AS1170.2 Local pressure questions

[Tomfh]

A couple of questions about Kl local pressure factors:

1. Do you use them for serviceability calculations?

2. Where there are two contributing external surfaces (e.g. top and bottom surfaces at eaves), do you apply Kl simultaneously to both surfaces?

 

[Agent666]

1. Yes most definitely. The higher pressures are a function/characteristic of the airflow around the building, and for the most part are independant of the wind speed whether at ULS or SLS.

2. I think this aspect is less clear, but taking a page out of the part in code related to canopies it's applied once to the total net pressure calculated. Which I guess is the same as stating its applied to both surfaces.

 

[Tomfh]

1. What purlin deflection limits do you recommend for localised serviceability pressures?

2. Yes I multiply the whole thing, as it’s simpler and conservative. I was wondering though if there was a clause somewhere which says you needn’t bother, since it doesn’t really make sense to be adding up local pressures across multiple surfaces.

 

[Agent666]

Same deflection limits as normal areas. Why should there be any relaxation if that's what you are angling for. It's a deflection under a design load, the limit should not be dependant on the level of that load if you like. They are independant things.

 

[Tomfh]

I meant what is a normal cladding/purlin deflection limit for serviceability wind speeds (inclusive of local pressure factors). L/150* seems to be a very common number. It's in all the purlin tables.


*Obviously you can tighten this up if you're supporting something brittle, or hanging a ceiling, etc.

 

[Settingsun]

Is it wrong to have a local pressure increase on two surfaces? My one book on wind loading didn't give me an answer. As1170.2 clause 5.4.3 does however rule out the combination factor (multi-surface reduction) for cladding and purlins.

 

[Tomfh]

Steveh49, That Kc limitation was in AS1170-2002 but I can't see it in the current version? Which version are you looking at?

 

[Agent666]

Span/150 seems to be used around here unless you have a ceiling then span/300 - span/360 seems to be the norm.

Keep in mind the local pressure can only have a 1:4 ratio for applying it on a span, so for longer spans it's only applied over part of the span assuming purlin spaced at 0.9-1.2m which seems to be the norm if there is a ceiling.

I'd say in the current version Kc would be 0.9 for internal and external coefficients for purlin design if there are two effective surfaces. To qualify it implies both internal and external pressures acting in the same direction, otherwise its 1.0. Refer to cases E & F in table 5.5

 

[Settingsun]

It was the 2002 version + A1 because I could download it from Parks Vic while using phone. Thought it was still current but maybe it's 1170.0 that's still from 2002 plus amendments.

 

[Tomfh]

The As1170.2-2011 Kc clause is different to 2002. It’s more rational in 2011 version. Kc is presented more purely in terms of the number of effective external surfaces that contribute to an action, as opposed to specific prescribed combinations of external surfaces.

Under 2011 I would read it that you can apply Kc=0.9 to the top and bottom surfaces of eaves purlin.

 

[Settingsun]

I agree with that but still curious whether the local increase is transient and possibly non-additive or a 'permanent' function of the geometry.

 

[Tomfh]

My opinion is that it doesn’t make sense to add local pressure factors across multiple surfaces, given that the whole point of local factors is they are transient isolated loads akin to point loads, but that the way the code is written and interpreted you probably should add them anyway.

 

[Agent666]

One way you could potentially argue less would be to apply the windward wall K_l of 1.25 (or more if near the corners) to the underside of the windward overhang as this is where the pressure load under is coming from, and a K_l of whatever would normally be on the roof on top. Kind of makes sense to me but then you have to argue a possible deviation from the literal interpretation of the code.

 

[Tomfh]

One way you could potentially argue less would be to apply the windward wall K_l of 1.25 (or more if near the corners) to the underside of the windward overhang as this is where the pressure load under is coming from, and a K_l of whatever would normally be on the roof on top. Kind of makes sense to me but then you have to argue a possible deviation from the literal interpretation of the code.

I would have thought that would be a literal interpretation, given that the code says to apply the wall pressures to the underside of eaves?

ps: windward wall local pressures are now 1.5, not 1.25

 

[Agent666]

Yeah agree totally with what you say.

Somehow when writing it I had canopies on the brain and the fact they calculate out to a single pressure coeff inclusive of equivalent internal/external pressures to which you then apply a single K_l factor presumably based on the roof (not part wall part roof). So was arguing you might be able to reduce it based on the underside pressure coming from the wall like you note.