monoslope free roof Cp values AS1170.2 1

[chenzo]

I am working on the structural design of a PV racking system which, for the purposes of determining wind loads, can be modelled as a mono slope free roof (empty under).
Unfortunately in the Australian standards wind load information for such structures is limited and after spending many hours of research I have not been able to find much additional information on the topic. Australian Standards provide information on mono slope free roofs inclined at up to 30 degrees. At 30 degrees a Cp value of 2.2 is tabulated. This is a very high figure and as I am also interested in roof slopes of greater than 30 degrees I need to find additional information. If anyone has comments on the Cp value of 2.2 for 30 degree mono slope free roofs or knows of any reference material for roof slopes greater than 30 degrees I would be most appreciative.

 

[dbuzz]

chenzo

What's a 'PV racking system'? What sort of roof slopes are you considering?

This explanatory note in the commentary to AS 1170.2 provides some background to the tables:

CD3.1 Aerodynamic shape factor for local net pressure on free roofs
The net pressure coefficients for monoslope, pitched or troughed free roofs are based mainly on wind tunnel tests described by Gumley (Refs. 4 and 5) and by Letchford and Ginger (Refs. 6 and 7). The roof pitches specified are those for which the tests were carried out. Some adjustment to Table D5 has been made based on the full-scale measurements by Robertson, Hoxey and Moran (Ref. 8).

REFERENCES
4 GUMLEY, S.J., ‘Panel Loading Mean Pressure Study for Canopy Roofs’, University
of Oxford, Department of Engineering Science, OUEL Report 1380/81, 1981.
5 GUMLEY, S.J., ‘A Parametric Study of Extreme Pressures for the Static Design of
Canopy Structures’, Journal of Wind Engineering and Industrial Aerodynamics,
Vol. 16, 1984, pp. 43-56.
6 LETCHFORD, C.W., and GINGER, J.D. Wind loads on planar canopy roofs —
Part 1: Mean pressure distributions, J.W.E. and Ind. Aero., Vol. 45, pp. 25 to 45,
1992.
7 LETCHFORD, C.W., and GINGER, J.D., Wind loads on planar canopy roofs—Part 2:
Fluctuating pressure distributions and corrections, J.W.E. and Ind. Aero., Vol. 51,
pp. 353 to 370, 1994.
8 ROBERTSON, A.P., HOXEY, R.P., and MORAN, P., ‘A full scale study of wind
loads on agricultural canopy structures and proposal for design’, Journal of Wind
Engineering and Industrial Aerodynamics, Vol. 21, 1985, pp. 167-205.

 

[chenzo]

A PV racking system is just a supporting structure for photovoltaic solar panels. The solar panels form a roof and the supporting structure is open on all sides. The tilt of the solar panels that I am having trouble finding pressure coefficients for is from 30 degrees up to 45 degrees.
Thanks for the references, I am trying to get a copy of these, however I think this reference material is only relevant to slopes up to 30 degrees. The commentary states: The roof pitches specified are those for which the tests were carried out. I am hoping however that there might be some additional information for slopes greater than 30 degrees.
I know that at 90 degrees I can model the structure as a hoarding/free standing wall. (Cp =1.39)
For one load case, (among others) I have the following information.

slope =0 Cp= -0.3
slope =15 Cp= -1
slope =30 Cp= -2.2
slope =90 Cp= -1.39

Plotting a curve through these shows that additional information above 30 degrees would let me know if the Cp starts to fall at 30 degrees (from 2.2 down to 1.39) or if it continues to increase. As 2.2 is quite a high pressure co-efficient I dont think it would continue to rise but some reference to that effect would be good.

 

[Tomfh]

What are your b/c/d/h values?

 

[chenzo]

for free roof
h/d varies as the shape of structure changes for tilt angles.
at tilt = 0 h/d=0.17
at tilt = 15 h/d=0.38
at tilt = 30 h/d=0.7
at tilt = 35 h/d=0.84
at tilt = 45 h/d=1.2

at tilt = 90
for hoarding
b/c=0.48
c/h=0.85
so cpn for hoarding =1.3,
not 1.39 as mentioned earlier

 

[dbuzz]

Suggest that you get in touch with John Holmes at JDH Consulting (

http://users.bigpond.net.au/jdholmes/drjdholmes.html).

If there's testwork or references available regarding pressure coefficients for steep monoslope free roofs, he's likely to know about it.