Wind load on canvas covered frame 1

[jeg]

I have a rectangular frame 2500mm x 1000mm made of circular hollow section. The frame is infilled with a strong canvas, blanket-stitched to the frame. Wind applies a load of 1 kN/m2 to the canvas. What is the loading on each frame member?

 

[wildehond]

Surely the loading is dependant on how much the canvas "blows out". This will cause bi-axial bending for the frame. Unless you're determined to get the exact answer, why not just take a "stab" at the deflected angle of the canvas, resolve the triangle of forces and then design your rectangular section for these loads.

 

[jeg]

Yes, that was my first reflex but then I wondered if I might be missing something - it seemed to be too easy!
Thank you Wildehond for confirmation.
Does anyone think we are barking up the wrong tree?

 

[WindX]

A canvas roof membrane(and probably with very light framing like a tent framework) would fit into what ASCE 7-02 refers to as "buildings and other structures, flexible " in Section 6.2; that means the gust factor,Gf, will be different from the normal defaut value of 0.85 given in Section 6.5.8.1.

There is a complicated formula given in Section 6.5.12.2.3 to calcuate Gf. So, first please check your assumption of 1 kN/m2 pressure. Or rather multiple the values you get by using 1 kN/m2 by the true pressure that takes into account a different gust factor.

As for the distribution, what you are describing sounds like a "sail" without further description, as one of the others above stated.

I would tend to treat the frame as a "sign" or "parapet" in order to get the pressures distribution (See Sections 6.5.13 and 6.5.11.5) and Commentary Section C6.5.8 on page 283 and Figure C6-9 on page 305).

The frames will probably take the distribution indicated but it is a complex problem and the structure is probably subject to a unbalanced loading which could shift loads to one side of the frame than the other. So I would apply unbalanced loading on it.

 

[jeg]

Yes, WindX, I am using BS6399 part 2, but came to similar conclusion. I followed several different wind loading routes including the parapet and sign scenarios and used the most onerous loadings. Interesting remark about the unbalanced loading - I did not apply unbalanced wind loads but do have large load factors on the wind loads and some of teh other load cases are unbalanced and applied separate from the wind loads. I believe all circumstances are adequetely covered.