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# AS/NZS 1170.2 : Action Combination Factor (Kc)

## AS/NZS 1170.2 : Action Combination Factor (Kc)

(OP)
Hi everyone,

I am a bit confused in trying to determine the Kc of a structure using the AS/NZS 1170.2. It states in Section 5 of the standards that internal surfaces with |Cpi| < 0.2 is determined as an ineffective surface. However, in the table for the design cases, it asked how many surfaces are effective. If the Cpi of a single storey building is -0.1, does that mean there is no Kc? Can anyone give advice on how to determine the Kc correctly? Thank you in advance. I appreciate your help.

### RE: AS/NZS 1170.2 : Action Combination Factor (Kc)

For starters I usually adopt Cpi = +0.2;-0.3 for a typical enclosed building. Thus Kci is always used. Net pressure coefficient on a surface is given by Cpn=Kci(Cpi)-Kce(Cpe). Remember to apply appropriate signs to pressures/suctions (+ve towards surface and -ve away from surface) and that the product of KaKc can't be greater than 0.8.
Example - take roof uplift on Rafter, whereby internal pressure and external suction combine
Cpe=-0.9 suction away from surface
Cpi=+0.2 pressure towards internal surfaces
Therefore Cpn=0.9(0.2)-0.9(-0.9)=0.9(1.1)=-1.0 (approx.) -- The negative indicates the net resultant is in the direction of the roof suction i.e. Upwards in this case.

When Cpi is less than 0.2, then yes Kce=1.0 and the internal pressure/suction is not included in the net resultant.
Generally
1 effective Surface causing structural response - 1.0 no reduction.
2 Effective surfaces causing structural response - 0.9 reduction
3 effective surfaces causing structural response - 0.8 reduction

Toby

### RE: AS/NZS 1170.2 : Action Combination Factor (Kc)

(OP)
Hi Toby,

Thank you very much for your reply. It is very helpful and I could understand it now.

### RE: AS/NZS 1170.2 : Action Combination Factor (Kc)

(OP)
Dear Toby,

I apologize for asking you again. I just wanted to know what constitute as surface as effective? For the internal surface I know that it is effective if |Cpi|> 0.2. But what about the other surfaces? Is it based on the situation and how the wind load acts on the building as shown in the diagrams? Thank you in advance for your help.

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