AS1170.2 Local Pressure Factors

[Trenno]

Hi all,

What is everyone's opinion on the value "a" according to Clause 5.4.4? In particular when you have the situation where you have one large building 'footprint' with smaller roofs at different heights.

Example

Assuming that the governing "a" is due to the overall building width impacts the purlin design greatly.

However you can get savings if you take the width and depth for each roof individually and such that these values reflect "what the roof sees from a certain wind direction."

In the case above "a" is around 8m and the local factor of 3.0 can occur 8m (each direction) in from the windward corner edge!

 

[rowingengineer]

I believe some engineering judgement is required, to get a good insight into how the wind loading would be working I would try to draw some stream lines across the building. I would also keep in mind that the Kl=3 case is for a 30-60deg wind direction, hence some judgement is required here in the application with 0 and 90 wind coeff.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

Thanks RE - where did you find the 30-60 deg information? I can't remember it being in the commentary?

 

[rowingengineer]

I thought it was in the new commentary: WIND LOADING HANDBOOK FOR AUSTRALIA & NEW ZEALAND – Background to AS/NZS 1170.2 Wind Actions

However I know it is in the wind loading handbook by Holmes.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

Arrr... I just did a quick search on the SAI Database.

I'm going to assume that book isn't publicly available. Would be good if the code gave a bit more insight into these things...

 

[rowingengineer]

yes it is, the AWES did the HB this time as the SAI process is fairly stuffed a the moment.

http://www.awes.org/products/

I checked my copy just now and it reference 'Wind Loading of Structures' by John D. Holmes, I have attached the relevant page.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

Thanks RE - might invest in these texts.

Just FYI - I just made a spreadsheet that adopts the "Design of Portal Frame Buildings" (Woolcock et al) approach for the design of purlins. Patch loadings etc etc.

Not sure if most designers know how great it can impact the purlin design. Not just the basic pressure x purlin spacing!

 

[rowingengineer]

i would hope most engineers doing steel design would know. weuse the moment method as we find this is better for us.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

Does anyone else have any thoughts on the questions I had above??

 

[asixth]

The "a" should be based on the whole building. When I do roofs I like to color highlight the different local pressure zones, corners, edges and internal. Just helps to understand the design pressure variations for c&c loadings

 

[asixth]

But it should also be from windward edges. So for all those steps the local pressures should be applied in the vicinity of the steps.

The actual purlin design is no different to the design of any other steel member. If you know the section properties than you can calculate it's capacity. Lysaght and Strammit give max span lengths for given idealized loadings and span configurations, I find the capacity's are generally driven by moment capacity and deflection criteria. I deal mostly in wind region B and generally have the purlin depth sized to about span/40 with a bridging member every 10-15 times the purlin depth.

 

[Trenno]

Thanks for replying asixth.

"The "a" should be based on the whole building."

I still find this hard to grasp... If I'm wind hitting a corner eave on say a smaller, isolated portion of roof, I don't care how big the roofs are on different levels ??

Also Appendix D, Paragraph D1.3 - states that the "a" value is now 20% of the smallest plan dimension of the free roof/canopy, thus again hinting towards the "a" value should be based on a roof by roof basis?

Or maybe I shouldn't be mixing the local factor from Appendix D with the one in Section 5...