ASCE 7-16 - Site Specific Study - Seattle Washington 1

[sticksandtriangles]

I am interested in how people out west address these high seismic regions...

I am trying to do equivalent lateral force procedure on a building out in Seattle Washington (outside of my typical area of the midwest)...

My typical procedure to do this is to go to the USGS/ASCE Hazard tool and get my seismic design parameters.
After running the report I get the following:

To my surprise Sd1 is not output, only an N/A.

Digging into this, it kicks you to ASCE 7-16 section 11.4.8.
Based on my data i think I need to do the following:

[ul]
[li]Based on item (3) of 11.4.8:

we are required to perform a ground motion hazard analysis in accordance with section 21.2.[/li]
[/ul]
[ul]
[li]Ok so onto 21.2 assuming none of the exceptions apply to me. Based on how I read 21.2, i need to create a probabilistic ground motion response per 21.2.1 and a deterministic ground motion response per 21.2.2.
Is there a website that produces both the probabilistic ground motion response per 21.2.1 and a deterministic ground motion response per 21.2.2?
[/li]
[/ul]
[ul]
[li]From here, it looks like a seismic response spectrum curve is spit out and I can multiply by 2/3*Fv per 21.3-1 and achieve Sd1?

[/li]
[/ul]

After all of this, I can then determine the seismic design category based off of Sds and Sd1?

If all of the above is the correct interpretation of the code, the only thing I need guidance on is how to obtain the probabilistic ground motion response per 21.2.1 and a deterministic ground motion response per 21.2.2.

Thanks again in advance for your help!

S&T

 

[msquared48]

I work in Site Class D in the Puget Sound area all the time using the Equivalent Force Method and have never had that output on the USGS site.

Heck, I live 10 miles off a major fault line...

How high is your building though? Over certain heights mandate the development of a response spectrum analysis.

I would have to check ASCE7, but I imagine there are other triggers too.





Mike McCann, PE, SE (WA)

 

[sticksandtriangles]

interesting, building is 96ft tall, but hopefully that does matter for getting seismic response parameters

all i input to the ASCE hazard tool was site class D, importance cat. II and Seattle Washington as the locale of interest and boom... no Sd1.

S&T

 

[msquared48]

You are looking on the wrong site.

Try Googling "USGS Seismic Design Values for Buildings", then click on the link "USGS - Ss & S1 - VP".

You can get the longitude and latitude to use for this USGS site from Google Maps.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

I've only skimmed the changes for ASCE 7-16 seismic since this doesn't affect my current work, but I believe this is whats happening (fascinating stuff really):

In preparing for ASCE 7-16, USGS and others found that the current ELF procedure produces un-conservative results when peak MCE[sub]R[/sub] occurs at periods greater than 1.0s for the site.

Thus, there's now a lot more areas in the country that require seismic site-specific analysis and I believe they've added a few more factors into the response spectra equation for ELF.

For OP: You appear to have the correct answer honestly, but likely this is more complex than your instructor intended. Verify that your instructor wants you to use ASCE 7-16, and not ASCE 7-10. Either way, bravo for digging into the code and learning; I had a few professors like that in university and they were the ones I learnt the most practical knowledge from.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[TLHS]

Site specific seismic studies are a specialist role. It's the sort of thing that a general structural engineer may learn the process of, but even if they do they won't generally do it enough to see the number of locations you need to get a real feel for it. So you part it out to a specialist geotechnical firm, usually (or accidentally part it out to a geotechnical firm that says they can do it, but can't really do it well)... Or more realistically you exclude it from your contract and tell your client that they have to have one done.

 

[kindlin]

I also saw this happening recently when I'm using the new Hazard Tool. Good find on that code requirement, but this still doesn't make sense.

We do simple designs on simple structures using ELF every week, and 99% of those do not have geotechnical reports, let along a site-specific seismic hazard analysis. Do the code writers really expect building owners to get a geotechnical report for their new deck or shed? Owner's don't even get geotechnical reports for massive renovations, let alone smaller projects.

For now I guess I'll just use ASCE 7-10 for the hazard tool.

 

[TehMightyEngineer]

I think they expect you to go to something other than ELF. Your point is still totally valid of course.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[kindlin]

I figured this out.

If you follow through with the exceptions in 11.4.8, stubby structures (low building periods) can still be designed with ELF with additional requirements. I think the 2nd exception is the most confusing, but all it's saying is if your building period, T, is less than 1.5Ts, nothing changes, but if T > 1.5Ts, increase Cs by a factor of 1.5.

Any structure not meeting the exceptions of 11.4.8, you will need a site-specific geotechnical/seismic hazard analysis.

 

[sticksandtriangles]

Thank you

S&T