ASCE 7-16 11.4.8 Site Specific

[bookowski]

I need some help working through whether or not I need a site specific study per asce 7-16. My project info: Ss = 0.991, S1 = 0.395, Site Class E, hn = 142 ft

The supplement to 7-16 has corrected tables 11.4-1 and 4-2, for Fa and Fv respectively. In those tables for Ss = 1.0 they send you to section 11.4.8.

Now to 11.4.8, a ground motion hazard analysis per 21.2 is required unless:
1. Site class E (yes) with Ss >= 1.0, Nope so doesn't help
2. Site classs D, doesn't apply
3. Site class E (yes) with S1 >= 0.2 (yes), provided that T < Ts and you use ELF.... Here's where I am stuck.

To calculate Ts I need Fa and Fv values, but I can't get those values because the table sends me to this section which is how I ended up there in the first place. I must be missing something here because I'm caught in a loop.

If I use the Fa from a site class D of 1.1 as the closest value I get a Ts of 0.87. If I use the code calc period (12.8-7) then I get a T of 0.82 and would meet the exception (barely).

I found a good article on this specific issue and the article also notes that a more aggressive reading of 11.4.8 would be to state your T such that it satisfies 11.4.8 and then design for that T.

I assume that I'm missing something (or a few things) here and there's a clear way to step through the code here and be sure of what is required.

 

[Deker]

ASCE 7-16 Supplement No. 1 addresses this issue: Link. There is also an SK Ghosh article that discusses the change along with some of the upcoming revisions in Supplement No. 3 that you'll need to use if you qualify for the exception: Link. I think you'll need the site-specific study (but just barely ).

 

[bookowski]

Thanks, Deker! I burned hours going in circles thinking I was missing something. You are right, I am just barely over - maybe close enough or maybe I'll convince them to shave a couple of feet off the building.