Transcript Low-rise buildings - LSU Hurricane Engineering

Wind loading and structural response
Lecture 18 Dr. J.D. Holmes
Low-rise buildings
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
•
Wind loads on roofs are very important
•
Internal pressures are important - especially for dominant openings
•
Resonant effects are negligible
•
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
Low-rise buildings
•
Full-scale studies
Dimensions in mm :
1
10
slope
1600
3050
h/zo=170
1500
(Jensen Number )
• Small shed used by Jensen in Denmark in 1950’s
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)
Cp=1.0
h/zo=13
rougher terrain
h/zo=170
h/zo=4400
h/zo=
smoother terrain
need to match correct Jensen Number (h/zo) to get correct mean pressure coefficients
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 :
Separation
“bubble”
Stagnation
Point
Shear layer positions:
High turbulence
Low turbulence
Fluctuating reattachment
point
• 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)
Low-rise buildings
• General flow characteristics :
Cp (t)
C
Cˆ
p
p
C

C
p
p
Time
Four values of pressure coefficients :
Cp 
p  p0
1
ρ a U h2
2
Cp  σ Cp 
p
2
1
ρ a U h2
2
ˆ 
C
p
pˆ  p 0
1
ρ a U h2
2

Cp 

p  p0
1
ρ a U h2
2
Low-rise buildings
• Mean pressure coefficients on pitched roofs :
5o roof pitch :
h/d = 1.0
wind tunnel
h/d = 0.4
Cp = 1.0
5 roof pitch
No separation at ridge. Higher negative pressures for greater h/d.
Low-rise buildings
• Mean pressure coefficients on pitched roofs :
12o roof pitch :
h/d = 1.0
h/d = 0.4
wind tunnel
h/d = 0.2
Cp = 1.0
12
Second separation at ridge. Higher negative pressures for greater h/d.
Low-rise buildings
• Mean pressure coefficients on pitched roofs :
18o roof pitch :
h/d = 1.0
h/d = 0.4
wind tunnel
h/d = 0.2
Cp = 1.0
Pressure on windward face is less negative at lower h/d’s.
18
Low-rise buildings
• Mean pressure coefficients on pitched roofs :
30o roof pitch :
h/d = 1.0
h/d = 0.4
wind tunnel
h/d = 0.2
Cp = 1.0
Positive pressure on upwind face of roof for lower h/d’s. Uniform
negative pressure on downwind roof.
30
Low-rise buildings
• Mean pressure coefficients on pitched roofs :
45o roof pitch :
h/d = 1.0
h/d = 0.4
wind tunnel
h/d = 0.2
Cp = 1.0
45
High positive pressure on upwind face of roof at all h/d. Uniform
negative pressure on downwind roof.
Low-rise buildings
• Fluctuating and peak pressures at corners of roofs :
2
0
-2
Cp
-4
-6
-8
-10
0
3
6
9
12
15
Time (minutes)
High negative pressure peaks (‘spikes’) near corners - associated with
formation of conical vortices
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 :
-1
-2
-2
-3
-2
-4
-3
-4
-3
-3
-5
15O
10O
-2
-2
-4
-3
-5
-3
-2
-3
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 :
-2.5
-4 -3
-2.5
-1.5
-1.5
-3
-4
-2
-2
-5
-7
-5
-4
30o
20o
-2 -3
-5
-2
-2.5
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 :
Bending
moment
Maximum value
Time
Minimum value
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
Low-rise buildings
• Structural loads :
Effective static pressure distribution for knee bending moment :
Expected pressure
distribution for maximum
bending moment at B
Range of
pressure
fluctuations
-
-
B
+
+
Load distribution determined from correlations of pressures/ influence lines
(Chapter 5/ Lecture 13)
Must fall within envelope of maximum and minimum pressures
Low-rise buildings
• Shelter and interference :
building height / spacing - critical parameter
three flow regimes : skimming flow (close spacing)
wake-interference flow (medium spacing)
isolated roughness flow (far 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 :
largest negative pressures
+-
Cp=1
+
Low-rise buildings
• Long low-rise buildings :
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
3 .0
2 .0
1 .0
B
Cp eak
0 .0
-1 .0 0
15.95
31 .9
47.85
63 .8

o
35
6m
-2 .0
-3 .0
-4 .0
-5 .0
D istanc e alo ng frame , (m )
AR =2.4
AR =4
AR=6
• Increasing suction on leeward roof slope and wall as AR increases
End of Lecture 18
John Holmes
225-405-3789 [email protected]