ASCE 7-10 - Drift and Modal Analysis

[RobertHale]

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:
[ol 1]
[li]The commentary is correct and the provision of the code is in error.[/li]
[li]The commentary is in error and the provision should be applied even when I am not "near a major active fault."[/li]
[li]The commentary is in error but the code provision should only apply where S1 equals or exceeds 0.6.[/li]
[/ol]
I am leaning toward the third reading since the provision was added in the 2010 code cycle, and it looks like the commentary was just the 7-05 commentary with a new coat of lipstick. I just wanted to get the opinions of some other engineers that are familiar with MRSA.

 

[mike20793]

I don't have commentary for that section in either of my digital copies of ASCE 7-05 or ASCE 7-10. Based on a thread earlier this week, it looks like they have added quite a bit of commentary to chapter 12 in the latest printing of ASCE 7-10. It looks like it may just be the commentary from the 2015 NEHRP Provisions.

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.

 

[MrHershey]

Would tend to lean toward 3 similar to you. Modal response is typically more useful for highrises or longer period buildings when you can actually get significant contributions from higher mode shapes. So assuming the induced load from shaking follows the first mode shape (as ELF generally results in) would tend to be less and less appropriate as you get taller. Would think this provision was included to get drifts scaled just as forces do when there's a large amount of shaking at periods closer to the building periods that modal response analysis typically gets run for (>1 sec).

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

 

[wannabeSE]

Mike,
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/documents/107646.

(and it looks like they recommend scaling the base shear to 100% of the ELF base shear)

12.9.1.4.1 Scaling of Forces
Where the calculated fundamental period exceeds C_uT_a in a given direction, C_uT_a shall be used in lieu of T
in that direction. Where the combined response for the modal base shear (V_t) is less than 100 percent of the
calculated base shear (V) using the equivalent lateral force procedure, the forces shall be multiplied by
V/V_t:
where
V = the equivalent lateral force procedure base shear, calculated in accordance with this section
and Section 12.8
V_t = the base shear from the required modal combination
12.9.1.4.2 Scaling of Drifts
Where the combined response for the modal base shear (V_t) is less than C_sW, and where C_s is determined
in accordance with Eq. 12.8-6, drifts shall be multiplied by C_sW/V_t.

C12.9.1.4.1 Scaling of Forces
The scaling required by Section 12.9.4.1 provides, in effect, a minimum base shear for design. This
minimum base shear is provided because the computed fundamental period may be the result of an overly
flexible (incorrect) analytical model. Recent studies of building collapse performance such as those of
FEMA P-695 (the ATC-63 Project), NIST GCR 10-917-8 (the ATC-76 Project) and NIST GCR 12-917-20
(the ATC-84 Project) show that designs based on the ELF procedure generally result in better collapse
performance than those based on MRSA with the 15% reduction in base shear included. In addition, many
of the designs using MRSA did not achieve the targeted 10% probability of collapse given MCE ground
shaking. While scaling to 100% of the ELF base shear and to 100% of the drifts associated with Eq. 12.86
does not necessarily achieve the intended collapse performance, it does result in performance that is closer
to the stated goals of this Standard. This change also corrects the misleading implication from the previous
scaling criteria that MRSA achieves improved collapse performance relative to the ELF procedure.
C12.9.1.4.2 Scaling of Drifts
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 displacements 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. The reason for requiring scaling when Eq. 12.8-6 controls the minimum based
shear is to be consistent with the requirements for designs based on the ELF procedure.