mean pressure coefficients on pitched roofs :

5o roof pitch :

5 roof pitch

wind tunnel

Cp = 1.0

h/d = 0.4

h/d = 1.0

No separation at ridge. Higher negative pressures for greater h/d.

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Low-rise buildings

Mean pressure coefficients on pitched roofs :

12o roof pitch :

Second separation at ridge. Higher negative pressures for greater h/d.

wind tunnel

Cp = 1.0

h/d = 0.2

12

h/d = 0.4

h/d = 1.0

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Low-rise buildings

Mean pressure coefficients on pitched roofs :

18o roof pitch :

Pressure on windward face is less negative at lower h/ds.

wind tunnel

Cp = 1.0

h/d = 0.2

h/d = 0.4

h/d = 1.0

18

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Low-rise buildings

Mean pressure coefficients on pitched roofs :

30o roof pitch :

Positive pressure on upwind face of roof for lower h/ds. Uniform

negative pressure on downwind roof.

wind tunnel

Cp = 1.0

h/d = 0.2

h/d = 0.4

h/d = 1.0

30

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Low-rise buildings

Mean pressure coefficients on pitched roofs :

45o roof pitch :

High positive pressure on upwind face of roof at all h/d. Uniform

negative pressure on downwind roof.

wind tunnel

Cp = 1.0

h/d = 0.2

h/d = 0.4

h/d = 1.0

45

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Low-rise buildings

Fluctuating and peak pressures at corners of roofs :

High negative pressure peaks (spikes) near corners - associated with

formation of conical vortices

0 3 6 9 12 15

Time (minutes)

Cp

2

0

-2

-4

-6

-8

-10

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Low-rise buildings

Fluctuating and peak pressures at corners of roofs :

Formation of conical vortices

30-60o

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Low-rise buildings

Cladding loads on pitched roofs :

Largest minimum pressure coefficients for any wind direction :

10O

-2-3

-2

-3

-3-4

-5

-4

-1

-2

-3

-2

-3

-4

-5

-2

-3

15O

-3-2

Contours converge towards corner of roof (effect of conical vortices)

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Low-rise buildings

Cladding loads on pitched roofs :

Largest minimum pressure coefficients for any wind direction :

-4-3

-2.5

-4

-2.5

-5

-1.5

-2

-4 -3

-2.5

-1.5

-2

-5-5

-7

-2 -3

20o

-2

30o

Gable end has highest minimum pressure coefficients

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Low-rise buildings

Structural loads :

Calculate peak structural loads and effective static load distributions :

Instantaneous load around frame will vary in magnitude and distribution

Codes and standards give simplified uniform distributions on surfaces

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Low-rise buildings

Structural loads :

Load effect e.g knee bending moment will experience maximum and

minimum values during a storm :

Either or both values may be critical - depending on b.m. due to dead load

Each peak value has an expected pressure distribution associated with it

Maximum value

Minimum value

Time

Bendingmoment

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Low-rise buildings

Structural loads :

Effective static pressure distribution for knee bending moment :

Load distribution determined from correlations of pressures/ influence lines(Chapter 5/ Lecture 13)

Must fall within envelope of maximum and minimum pressures

Range ofpressure

fluctuations

+ +

- -

--

Expected pressuredistribution for maximum

bending moment at B

B

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Low-rise buildings

Shelter and interference :

building height / spacing - critical parameter

wake-interference flow (medium spacing)

isolated roughness flow (far spacing)

three flow regimes : skimming flow (close spacing)

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Low-rise buildings

Multi-span buildings :

pitches less than 10 degrees are aerodynamically flat :

+-

+ +++

+-

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Multi-span buildings :

Saw-tooth roofs - magnitude of negative pressures reduces downwind :

Cp=1

-

-+

+

largest negative pressures

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Bulk Sugar Storage Shed :

Span (d) = 46m, Length (b) = 303m, = 35o

Low-rise buildings

Long low-rise buildings :

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Peak Cps on = 35o Building, Frame B, = 45o

Increasing suction on leeward roof slope and wall as AR increases

B

6m

35o

-5.0

-4.0

-3.0

-2.0

-1.0

0 .0

1 .0

2 .0

3 .0

0 15.95 31.9 47.85 63 .8

D istance a long frame, (m)

C

peak

AR =2.4 AR =4 AR=6

Low-rise buildings

Long low-rise buildings :

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End of Lecture 18

John Holmes225-405-3789 JHolmes@lsu.edu