AS1170.2 2011 Local Pressure Factor Area

[Drapes]

There is a note on page 39 of AS1170.2 2011 pertaining to local pressure factors that says "The side ratio of any local pressure factor area on the roof shall not exceed 4". This was not included in the earlier edition of the code. Previously, as far as I know, you would apply small localised high pressure areas, either (a x a) or (a/2 x a/2), along the length of the purlin, and it would be these areas that we would apply the local pressure factor k[l].

What are the implications of the above note in the more recently published code? Do we now also consider an area of (a x a/4) when designing purlins?

 

[rowingengineer]

Drapes,
Recommend if your designing in Australia that you get a book Design of Portal Frame Buildings, 4th Edition, S. T. Woolcock et al.

Commonly what you are referring to is called patch loading. basically the patch in the older codes was undefined in dimensions so you could design with a long slender patch if this gave you the worst loading position. This new requirement limits the ration of the side dimensions to 1 : 4. so if you patch ahd a side legnth of 1m the other side length would be limited to 4m max in length.

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."

 

[Drapes]

Thanks rowingengineer. I just ordered the book, was exactly what I was looking for.

 

[yippyahh]

Hi fellas, what is your take on the kl=3.0 local pressure factor for roofs with pitch less than 10 degrees with longitudinal wind? Expanding on this, do you apply kl=3.0 for ALL LONGITUDINAL wind cases because roof pitch is always taken as zero (as per Cpe section of the code)? I know the code makes that clear for Cpe values, but it doesnt actually state that in the local pressure section. I would love for someone to say that kl=3.0 for gable roofs (eg portal frames) with roof pitches equal to or greater than 10degress does not apply for long wind. But I dont think I will be that lucky. It just makes purlin/top hat designs spacings/fixings for single span sheds, for example, INSANE. Your thoughts??

 

[rowingengineer]

Kl=3 is required by the codes, so no-one should just ignore this value. I dont find it makes my purlin spacing insane, i do however make sure i use the right methods of analysis for both my sheeting and purlins. Since it is wind at the 45 on flat roofs that cause the double turbulence streams then i think that kl=3 isnt applicable to roof pitches greater than 10 degrees.

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."

 

[yippyahh]

thanks rowing. wind at 45 that cause the double turbulence streams? excuse the ignorance. Where do I read up on this? Cannot say I have analysed wind on the 45.

 

[yippyahh]

so for a portal frame shed with a roof pitch of 15degrees...am i applying kl=3.0 for longitudinal wind???

 

[rowingengineer]

The code doesn't do a 45 degree analysis case, maybe it should. There are a few references that discuss this effect of double turbulence streams, I would start with John Holmes book, wind loading of structures.

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."

 

[flyingdebris]

yippyahh, i use kl=3.0 for long wind cases, as the roof pitch is less than 10degrees, as defined by 1170. Well thats how I read it, and I would rather be over than under I guess. Would be happy to hear others opinions on the matter. I do agree with you that it does make a difference to project costs, especially the humble old backyard shed.

 

[rowingengineer]

I think the standard is fairly clear:
extract from clause 5.4.4 "The RC1 case only applies to flat or near-flat roofs (slope less than 10°)".

I don't believe that the standard define two different roof pitch's a roof. clause 5.4.1 diagram top right the roof pitch is unchanged. The standard clearly defines gable roofs not a change in roof pitch.

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."

 

[flyingdebris]

roweng, reading the clause, I would say you have a point. ty