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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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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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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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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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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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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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Fluctuating and peak pressures at corners of roofs :
Formation of conical vortices
30-60o
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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 [email protected]
