Code-based Spectra for ASCE 41-17

03-Analytical/modelling capabilities
fernnie_gae
Posts: 7
Joined: 10 Aug 2023, 01:43

Code-based Spectra for ASCE 41-17

Under Seismic Action Tab, the program is asking for values of the following:
SXS 5%/50y and SX1 5%/50y (which is BSE-2E)
SXS 20%/50y and SX1 20%/50y (which is BSE-1E)

Now, for example we have SS 5%/50y = 1.20 and Fa = 1.2 (Site Class C),
then per ASCE 41-17 Eqn. 2-1, SXS 5%/50y = 1.20*1.2 = 1.44.

If we input SXS 5%/50y = 1.44 to the program, it seems that the generated spectrum is not correct.
The y-intercept should be 0.4*SXS as shown in Fig. 2-1 of ASCE 41.
From above, 0.4*SXS 5%/50y = 0.4*1.44 = 0.576, but the generated spectrum has y-intercept at 0.6912.

So I think that the SXS 5%/50y that the program is asking is actually the SS 5%/50y. Same is true for SX1, which should be S1.
It seems that the site factors are multiplied twice is we input the "SXS" calculated per ASCE 41-17 Eqn. 2-1.
If I input the SS 5%/50y = 1.20 into the program, then everything seems correct.

I would like to ask confirmation of the above.

P.S.
I hope that you also consider adding a site class of SITE CLASS D (Default) into the program.
For Ss values > 0.75, Site D has Fa values less than 1.2. For example, Fa = 1.0 for Ss > 1.5.
However for Site D (chosen as Default), Fa shall not be less than 1.2. Thank you very much.
Kirkaguel
Posts: 63
Joined: 31 Dec 2018, 10:04

Re: Code-based Spectra for ASCE 41-17

Tried the values you suggested but it is working fine with me got the correct values ofr 0.4SXS = 0.576
fernnie_gae
Posts: 7
Joined: 10 Aug 2023, 01:43

Re: Code-based Spectra for ASCE 41-17

Are you sure? Did you check the coordinates, specifically the y-intercept of the generated spectrum?

Under Seismic Action Tab, select Other Spectral Shapes > Other Codes > ASCE 41-17

Input Damping = 5, Site Class = C and use SXS = 1.440, SX1 = 1.105.

The following coordinates are generated:
Period (sec) Sa (g)
0.00 0.6912
0.02 0.8066
0.04 0.9219
0.06 1.0373
0.08 1.1526
0.10 1.2680
0.12 1.3833
0.14 1.4987
0.16 1.6140
0.18 1.7239
0.20 1.7239

0.4SXS = 0.44*1.440 = 0.576, but the generated spectrum as shown above gives you 0.6912.

My clarification is that, the program is asking the value of SXS, and NOT SS. Where in fact, I think that the value you should input in the program is the SS. You can read below for a similar topic raised in 2020, but I think that it was not answered well.

viewtopic.php?p=15639&sid=7c5e0ca49d462 ... 51c#p15639
Kirkaguel
Posts: 63
Joined: 31 Dec 2018, 10:04

Re: Code-based Spectra for ASCE 41-17

fernnie_gae wrote: 28 Nov 2023, 01:07 Are you sure? Did you check the coordinates, specifically the y-intercept of the generated spectrum?

Under Seismic Action Tab, select Other Spectral Shapes > Other Codes > ASCE 41-17

Input Damping = 5, Site Class = C and use SXS = 1.440, SX1 = 1.105.

The following coordinates are generated:
Period (sec) Sa (g)
0.00 0.6912
0.02 0.8066
0.04 0.9219
0.06 1.0373
0.08 1.1526
0.10 1.2680
0.12 1.3833
0.14 1.4987
0.16 1.6140
0.18 1.7239
0.20 1.7239

0.4SXS = 0.44*1.440 = 0.576, but the generated spectrum as shown above gives you 0.6912.

My clarification is that, the program is asking the value of SXS, and NOT SS. Where in fact, I think that the value you should input in the program is the SS. You can read below for a similar topic raised in 2020, but I think that it was not answered well.

viewtopic.php?p=15639&sid=7c5e0ca49d462 ... 51c#p15639
Hi Fernnie, I double checked and you are right it seems that the program is multiplying twice the site coefficient Fa and Fv. Since it is asking for Sxs and Sx1 which is already multiplied by the site coefficients. Developers comment on this issue is critical.
seismosoft
Posts: 1168
Joined: 06 Jul 2007, 04:55

Re: Code-based Spectra for ASCE 41-17

Thank you both for your feedback.
We sent this post to the developers, they will check things and we will come back to you as soon as possible.
Seismosoft Support
seismosoft
Posts: 1168
Joined: 06 Jul 2007, 04:55

Re: Code-based Spectra for ASCE 41-17

Hello, Kirkaguel and fernnie_gae,

We updated today the SeismoBuild release that is currently available. In the new release (v2024 Release-1, build-5) the labels of the textboxes have been changes to Ss and S1 and now it is clear that Fa and Fv are then applied to these values. Apologies for the time you spent, and thank you for your contribution to make SeismoBuild better.
Seismosoft Support
fernnie_gae
Posts: 7
Joined: 10 Aug 2023, 01:43

Re: Code-based Spectra for ASCE 41-17

Thank you very much for your confirmation and action.
I appreciate it! As I have suggested too, I hope you include a DEFAULT SITE CLASS D in the next releases which limits Fa to be at least 1.2.
Thank you very much!