Modal Response Spectrum Analysis Drift in ASCE 7-16

[RobertHale]

I am trying to figure out what the ASCE 7-16 code intends for MRSA drift calculations.
Looking at section 12.9.1.4.2 of the code you have the following:

Furthermore, Looking at Equation 12.8-6, you have:

Based on those two provisions, is the intention NOT to scale the base shear for drift if you have a structure on a site where S1<0.6g or where Equation 12.8-6 doesn't control? Or is the intention to artificially perform a base shear calculation with the equation referenced (i.e. V' = [0.5*S1*Ie/R ]*W), then compare that with the modal base shear and scale if appropriate?

Thanks for any insight in advance.

Robert Hale, PE

 

[JoshPlumSE]

Robert -

From the commentary to 12.9.1.4:

Displacements from the modal response spectrum are only scaled to the ELF base shear where Vt is less than CsW and CS is determined based on Eq. 12.8-6. For all other situations, the displacement need NOT be scaled because the use of an overly flexible model will result in conservative estimates of displacement that need not be further scaled.

So, my interpretation is that ONLY when Eq 12.8-6 controls and Cs*W > Vt is the RSA displacement to be scaled up.

Note: I should point out that in past codes, we were required to scale up the forces (but not the drifts) for cases where Vt < 0.85*Cs*W. That's been removed because it was thought that RSA designed building did not have level of safety equivalent to those designed using ELF. The code makers went even further to suggest that in the one particular case, we now need to also scale up the drifts.

 

[HTURKAK]

I agree with your interpretation .

If we consider together with Response Spectrum ( FIGURE 11.4-1), apparently the basic formula to determine Cs is 12.8.2 and nominator Sds implies that T ≤ Ts and the upper limits for this value the formula 12.8.3 ( which is applicable when Ts <T ≤ TL) and the formula 12.8.4 ( which is applicable when T > TL )

The threshold values are calculated with formulas 12.8.5 and 12.8.6.

Eq. 12.8.6 is applicable when S1 is equal to or greater than 0.6g which can control at areas of high seismicity .
IMO, the RSA displacements should be scaled up for Eq.5. also. Consider a case S1=0.59g , the displacements will not be scaled up ???.

 

[haynewp]

What I find interesting is that ASCE 41 has no such minimum base shear limits for dynamic analysis and it is applied to all types of construction and of historic age.

 

[r13]

haynewp,

I think the difference is because ASCE 41 is for structural rehabilitation, while ASCE 7 is for new construction.

 

[haynewp]

Correct, but that is still a major difference to not have any minimum at all. I realize existing buildings are of course not held to the same requirements as new buildings are. But you can get a very low base shear in comparison, depending on the structure you are performing the dynamic analysis on.

 

[r13]

This linked article has a thorough review on ASCE 41. Pay attention to p25 and after. Link

 

[haynewp]

I don't see anything about scaling base shears. I have talked to the ASCE 41 committee head before and used ASCE 41 several times. I don't want this to go too far off the original topic.