ASCE 7-10 - Drift and Modal Analysis
ASCE 7-10 - Drift and Modal Analysis
(OP)
I am working on a moderately tall building with 6 levels of parking that are more or less 3x the size of the typical 12 stories of tower plate. I thought it would be a good idea to run a modal analysis in order to get a better vertical distribution of force. Everything with the strength analysis has been pretty straight forward, but I have been perplexed by provision 12.9.4.2 Scaling of Drifts. The provision points back to a base shear calculated using equation 12.8-6 which only applies when S1 is greater than 0.6 (and the seismic commentary bares this out stating the equation is only for sites "near major active faults). Additionally if you go to the commentary section C12.9.4.2, it cryptically states "Displacements for the modal response spectrum are not scaled because the use of an overly flexible model results in conservative estimates of displacement that need not be further scaled." I am left considering three options:
- The commentary is correct and the provision of the code is in error.
- The commentary is in error and the provision should be applied even when I am not "near a major active fault."
- The commentary is in error but the code provision should only apply where S1 equals or exceeds 0.6.






RE: ASCE 7-10 - Drift and Modal Analysis
Anyways, I can see where the confusion comes from. Since the commentary states, "The reason for requiring scaling when Eq. 12.8-6 controls the minimum base shear is to be consistent with the requirements for designs based on the ELF procedure," it's hard to tell whether the code is trying to refer back to either equation 12.8-5 or 12.8-6, whichever applies, or 12.8.6 is a limit regardless of your location. Since they are trying to be consistent with ELF, I'm thinking it could be either 12.8-5 or 12.8-6 since they are the minimum for the ELF procedure.
RE: ASCE 7-10 - Drift and Modal Analysis
An article from SK Ghosh in PCI would seem to back this up:
Provision has been added for scaling of drifts where the near-fault minimum base shear equation (ASCE 7-10 Eq. [12.8-6]) governs.
Link
RE: ASCE 7-10 - Drift and Modal Analysis
ASCE 7-10's expanded commentary is similar to (same as) the FEMA P-750, 2009 NERHP provisions. I just noticed the 2015 NEHRP provisions, FEMA P-1050. They have a little more discussion on scaling the drift, https://www.fema.gov/media-library/assets/document.... (and it looks like they recommend scaling the base shear to 100% of the ELF base shear)