Specific Seismic Question

[plainjane]

Hi all! I have a question about the seismic response coefficient, Cs, which I've been looking for the answer to for a while but can't seem to figure it out.

In the manual that I'm using, it says that Cs should be "conservatively calculated" (see below) if S1 >= 0.2 and Site Class is D. Howeverrrr, at some point it says something along the lines of - if values for Fv come from ASCE 7 Table 11.4-2, then the conservative values should still be used (see below).






Does anyone know which requirements should be satisfied in order to use these "1.5" conservative values for Cs? Like I said, I've tried to make sense of it on my own and asked other test-takers as well, but nobody seems to know a clear answer. In practice problem solutions, I see it used sometimes in these situations, but not always, so I just want to make sure I'll only be using the conservative equations when necessary.

Any help is appreciated! Thanks!

 

[as_botd]

The context that I understand these relationships is that they are used to calculate Cs when you already are using the Fv from table 5.2 to calculate your SD1.

Basically what all this is saying is that to use the SD1 that you get from using the Fv from table 5.2, you would then have to use a Cs formula based upon the period, as you pasted above, when calculating the base shear and limits later on. Honestly this never came up for me on the exam and I don't recall having to deal with it in the homework because I was almost always able to use the alternative method where you determine the Cs formula by checking T against Ts.

Disclaimer: I did pass seismic but I'm not an expert. My knowledge solely comes the AEI course.

 

[plainjane]

The context that I understand these relationships is that they are used to calculate Cs when you already are using the Fv from table 5.2 to calculate your SD1.

Basically what all this is saying is that to use the SD1 that you get from using the Fv from table 5.2, you would then have to use a Cs formula based upon the period, as you pasted above, when calculating the base shear and limits later on. Honestly this never came up for me on the exam and I don't recall having to deal with it in the homework because I was almost always able to use the alternative method where you determine the Cs formula by checking T against Ts.

Disclaimer: I did pass seismic but I'm not an expert. My knowledge solely comes the AEI course.

Thank you for this reply!

I was kind of thinking that it was used whenever the Fv from the table was used, but then it seems like you'd hardly ever have to use the "normal" formulas for Cs. With these requirements in mind, it seems like the only time you would use the "normal" formulas for Cs is when SD1/SDS etc. are given to you as known variables rather than something you have to compute.

Do you think that sounds correct?

 

[Reverse Polish]

Hi all! I have a question about the seismic response coefficient, Cs, which I've been looking for the answer to for a while but can't seem to figure it out.

In the manual that I'm using, it says that Cs should be "conservatively calculated" (see below) if S1 >= 0.2 and Site Class is D. Howeverrrr, at some point it says something along the lines of - if values for Fv come from ASCE 7 Table 11.4-2, then the conservative values should still be used (see below).

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Does anyone know which requirements should be satisfied in order to use these "1.5" conservative values for Cs? Like I said, I've tried to make sense of it on my own and asked other test-takers as well, but nobody seems to know a clear answer. In practice problem solutions, I see it used sometimes in these situations, but not always, so I just want to make sure I'll only be using the conservative equations when necessary.

Any help is appreciated! Thanks!

Not sure what manual you're using, but it looks like the "1.5" factor may be Cu, the Upper Limit on Calculated Period from Table 12.8-1 in ASCE 7? If S1 > 0.2, then Cu < 1.5, and thus using a value of 1.5 for Cu is "conservative" for calculating period (but actually results in a lower value of Cs).

I'm not entirely sure, here. I've never seen this approach with 1.5 fudge factor, so I can't tell the author's intent. ASCE 7 Equations 12.8-3 and 12.8-4 do not have this 1.5 factor.

 

[plainjane]

Not sure what manual you're using, but it looks like the "1.5" factor may be Cu, the Upper Limit on Calculated Period from Table 12.8-1 in ASCE 7? If S1 > 0.2, then Cu < 1.5, and thus using a value of 1.5 for Cu is "conservative" for calculating period (but actually results in a lower value of Cs).

I'm not entirely sure, here. I've never seen this approach with 1.5 fudge factor, so I can't tell the author's intent. ASCE 7 Equations 12.8-3 and 12.8-4 do not have this 1.5 factor.

I agree, I couldn't find it in ASCE 7 - however, both the AEI and Hiner manuals include this "conservative" method of finding Cs. Not quite sure where it comes from!

 

[EBAT75]

Cu is used to calculate T as a refinement of Ta which is approximate. Cu can be anywhere from 1.4 to 1.7. But ASCE says Ta can be used by itself in place of T in the Cs equations. T=Cu x Ta ; 1.4 to 1.7 times Ta. If used Cs involving T will be lower. Even though these are not-less-than conditionals, and in some cases immaterial, I am not sure if exam graders would be happy.

I have also not seen the 1.5 multiplier used in this manner. If using it this way can be conservative as said in the original post, how can a lower value of Cs which translates to a lower base shear be conservative seismic design?

Another point is - Cu has to do with SD1 (1-sec) and not S1 as mentioned in a reply. S1 has to do with MCE whereas SD is design basis. Also, what if the Site Class is other than D.

I am sticking with the equality and the inequality conditionals/caveats to be safe than sorry.

 

[PlanCheckEng]

This comes from ASCE 11.4.8.

The way to apply it:

If your Ta > 1.5Ts then use Cs = 1.5Sd1/T * I/R.
If Ta < 1.5Ts, use the normal Cs = SdsI/R

(Ts = Sd1/Sds)

 

[EBAT75]

You mean ASCE 11.4.6? I do not see 11.4.8 in the version I have.

11.4.6 is for Risk-Targeted Maximum Considered Earthquake (MCEsubR) Response Spectrum.

When an MCE,R response spectrum is required, it shall be determined by multiplying the design spectrum by 1.5.

We are looking at Design Spectral, not MCE which is required for liquifiable soils mostly in Site Class E, F but not necessarily limited to E, F only.

11.4.3 also refers.

 

[plainjane]

He's correct - 11.4.8:



I am waiting for an response from Dr. Ibrahim (AEI manual author) to clarify. From what I've read, it seems as though there are two conditions:

1. SD1 is given or calculated via a ground motion hazard analysis (you don't have to use Fv table to find SD1).
2. Site class is D and S1 is greater than or equal to 0.20.

My question to him - if you do use Fv table to find SD1, but site class isn't D or S1 is less than 0.20, do you still use the 1.5 equations?

 

[EBAT75]

Is this 11.4.8 in the 7-10 Third Printing- current specification for the exam? Or is it in 7-16? I cannot find it in 7-10.

 

[plainjane]

Is this 11.4.8 in the 7-10 Third Printing- current specification for the exam? Or is it in 7-16? I cannot find it in 7-10.

This is 11.4.8 in 7-16, which is the current code for the exam, from Special Civil Engineering Examinations Reference List - Board for Professional Engineers, Land Surveyors, and Geologists

Minimum Design Loads for Buildings and Other Structures
American Society of Civil Engineers, ASCE/SEI 7-16

 

[EBAT75]

Thank you for the clarification.

I am taking the SE exam. 7-10 is the specification for that.

Good to know for future reference. I think 7-16 will become the ACI specification for the October exam. I am curious to know what if Class E or F. I think why it is left out of this is because Site Specific, Ch 21, MCE etc would kick in for E or F

Please post if you get a reply from Dr. Ibrahim.

 

[EBAT75]

Meant to say ASCE not ACI.

 

[plainjane]

OK - here’s what I received as clarification from Dr. Ibrahim:

Basically, if you use Fv table to get SD1 value (aka it is not given or calculated through ground motion hazard analysis), AND you have Site Class = D AND S1 > or = 0.20, then you will use the conservative equations (1.5 etc.) to find the Cs value.

If SD1 value is given or calculated through ground motion hazard analysis, or Site Class ≠ D, or S1 < 0.20, then you use the normal equations to find the Cs value.

I suppose that’s as clear as it can get. Quite confusing the way it’s worded in the manuals and all that, but oh well. Thanks for everyone’s help!