AS1170.2 Local Pressure Factors
AS1170.2 Local Pressure Factors
(OP)
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!
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!





RE: AS1170.2 Local Pressure Factors
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."
RE: AS1170.2 Local Pressure Factors
RE: AS1170.2 Local Pressure Factors
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."
RE: AS1170.2 Local Pressure Factors
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...
RE: AS1170.2 Local Pressure Factors
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."
RE: AS1170.2 Local Pressure Factors
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!
RE: AS1170.2 Local Pressure Factors
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."
RE: AS1170.2 Local Pressure Factors
RE: AS1170.2 Local Pressure Factors
RE: AS1170.2 Local Pressure Factors
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.
RE: AS1170.2 Local Pressure Factors
"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...