ASCE 7-16 Section 11.4.8 Site Class D Exception 1

[TS59401]

Hello,
I am sure there has been plenty of discussion regarding this section of ASCE 7-16, however I have not been able to locate a clear answer. I am looking at section 11.4.8, Exception (2) that discusses Site Class D sites with S1 greater than 0.2.

As I read the exception, I believe it says:
If the structure period (T) is less than 1.5*Ts, then calculate Cs per Eq12.8-2 as normal.
If the long period transition (TL) is greater than the structure period (T), and the structure period is greater than 1.5*Ts than Cs is 1.5*Eq12.8-3.
If the structural period (T) is greater than TL, than calculate Cs is 1.5*Eq12.8-4

I also assume that seismic design per chapter 13 and 15 is unchanged, except where chapter 15 references the requirements of Section 12.8.

Am I reading this correct - If T < 1.5*Ts then calculate Cs per section 12.8 as usual?

Thanks in advance.

 

[CaliEng]

Hey there,

I am having some trouble interpreting this same section as well.
I think you are on the right track though, your interpretation seems correct to me.

My questions are:
-is if these exceptions are met, would you revert back to Table 11.4-2 to find Fv and calculate SD1?
-And if the exceptions are not met can you simply make Fv=2.5 per Section 21.2.2 to calculate SD1?

 

[EDub24]

@TS59401 - You’re interpretation of 11.4.8 for site class D is correct in my view. I also agree that Chapters 13 and 15 aren’t changed unless they refer back to 12.8 with the exception of tank designs. For that the impulsive mode must be checked per the exceptions while the convective mode is increased 50% regardless.

@CaliEng - If you meet these exceptions then yes you can use the values from table 11.4-2. If you choose not to use the exceptions then you (the geotech) will have to perform a ground motion hazard analysis. The Fv=2.5 you mention in section 21.2.2 for site class D is used along with Fa=1.0 to determine a lower limit response spectrum. At any period of the response spectrum determined from the ground motion hazard analysis the corresponding acceleration can’t be lower than the acceleration at the same period from the response spectrum using Fv=2.5 and Fa=1.0 for site class D.

 

[CaliEng]

@EDub24 - Thanks for the reply.

Is this not a bit of a circular argument/check?: The exception is asking value of Ts which is defined as SD1/SDS, but SD1 cant be calculated until after sorting out this exception criteria. Or can Ts be assumed to be 0.2sec?

How would this apply if I am using a reference document (API 650) to calculate Cs (called Ai in API650)? Equation E.4.6.1-1 (650) is equal to Eq 12.8-2 (7-16). However my building period (convective period Tc) is approx 2-3 seconds with TL either 8 or 12 seconds. So I would be looking at 1.5*Eq12.8-3. Or does this Criteria even still apply since I am using an outside reference?

TS59401 - Sorry if I'm hijacking your thread! I can start a new thread if needed.

Cheers!

EDIT: @EDub24 - Just noticed your comment to TS59401 regarding the tank design aspect. So only the impulsive mode is check in regard to the 11.4.8 exceptions? In this case the impulsive period is very small (<0.2 seconds) and the exception is satisfied with comparison to Ts.

 

[TS59401]

@EDub24, thanks for the confirmation and the heads up on tank design.

@Calieng, No problem. I am curious about your question as well. I wonder how many man-hours have been spend on this new ASCE requirement?

 

[EDub24]

Is this not a bit of a circular argument/check?: The exception is asking value of Ts which is defined as SD1/SDS, but SD1 cant be calculated until after sorting out this exception criteria. Or can Ts be assumed to be 0.2sec?

Yup it is a bit circular. You go to the table for Fv then that leads you to the section 11.4.8 which then leads you back to the table to determine Fv and by extension Sd1 and Ts (if you choose to apply the extension) and then back to the section 11.4.8 for the modifications to the Cs equations in Chapter 12. It's a nightmare.

How would this apply if I am using a reference document (API 650) to calculate Cs (called Ai in API650)? Equation E.4.6.1-1 (650) is equal to Eq 12.8-2 (7-16). However my building period (convective period Tc) is approx 2-3 seconds with TL either 8 or 12 seconds. So I would be looking at 1.5*Eq12.8-3. Or does this Criteria even still apply since I am using an outside reference?

It still applies from what I can gather. I traded a few e-mails with ASCE when this code first came ask asking for clarification on how to apply section 11.4.8 to tank design. My understanding of the conversation is that if the design code reduces the impulsive seismic coefficient based on the period of the structure then the exceptions would apply. If the code doesn't reduce the impulsive coefficient and just uses the equivalent of eqn 12.8-2 in ASCE 7 then you don't need to do anything since that's already the maximum seismic coefficient you can have per the code. The convective mode is based on the long-period seismic acceleration so the convective seismic coefficient, Cc (or Sac) needs to be bumped up by 50% regardless of the structure period. If you read the ASCE 7 commentary they basically said that the long period seismic acceleration was off so if you have a long-period structure the accelerations are higher. If you don't do a ground hazard motion analysis you need to take a 50% increase of the seismic coefficients but you're still capped at the max value which is eqn 12.8-2. Anyways, to answer your question I haven't used API 650 (mostly ACI 350.3 and AWWA) so you would need to check that code to see if they reduce Ai based on the structure period. If not then yup you don't need to do anything for Ai but the convective coefficient will still need to be increased 50%.

EDIT: @EDub24 - Just noticed your comment to TS59401 regarding the tank design aspect. So only the impulsive mode is check in regard to the 11.4.8 exceptions? In this case the impulsive period is very small (<0.2 seconds) and the exception is satisfied with comparison to Ts.

Yup. But remember the convective coefficient will need to be increased 50%.

I wonder how many man-hours have been spend on this new ASCE requirement?

Seriously! I'm hoping they revise the tables when the next edition comes out and get rid of this section. It's been a nightmare. I'm working on a project with a couple of metal buildings and I'm still trying to determine the best way to put on the drawings/specs that the metal building mfr has to apply section 11.4.8 to his/her design.

 

[CaliEng]

Yup. But remember the convective coefficient will need to be increased 50%.

There is a 1.5 factor for the convective coefficient within Eq 15.7-10 (7-16) and similarly within Eq E.4.6.1-4 (API 650) (I am getting a feel for the overlap between these two codes).
Is this the 50% increase you are referring to?

 

[EDub24]

I don't think so. That equation was also in ASCE 7-10 before the whole section 11.4.8 fiasco. It's also the same equation in ACI 350.3 (eqn 9-37). If you don't do a ground motion hazard analysis you would need to add an addition 50% to that equation.

 

[EDub24]

To add to this. Here is an excerpt from an e-mail I got from ASCE:

Section 11.4.8 (or more specifically, the exception in 11.4.8) applies to both the determination of impulsive and convective accelerations. Please note that all national standards for tank design (AWWA D100, API 620, API 650, ACI 350.3, etc.) set Sai equal to Sds. So if you are using a national standard, the impulsive acceleration does not change when applying the exception in Section 11.4.8. For the convective acceleration, you would need to multiply Sac by 1.5. This increase is in addition to the 1.5 increase shown in Equations 15.7-10 and 15.7-11 used to convert from 5% damping to 0.5% damping.

ACI 350.3 actually does reduce Sai based on the structure period but the intent is the same. Ground-bottom tanks will almost always have a lower period of vibration so Sai wouldn't be affected but elevated tanks might be affected.

 

[CaliEng]

To add to this. Here is an excerpt from an e-mail I got from ASCE:

Section 11.4.8 (or more specifically, the exception in 11.4.8) applies to both the determination of impulsive and convective accelerations. Please note that all national standards for tank design (AWWA D100, API 620, API 650, ACI 350.3, etc.) set Sai equal to Sds. So if you are using a national standard, the impulsive acceleration does not change when applying the exception in Section 11.4.8. For the convective acceleration, you would need to multiply Sac by 1.5. This increase is in addition to the 1.5 increase shown in Equations 15.7-10 and 15.7-11 used to convert from 5% damping to 0.5% damping.

Awesome, this is very helpful. However, can this be found any any more official format, rather than an internet forum and third hand email?

Thanks for all the help!

 

[EDub24]

Awesome, this is very helpful. However, can this be found any any more official format, rather than an internet forum and third hand email?

I doubt it. I had to e-mail ASCE directly to get direction. If you want something more official shoot them an e-mail. They're pretty responsive. There's a chat feature on the ASCE website. Just ask them for the e-mail of the technical person to contact.

 

[maest]

Am I reading this correct - If T < 1.5*Ts then calculate Cs per section 12.8 as usual?

I don't think that's exactly correct in that the usual method of calculating Cs per section 12.8 puts an upper limit on Cs per Eq (12.8-3/4). Since the new verbiage of Exception 2 says "Cs is determined by Eq (12.8-2) for values of T <=1.5Ts", I don't think there is an upper limit on Cs if using this exception.

 

[maest]

If you've got a rigid T<0.06s non-building structures per 15.4-5, and you read 11.4.8 Exception 2 as literally as I do above, does this mean you can't calculate the base shear using Eq 15.4-3? 0.3*SDS*W*I