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Action Combination Factor

Action Combination Factor

Action Combination Factor

Hi everyone,

I have some questions in regards to the rules stated in the AS/NZS 1170.2 under Section 5 on the Action Combination Factors (Kc). The standard states that when the building has

2 effective surfaces: Kc = 0.9
3 or more effective surfaces: Kc = 0.8

However, I am a little bit confused on what constitutes as an effective surface. For the internal surface, I understood that it is considered effective if the |Cpi| > 0.2. But I am not sure about the Cpe. Can someone explain this to me? I have read several handbooks on the AS/NZS 1170.2 but still fail to understand this part. Thank you in advance for your help. I will appreciate it very much.

RE: Action Combination Factor

It's exactly like the diagram. If looking at global building response, if you have wind on a surface (walls, roof, etc) then it's effective.

If looking at wind on a smaller element like a purlin for example, external pressure counts as one effective surface, internal as another if Cp,i is greater than 0.2. If only one surface then Kc =1 anyway so no effect.

If looking at an entire structure under wind then it's broken down into wind on roof being one effective surface, walls being another etc (as per the diagrams.

It's useful to understand the purpose of the factor, it's meant to represent the fact that the maximum wind pressures internally and externally over the full structure may not occur concurrently (due to the variations in pressure due to fluctuations in wind speeds), but on a local (smaller) element like an isolated beam, it's more likely that the wind pressures could be at their maximums.

RE: Action Combination Factor

Dear Agent666,

Thank you for your reply. So, does that mean it the diagram has nothing to do with the location of the openings? For example design case 1 applies if the location of the opening is on the windward and leeward wall?

RE: Action Combination Factor

Openings only come into it to determine the internal and external pressure coefficients. Kc factor is assessed independently of any openings that might be present.

RE: Action Combination Factor

To echo Agent666. The more surfaces that contribute to a "Structural Action Effect" i.e. moment, shear, deflection etc... the less likely it is the surfaces will simultaneously be subject to maximum values of wind pressure/suction.

RE: Action Combination Factor

Hi Agent666,

I apologize for the late reply and thank you so much for your explanation.

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