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# AS1170.4 Section 5.4.2.1

## AS1170.4 Section 5.4.2.1

(OP)
I need an explanation of
AS1170.4 Section 5.4.2.1 say: "For structural components and footings that participate in resisting horizontal earthquake forces in both major axes of the structure, the effects of two directions determined separately shall be added by taking 100% of the horizontal earthquake forces for one direction and 30% in the perpendicular direction"

Does it mean that I have earth load in two directions x and y of Ex and Ey. And the RC column and shear (core) walls will be designed by (Ex+0.3Ey) and (Ey+0.3Ex) when considering the effect of earthquake in 2 directions ???
Replies continue below

### RE: AS1170.4 Section 5.4.2.1

I don't reckon so. If you perform a 3d response spectrum analysis it consider's the global response of the building and when assessing your "orthogonal" directions you will get some perpendicular interaction. I don't see a need to further combine the two.

### RE: AS1170.4 Section 5.4.2.1

I disagree Asixth, 100% & 30% loadcases definitely required here in NZ as well.

A 3d analysis is still looking at the response to an orthogonal loadcase, just happens for most structures you will get some response in the perpendicular direction from the irregularities. This needs to be combined in 100%/30% loading scenario.... Earthquakes are rarely aligned with the orthogonal axes of a structure, the 100%/30% concurrency accounts for this in elastic structures (adds a degree of resilience)

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