Scaling Factor for Response Spectrum 1

[nateTdot]

Hi,

When modeling with ETABS, what should be the scaling factor for the FIRST iteration of the response spectrum function?

Thanks!

 

[dawn836]

it should be the gravity acceleration depedning on units which yu use in modeling

 

[Taro]

If you are doing an elastic analysis, you would use gravity acceleration, g, divided by the response modification factor, R.

 

[Murali27]

Hi Taro,

When we scale up the modal shear to the static base shear, do the scale factor need to be multiplied with g/R and then multiply with spectral accelerations. Isn't?

I have one more doubt on the scaling the base shear. In one of my project, the flexible tower 50 story stands on the flexible podium (12 story), where do we need to compare the base shears. Is at the top of podium or at the base of the structure.

I am using UBC 1997 code

Thanks
Murali

 

[dawn836]

Well Murali check clause number 1631.5.4 it tells that you must scale reponse spectrum Parametes(shear and moments and displacments) with Factor equals to Base Shear calculated from Response Spectrum Analysis divided by Base Shear obtained from Eaerth quick Analysis as per caluse 1630.2

For this i understand that Base Shear must be equal in both methods for design purpose.

 

[Murali27]

Thanks dawn836

When we compare the base shears @ the base of structure, the scale factor will be 2.5. With this scaled spectrum forces, the total structure can be designed.

When we compare the cumulative seismic shear between static method & dynamic method @ top of podium, the scale factor will be 6, which is really over conservative for total structure.

Does the code requirement to match the story shear at each level?

By static method, Ft value is calculated based on the total mass of the structure, usually 25% Base shear, which will be acting at the top in the static method vertical shear distribution.

Dynamic method of vertical distribution is realistic with respect to spatial distribution of mass & stiffness.

Im my understanding, base shear to be compared at the base level of structure and the distribution of this shear needs to follow dynamic method distribution.

Thanks
Murali27