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09/04/2024 1:25 pm
We have encountered differing interpretations of NZS1170.5 regarding the application of eccentricity for the two seismic load directions, namely the 100% and 30% components.
One reviewer has interpreted that according to NZS1170.5 5.3.2, accidental eccentricity is required for each required direction of earthquake loading. They noted that while accidental eccentricity lies on the outline of an ellipse, the 100% and 30% loadings are not applied at the same location on the ellipse. They argue that the 100% load direction has an eccentricity related to that specific load direction, while the 30% load direction has its own perpendicular eccentricity, resulting in both loadings being considered at two different locations on the ellipse.
On the other hand, our interpretation is that the 100% and 30% loads are superimposed at the same location on the ellipse, with the 0.1b eccentricity related to the 100% direction.
We have pointed out to the reviewer that clause C6.2.2 of the standard refers to only one point on the ellipse. It states, “For an arbitrary direction of loading, the load may be assumed to lie on an ellipse…” and implies that the point of application of the equivalent static forces lies on this ellipse. These forces are to be applied simultaneously in two orthogonal directions, as stated in C5.3.1, which notes “…with 30% of the design actions on the axis at right angles”.
The reviewer is working for the territorial authority, and has requested evidence mentioning the same point on the ellipse for two directions, otherwise we need to reanalyse to consider their interpretation.
We have been unsuccessful in finding explicit documentation on the interpretation of accidental eccentricity within NZS1170.5. Does anyone have any evidence or insights into what the correct interpretation of NZS1170.5 is in this regard? Any references to papers, research, or course notes would be greatly appreciated.
18/04/2024 5:01 pm
Johnathan,
Some further information related to similar 100% – 30% rules can be found in the commentary to ASCE7-22. This contains a commentary around this Clause C12.5.1. https://archive.org/details/asce-7-22-minimum-design-loads-and-associated-criteria-for-buildings/page/659/mode/2up
Jonathan Haagh liked
29/04/2024 4:28 pm
Hi, Jon.
I agree that 1170 is not 100% clear and agree with your interpretation as it seems illogical to consider the mass to be in two different locations at a single point in time.
Considering the 30% load with an eccentricity as well, would position the centre of mass on the external points of a rectangle rather than on an ellipse which is referenced in 5.3.2b).
02/05/2024 10:18 pm
Thanks to those who reached out, on the forum and privately, it’s seems this query was coming up on several building consents as part of the “regulatory review”.
It is clear that NZS1170.5 cl 5.3.1 and 5.3.2 are ambiguous, although it seems that design engineers (who I have spoken to) are generally in agreement that the intention was apply those provisions as stated in the ASCE7-22 commentary.
Interestingly, I came across a 2019 ASCE paper, in which the abstract suggested that the US was having similar struggles with their interpretation of these clauses, which is possibly why ASCE7-22 is so explicit.
SESOC have noted that they will try and get this clarified in the next revision of the Interim Design Guidance, although there is currently no timeframe for it yet.
05/07/2024 10:09 am
Closing this out – as reported in the recent newsletter, SESOC have been working with an expert group to resolve the ambiguity in NZS 1170.5 regarding how accidental eccentricity provisions (clause 5.3.2) should be applied to the “100% + 30%” earthquake action set used for nominally ductile structures (clause 5.3.1.2). The clarification arrived at, which will be added to our Interim Design Guidance in due course, states:
For nominally ductile and brittle structures the 0.1b accidental eccentricity provisions of Clause 5.3.2 are to be applied to the actions along the 100% orthogonal axis only and are to be applied in conjunction with the 30% perpendicular action applied through the nominal centre of mass for that direction (i.e. with zero eccentricity from that perpendicular action).
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