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Low-rise buildings
Wind loading and structural response
Lecture 18 Dr. J.D. Holmes
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 1970’s, 1980’s and 1990’s - wind tunnel and full scale
• Wind loads on roofs are very important
• Internal pressures are important - especially for dominant openings
• Resonant effects are negligible
Low-rise buildings
• Full-scale studies
• Small shed used by Jensen in Denmark in 1950’s
110
slope
1600
15003050
Dimensions in mm :
h/zo=170
(Jensen Number )
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
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
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
Low-rise buildings
• General flow characteristics (0o to wall):
(movie by Shimizu Corporation, Tokyo, Japan)
Low-rise buildings
• General flow characteristics (45o to wall):
(movie by Shimizu Corporation, Tokyo, Japan)
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)
Separation “bubble”
Stagnation Point
Fluctuating re-attachment point
Shear layer positions:High turbulenceLow turbulence
Low-rise buildings
• General flow characteristics :
Four values of pressure coefficients :
2ha
0p
Uρ21
ppC
2ha
0p
Uρ21
ppC
2ha
0p
Uρ21
ppC
2ha
2
Cpp
Uρ21
pσC
Time
Cp (t)
Cpˆ
Cp
C p
Cp
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.
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
Low-rise buildings
• Mean pressure coefficients on pitched roofs :
18o roof pitch :
Pressure on windward face is less negative at lower h/d’s.
wind tunnel
Cp = 1.0
h/d = 0.2
h/d = 0.4
h/d = 1.0
18
Low-rise buildings
• Mean pressure coefficients on pitched roofs :
30o roof pitch :
Positive pressure on upwind face of roof for lower h/d’s. Uniform negative pressure on downwind roof.
wind tunnel
Cp = 1.0
h/d = 0.2
h/d = 0.4
h/d = 1.0
30
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
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
Low-rise buildings
• Fluctuating and peak pressures at corners of roofs :
Formation of conical vortices
30-60o
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)
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
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
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
Be
nd
ing
m
om
en
t
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 of pressure
fluctuations
+ +- -
- -
Expected pressure distribution for maximum bending moment at B
B
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)
Low-rise buildings
• Multi-span buildings :
pitches less than 10 degrees are ‘aerodynamically flat’ :
+
-
+ +++
+-
Low-rise buildings
• Multi-span buildings :
Saw-tooth roofs - magnitude of negative pressures reduces downwind :
Cp=1
-
-+
+
largest negative pressures
Bulk Sugar Storage Shed :
Span (d) = 46m, Length (b) = 303m, = 35o
Low-rise buildings• Long low-rise buildings :
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 along frame, (m )
Cp
ea
k
AR=2.4 AR =4 AR=6
Low-rise buildings• Long low-rise buildings :
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