Scaling up Dynamic Base Shear

[KenStructural]

Scaling the dynamic base shear as per section 12.9.4 of ASCE Seismic Design criteria is not that clear for me on which mode shape is to be considered for the scale up factor (0.85V/Vt), i believe it has something to do with the mass participation of the building. Say for example mode shape 1 and 2 is translation about orthogonal directions and mode shape 3 is rotation, if you will try to look at the mass participation of these first three mode shape is less than 90% as required by the code. I don't have the complete understanding on scaling up the dynamic base shear, is the dynamic base shear (V) and static base shear (Vt) with mode shape of at least 90% mass participation would i consider for the computation of scale factor U=0.85(V/Vt), by the way im using ETABS software for the analysis. Any explanation is much appreciated.

 

[structSU10]

You should use as many modes as needed to achieve 90% mass participation, be in 3, 5, 10 etc. The base shear reported from the combination of the modes is what would be adjusted per ASCE.

 

[UcfSE]

You don't scale an individual mode shape. You scale the final base shear, which is your combined value (SRSS or CQC) based on all mode shapes considered.. Just make sure you include enough mode shapes in the analysis to get the 90% participation.

 

[KenStructural]

strucSU10/UcfSE,

Thanks, now I have a clear picture for the required number of mode shapes to attain at least 90% mass participation, though i have a follow up question for this. Let say i have 10 number of mode shape and upon running the analysis say mode shape 7 has already attain the 90% mas participation, is these simply means that 7 modes will do? or since it was explicitly stated in the code the word "at least 90%" the desirable mass participation should be 100%.

 

[structSU10]

Yes, by code as long as you have 90% participation the analysis is adequate. If you have a simple building, and if its modeled with rigid diaphragms, it will not take many modes to achieve this (3 DOF per floor for rigid diaphragms, 2 translation, 1 rotation). If you have a semi-rigid diaphragm modeled most programs have more DOF's per floor (2 translation per discrete node), and would require more modes. Basically the 90% rule helps when you have a lot of DOF.

 

[UcfSE]

Yes, if you get 90% participation in 7 modes, you don't need to run further modes. It doesn't hurt if you do either.

 

[DCBII]

Use at least enough modes to get 90% mass participation between all of them when combined. CQC or SRSS is required by code for combining modes. I usually use CQC for 3D models. It better accounts for closely spaced modes.

The scale factor is used to acount for two effects:
(1) Importance
(2) Ductility

To scale results, simply multiply them by I/R.

Usually a response spectrum analysis results in a smaller base shear than the equivalent lateral force procedure base shear. The code limits the amount of base shear reduction you can take to prevent you from having less than 85% of the equivalent lateral force procedure base shear.

 

[KenStructural]

DCBII,

Thanks a lot for further explaining, is it okay if say the ratio of Static Base Shear EQX and dynamic base shear SPECX (EQX/SPECX) is almost 90, coz some of my recent job I'm arriving with a scale factor of 10 or lesser, but this time I'm arriving with scale factor of 90.