Diaphragm Design Forces from Analysis

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ASCE 7 cl. 12.10.1.1 provides an equation for diaphragm design forces (Fpx) but also says they shall not be less than the forces from the structural analysis. Often, this is taken as the equivalent static force (Fx).

I am wondering if anyone uses the results from response spectrum analysis for their diaphragm design - either obtaining the diaphragm force transferred at each level, or using semi-rigid diaphragms and looking at membrane forces directly?

I find that there can be pretty significant differences between the two assumptions, even though they are scaled to the same base shear. Main factors seem to include if the diaphragm has an irregular shape, and if there are multiple SFRS elements, which lead to them interacting through the diaphragm. The differences are large enough that I wonder if this should be revisited in future editions of ASCE 7.

Thoughts?

-JA
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[driftLimiter]

I am seeing Fpx control a lot more often than the Fx from ELF. Even for single story buildings, this is because I work in an area with a high Sds and the Fpx min often controls the diaphragm force.

For mutli story buildings I would expect more often than not, the Fpx controls the design especially at lower levels.

I think it would be reasonable to use the diaphragm force determined by modal analysis as Fx, you might solve for this by calculating the story shears due to the modal response spectrum analysis.

Even still I would expect that Fpx controls for lower levels etc. Because the modal response of multistory buildings that are regular should be pretty similar to the vertical distribution of equivalent lateral base shear.

For structures with large mass or stiffness discontinuities perhaps the modal response varies quite a bit from the ELF.

I would suggest that the Fpx equation kicks into gear more with higher Sds. Effecting both the base shear and the minimum Fpx load (0.2SdsIe).

It would be valuable for this discussion to be able to provide an example where the diaphragm forces resulting from the modal analysis vary significantly from the equivalent lateral force procedure.

 

[KootK]

Clearly articulated or not, my expectation is that engineers would interpret ASCE7 as requiring design to the larger of:

a) the diaphragm forces Fpx or;

b) the structural analysis forces produced by the analysis method deemed appropriate for the rest of the building.

It doesn't feel rational to me to come to the conclusion that RSA is needed for the rest of the building but, then, ELF is appropriate for the diaphragms.

 

[Deker]

I haven't done the proof, but for ELF I don't think it's mathematically possible for Fx to be larger than Fpx with respect to inertial forces. If you have a transfer condition, then Fx may control over Fpx.

I've seen engineers use MRSA for diaphragm design, but the MRSA story forces aren't really compatible with the diaphragm forces determined by Eq. 12.10-1 which are based on ELF. If you're modeling the diaphragm elements directly, the forces produced in MRSA are already capturing higher mode effects in a way that's compatible with the analysis used for the vertical elements.

If you're not modeling the diaphragm, it gets a bit trickier as you have to determine the story forces for each individual mode and then perform the modal combination. There's also the question of whether it's acceptable to reduce all modes by the R factor or whether it's more appropriate to reduce the first mode only and use R=1 for all other modes.

It may also be worth taking a look at the commentary and resource papers on the development of the alternative diaphragm design procedure of 12.10.3 which is meant to produce more realistic diaphragm demands.

 

[HTURKAK]

My opinion is, as far as i understand ,

- Section 12.10.1.1 ( Diaphragm Design Forces) and Equation (12.10-1) the force Fi is the ELF determined from Equations (12.8-11) and (12.8-12). Apparently this is procedure for determining diaphragm forces and threshold value and upper limit in buildings. (Equation 12.10-1, 12.10-2, 12.10-3)

- Clause 12.10.3 of ASCE 7-16 is alternative method to determine diaphragm forces instead of the use of the ELF procedure.

- IMO, The diaphragm forces can be determined with section cuts above and below the floors of seismic forces
determined from the MRS analysis . But keeping in mind the force determined from shall not be less than equation (12.10-2). The use of R=1.0 seems overkill but the diaphragms shall not yield before MFRS..

- The following document is essentially for Precast Concrete Diaphragms but still a valuable document and it it is worth to look for CIP diaphragms also..

https://nvlpubs.nist.gov/nistpubs/gcr/2017/NIST.GCR.17-917-47.pdf

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