AS1170.2: 2011- Wind Actions Combination factors kc,e and kc,i 1

[Rajshrn06]

Are we permitted to use the action combination factors kc,e and kc,i from Table 5.5 of AS1170.2: 2011, when designing the Roof cladding and Purlins for internal pressure?

This is particularly for buildings designed in region C and D, which can have very large internal pressures.

Previously, in 2002 version of the code, combination factors were only valid for major structural elements. The 2011 code is silent on this. I feel that we cannot use kc,e and kc,i for Purlins and Claddings as these members may not get adequate time to develop combined actions as the gust winds would be instantaneous. Similar to the fact that we have to use directional multiplier Md as 1.0 for cladding and immediate members.

 

[Agent666]

It's about effective surfaces, which are defined as side wall, roof, leeward or windward wall etc acting together to produce some effect in combination on structural elements (subtle wording change from 2002 version).

I'd say case (f) is the same as your single surface like purlins/cladding with two effective surfaces isn't it? Having said that I barely every seem to recall ever taking account of the Kc factor for this, maybe out of habit and having grown up under the 2002 version of the code. Usually you are chasing some lower lateral drift or deflection limit of the main structure and end up factoring it in there.

By it's very definition it is inferring that these effective surfaces load the surfaces resisting the lateral and/or vertical loads. Your cladding and purlins are part of this load path. For external only pressures, or internal only you cannot use Kc=0.9 because this is effective case (e), but for both acting simultaneously I think the intent is that you can adopt the 0.9 reduction.

That limit on Md being 1.0 only applies for the B, C and D regions. Otherwise you can use the direction winds.

I think they have made this much clearer in the draft for the 2020 version of AS/NZS1170.2:-

 

[blihpandgeorge]

i would be interested to hear what others do. As for me, rightly or wrongly, I do include Kc when designing purlins as i am not aware of anything that precludes it.

At Agent666 - i hadnt realise AS1170.2 was under revision, i will now track down the draft to see what is coming. Interesting that is introducing a limit on internal effective surfaces of 0.4. Any ideas when AS1170.2 may be published?

 

[rapt]

blihpandgeorge

The code itself would appear to say it is not included in purlin design. How do 2 surfaces acting together come into calculating the moments for purlin design? It is purely the loading on that surface that effects the moments for purlins.

 

[Agent666]

Rapt, it's two effective surfaces, wind pressure above and below (external and internal) acting on the cladding are counted as two effective surfaces. Least that's the way its stated to me, and how it's interpreted generally.

 

[Agent666]

Any ideas when AS1170.2 may be published?

Unsure, draft came out in March 2020, so likely to be a while.

 

[rapt]

Agent666,

Sorry, you are correct, there is a diagram showing that, but not if |Cpi| < .2.

 

[Rajshrn06]

Thanks for your reply guys. We design for Cpi = 0.65 for majority structures in cyclone regions, which results in high wind forces. Often, for tall structures, the local pressure zones become highly loaded and start requiring concrete roofs.

This is exactly the difficulty which I am facing now where I have used kc for purlins and cladding to get the roof cladding to work, however the peer reviewer is adamant to use kc only for major structural elements possibly due to his experience with the 2002 code terminology. I also ignore the kc factors for immediate members but for this one instance I decided to use it based on the info provided in 2011 code alone.

He has further challenged me that the schematic diagrams provided in the tables only show major structural elements and the wording is verified by 2002 code. To top that, he has argued that if the structure was designed pre-2011, you would need a concrete roof to cater for kl=3.0 loading.

Is there any other code which uses combination factors and clearly states using it for purlins and claddings?

 

[Settingsun]

Perhaps point to the exract from the draft code provided in this discussion. It specifically refers to cladding.

Falling that, request that the reviewer identify which provision of the current code is violated by your design.

 

[Agent666]

Yeah it's pretty clear that the intent in the draft is a move towards (or has always been) that the purlins and cladding can account for the Kc factors effect.

The fact that they removed in the 2011 version that 'major structural elements' definition is a move towards this. Unfortunately removing something like that only implies the intent that it applies to secondary elements rather than coming out and saying it directly that you can use it for this and that. 2020 draft supports a much clearer definition of the intent, if your reviewer cannot make that leap of understanding what is the world coming to.

Failing that you can always get in contact with the standards committee and get a confirmation that it is intended to apply to purlins and cladding. Certainly this would satisfy the reviewer if nothing else will.

 

[BJI]

Another consideration, which may have led to this change, is the effect of building envelope flexibility. The pressure coefficients assume the internal volume is relatively constant, which is never the case, but especially under cyclonic conditions. Therefore, I would think if you don't also apply these factors to the members that make up the pressure envelope, then your stiffness won't adequately increase proportional to the wind loading.

There has been significant research and changes to the wind loading codes since the 2002 version, so it wouldn't be appropriate to go back that far to assess the intent of the current codes and what is now considered good practice.

 

[blihpandgeorge]

i agree with steveh49 and ask where the design violates the standard. A peer review should only make reference to published guidelines etc and not be used to push personal interpretations. You can also agree to disagree with the peer reviewer, at which point when it gets elevated having the examples of the new draft and that the standard does not explictly say you can't will come into play

 

[rowingengineer]

I agree with others the draft code is the reference to supply to your revised.

However I do wonder if you using cyclone washes for your design for cladding? The reduction in pressure seems minimal in a go/no go situation.

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."

 

[Rajshrn06]

Thanks everyone for your replies. Peer reviewer has reluctantly accepted based on the wording from the draft code.

rowingengineer - yes, the use of cyclone washers is a no miss for us.

 

[rowingengineer]

Good, but are you also aware that in a lot of instances the max reported sheeting strength for cyclone washers is due to a limit in the air box pressure levels and if you give the tech department a call they can often help get a little more.

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."