Wind Loading: Shielding on multiple parallel frames 1

[RPLJohnson]

I have a situation where multiple bays of racking will be subject to wind loads.

Looking at the shielding factor (Ksh) for multiple parallel frames given in AS1170.2 Appendix E, it appears that Ksh does not reduce for multiple parallel frames. For equally spaced, identical frames according to Table E2, lambda and the solidity ratio stay the same and so the shielding factor for every leeward frame is the same.

It seems counterintuitive that the 20th bay of racking (counting from the windward frame) receives the same load as the 2nd.

I have very many bays of racking and the overall load is many times higher than an equivalently sized clad building.

Am I calculating lambda correctly (spacing between adjacent frames/smallest overall dimension)?

Or should the "frame spacing" for each frame be its distance from the windward frame?

Thanks
Richard

 

[Lomarandil]

I'm not familiar with that code, but the possibility that you have a counter-intuitive result due to application of wind shielding effects doesn't surprise me.

I have a little cheat sheet on my office wall to show which codes allow partial shielding, no shielding, full shielding. None of the codes I work with in the US are consistent about it.

Hopefully one of our southern hemisphere contributors will be able to more specifically address your question.

 

[Agent666]

Sounds like you are interpreting lambda correctly. If you used the spacing from the first frame, lambda is larger and hence Ksh is larger, which would mean more load on frames further from the first for a given solidity ratio. (this would definitely be counterintuitive!)

If you think about first frame being in undisturbed air, second and subsequent frames are in disturbed air and still subject to the same wind speeds/pressures it all seems to make sense to me. A further reduction for each subsequent would mean lower wind speeds/pressures as you get further from the first frame, this isn't the case.

 

[SAIL3]

ASCE has an excellent publication on the subject..."Wind Loads and Anchor Bolt Design for Petrochemical Facilities"...it has alot of good practical wind design methods in it....probably one of the most useful books that I have besides books by Blodget....