Notifications
Clear all
14/01/2019 3:40 pm
Hi All,
I am wondering how to go about calculating the Peak Ground Acceleration (PGA) for the lower region of the pESA method envelope when calculating the inertial forces of a concrete diaphragm. Typically ground acceleration would be found by using bracketed spectral shape factors in equation 3.1(1) of NZS 1170.5:
C(0) = Ch(0).Z.R.N(T,D)
And as per the guidance in NZS1170.5 C5.7.A2.3, NOT multiplying that acceleration by the building’s overstrength and instead using a strength reduction factor of 0.75 for design. However, the paper by Gardiner (2011) which outlines the pESA method uses a different equation for calculating the lateral force coefficient of the lower region of the pESA envelope:
Lfc(T1) = Chmod(T).Z.R.Sp.Øob/Ku (Equation 5-7)
This method uses a modified spectra based on the structure type (based on NLTHA analysis given in the paper), neglects the near-fault factor N(T,D) (no explanation as to why – but this would affect all tall buildings in Wellington), and accounts for the superstructure ductility & overstrength factor (part of the superstructure response & not related to peak ground acceleration). Can anyone offer any insight into why these methods are different, and which is preferred in design of diaphragms in multistorey buildings?
Cheers
30/01/2019 9:44 am
I believe the PGA should be calculated as per the 1170.5 spectra.
Gardiner’s work occurred during development of the PESA method. It predates the recent amendment to 1170.5, and was familiar to the authors of the amendment – it was evidently not considered necessary to include the modified spectra.
25/07/2023 11:50 am
Hi, I am in a similar boat here and not entirely sure how to determine PGA.
Would you not just use the tabulated PGA from “Earthquake Geotechnical Engineering Practice Module 1 Overview of the Guidelines”?
Also, am I right in thinking that you would use the 2500-year event?
If so, the tabulated value is 0.39 for Invercargill in Type C ground, but calculating C(0) from 1170.5 gives 2.36*0.17*1.8 = 0.72. These two values are clearly wildly different so it isn’t overly clear to me which one is the correct value, and reading the guidance doesn’t provide any clarity in my opinion.
10/08/2023 9:55 am
I think you might need to use 1.33 instead of 2.36, since you multiply your floor weight to PGA is regarded as one of the many modes of excitations, aka “Modal analysis”.
Forum Statistics
3
Forums
85
Topics
200
Posts
1
Online
5,175
Members
Latest Post: SESOC Interim Design Guidance Design of Conventional Structural Systems – Update Our newest member: Julian Rincon Recent Posts Unread Posts
