1. Wind loading and structural response Lecture 18 Dr. J.D. Holmes
Low-rise buildings

2. 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

3. Low-rise buildings Full-scale studies (Jensen Number )1
10 slope
1600
1500
3050
Dimensions in mm :
h/zo=170
(Jensen Number )
Small shed used by Jensen in Denmark in 1950’s

4. Low-rise buildings Full scale studies
Aylesbury Experimental Building, U.K
Variable pitch roof (adjustable between 5 and 45 degrees)
Use for an international comparative wind tunnel experiment

5. Low-rise buildings Full scale studies
Texas Tech Field Experiment , U.S now
Flat roof. Can be rotated on turntable.
High quality data on fluctuating local and area-averaged pressures

6. Low-rise buildings Wind-tunnel studies
Comparison of mean pressures on centerline by Jensen (1958)
Cp=1.0
h/zo=13
h/zo=170
h/zo=4400
h/zo=
need to match correct Jensen Number (h/zo) to get correct mean pressure coefficients
rougher terrain smoother terrain

7. Low-rise buildings General flow characteristics (0o to wall):
(movie by Shimizu Corporation, Tokyo, Japan)

8. Low-rise buildings General flow characteristics (45o to wall):
(movie by Shimizu Corporation, Tokyo, Japan)

9. Low-rise buildings General flow characteristics :
Separation “bubble”
Stagnation Point
Fluctuating re-attachment point
Shear layer positions:
High turbulence
Low turbulence
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)

10. Low-rise buildings General flow characteristics :
Time
Cp (t)
Four values of pressure coefficients :

11. 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.

12. Low-rise buildings Mean pressure coefficients on pitched roofs :
12o roof pitch :
wind tunnel
Cp = 1.0
h/d = 0.2
12
h/d = 0.4
h/d = 1.0
Second separation at ridge. Higher negative pressures for greater h/d.

13. Low-rise buildings Mean pressure coefficients on pitched roofs :
18o roof pitch :
wind tunnel
Cp = 1.0
h/d = 0.2
h/d = 0.4
h/d = 1.0
18
Pressure on windward face is less negative at lower h/d’s.

14. Low-rise buildings Mean pressure coefficients on pitched roofs :
30o roof pitch :
wind tunnel
Cp = 1.0
h/d = 0.2
h/d = 0.4
h/d = 1.0
30
Positive pressure on upwind face of roof for lower h/d’s. Uniform negative pressure on downwind roof.

15. Low-rise buildings Mean pressure coefficients on pitched roofs :
45o roof pitch :
wind tunnel
Cp = 1.0
h/d = 0.2
h/d = 0.4
h/d = 1.0
45
High positive pressure on upwind face of roof at all h/d. Uniform negative pressure on downwind roof.

16. Low-rise buildings
Fluctuating and peak pressures at corners of roofs :
Time (minutes) Cp 2 -2 -4 -6 -8
-10
High negative pressure peaks (‘spikes’) near corners - associated with formation of conical vortices

17. Low-rise buildings
Fluctuating and peak pressures at corners of roofs :
Formation of conical vortices
30-60o

18. Low-rise buildings Cladding loads on pitched roofs :
Largest minimum pressure coefficients for any wind direction :
10O -2 -3 -4 -5 -1
15O
Contours converge towards corner of roof (effect of conical vortices)

19. Low-rise buildings Cladding loads on pitched roofs :
Largest minimum pressure coefficients for any wind direction : -4 -3
-2.5 -5
-1.5 -2 -7
20o
30o
Gable end has highest minimum pressure coefficients

20. 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

21. Low-rise buildings Structural loads :
Load effect e.g knee bending moment will experience maximum and minimum values during a storm :
Maximum value
Minimum value
Time
Bending moment
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

22. Low-rise buildings Structural loads :
Effective static pressure distribution for knee bending moment :
Range of pressure fluctuations + -
Expected pressure distribution for maximum bending moment at B B
Load distribution determined from correlations of pressures/ influence lines (Chapter 5/ Lecture 13)
Must fall within envelope of maximum and minimum pressures

23. 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)

24. Low-rise buildings Multi-span buildings :
pitches less than 10 degrees are ‘aerodynamically flat’ : + -

25. Low-rise buildings Multi-span buildings :
Saw-tooth roofs - magnitude of negative pressures reduces downwind :
largest negative pressures
Cp=1 - +

26. Bulk Sugar Storage Shed :
Low-rise buildings
Long low-rise buildings :
Bulk Sugar Storage Shed :
Span (d) = 46m, Length (b) = 303m,  = 35o

27. Peak Cps on  = 35o Building, Frame B,  = 45o
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

28. End of Lecture 18 John Holmes 225-405-3789 JHolmes@lsu.edu