RESPONSE SPECTRUM ANALYSIS WITH ETABS 1

[nateTdot]

I am performing a RSA in a concrete shearwall building and would like some advice to see if I'm on the right track.

I defined the response spectrum function as per the the geotechnical report (site specific) and have created a response spectrum load case with scale factor of 1.

After running the analysis, I compare the static analysis base shear with the response spectrum base shear in both x and y directions and if the dynamic base shear is less than 85% of the static base shear I use the ASCE7-02 equation 9.5.6.8 (085 x V/Vt) to find a modification factor that I will plug into the Etabs Response Spectra Case Data as the scales factors for both X and Y directions and run the analysis once more.

After the analysis is done, I get the forces for each story and create a "pseudo" static user define load case and run my analysis again.

Would someone please let me know if those steps make any sense. I also would be happy if you could expand a little bit more. Thanks a lot.

 

[jen4950]

You might notice that if you have a irregular building, the static equivalent seismic forces derived from the response spectra cases are kinda all over the board. Really, it's pretty hard to even tell if you are conservative or not with a static equivalent.

If you dive into the theory a bit- there is an SRSS involved in the response spectra analysis. Square-Root-of-Sum-of-the-Squares. Basically you loose the sign. For instance, all of your story shears are positive, and all reactions from the response spectra at the foundation are positive.

If your building isn't too funky, it is reasonable. Otherwise you are making pretty general assumptions on the sign.

...

Scaling the response spectra results to the code minimum base shear is correct.

So what I do:

- Build the Model
- Run it
- Pull Pesponse Spectra Base Shears
- Pull Assembled point masses
- Derive Required base shear
- Input New Scale factor ([Required BS / Spectra BS] x Original SF) -> Repeat to converge
- Run Again, calculate accidental torsion amplification factors
- Apply Magnified Accidental Eqrthquake Torsions to model
- Run again, get final forces and drifts.

 

[nateTdot]

Thanks Jen,

Just one more thing, what do you mean by assembled point masses? is that the center of mass for each story?

Thanks again,

Nate

 

[jen4950]

Assembled Point Masses is the total mass of the struture; at the end of the table it has a total.

For instance -> 30,000 kilo-slugs from APM

30,000 kilo-slugs x 32.2 (g) -> 966,000 kips - Effective Weight

966,000 kips x 3% -> 28,980 kips - Minimum Base Shear by Code

20,000 kips total base shear from response spectra case

multiply old scale factor by 1.449 for the new scale factor.

Lot's of intermediate steps left out for you to figure out, but that's the basic idea.

 

[Murali27]

Thanks Jen

How do we design the foundation for seismic forces out of response spectrum method.

Can't we apply the story shears to get the right foundation forces?

Thanks, Murali

 

[dawn836]

well i would also to add my comment according to UBC97
here is statement from UBC97

1631.5.1 Response spectrum representation and interpretation of results.
The ground motion representation shall be in accordance with Section 1631.2. The corresponding response parameters, including forces, moments and displacements, shall be denoted as Elastic Response Parameters. Elastic Response Parameters may be reduced in accordance with Section 1631.5.4.

1631.5.4 Reduction of Elastic Response Parameters for design.
Elastic Response Parameters may be reduced for purposes
of design in accordance with the following items, with the limitation that in no case shall the Elastic Response Parameters be reduced such that the corresponding design base shear is less than the Elastic Response Base Shear divided by the value of R.

From this ubc97 statements i think we should apply the scaling factor(either magnification or reduction) to the ultimate or working load combinations.
for example if we have load combination 0.9D+1.0E
it would be 0.9D+ Scaling factor*E

Does people agree with me in this?

 

[Murali27]

Yes, you are correct.

Scaling factor to be applied on Eh (Horizontal Earthquake). Usually what we do, static method shear is to be compared to response spectrum modal shear and modal shears are scaled up or down to static method shear value.

-Murali