Minimum seismic diaphragm force

[wikidcool]

ASCE7-16 Section 12.10.1.1 provides horizontal diaphragm design force equations. For a simple one-story building, I've never seen the minimum force equation 12.10-2 applied in design (Fpx = 0.2*Sds*Ie*Wpx). In talking to an SE friend who works on schools (subject to pretty robust plan reviews), she said she's never used it either and has never been called on it. Her opinion was it didn't really apply to single-story buildings but I don't see any language saying it wouldn't. The minimum force level governs on a lot of projects I work on. What am I missing?

 

[JoshPlumSE]

Yeah, that's my impression too. For multi-story buildings the acceleration you apply to each floor level varies with height, right? So, there is a danger that the lower floors will be designed for such a low acceleration that the diaphragm could still experience localized forces that could damage it. So, just a lower bound to make sure those diaphragms are safe.

My impression (though I don't have any publications that back this up) is that you have to be about a 5 story building before it's likely to kick in. But, I've never tried to verify that with calculations.

 

[driftLimiter]

The code doesn't offer any exception that I read out of the minimum level of diaphragm force, even the alternate method uses the same required minimum 0.2 Sds Ie.
We always use the minimum diaphragm force from this equation wherever it controls over the direct MLFRS diaphragm force.

If you compare the MSFRS force F = Cs * W for peak acceleration vs the Diaphragm force Fpx = 0.2 Sds I Wp.(see attached) The diaphragm force doesn't control until you are using an R factor of 5 or greater.

Equation 12.10-1 applies to one story buildings as well, you just arrive on the same force for the MLFRS and the Diaphragm Component. I.e. Fp = F, so then you check against 12.10-2 and 3.

From a seismic principles perspective, we don't want the diaphragm to be the weak link in the chain (i.e. no yielding at all), and we need it to be able to transmit load up to the yielding element's ductile load level and account for material overstrength effects, and uncertainty.

TL;DR I don't think your missing anything in my practice this force governs on a lot of projects too even 1 story homes.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1649462173/tips/diap_force_comparison_hrvov3.pdf[/url]

 

[ggcdn]

I agree with @driftLimiter, I find it comes into play quite often. But I am usually designing Stories>1 and R>5

A related question for the diaphragm experts out there - how are you treating a diaphragm below grade (below the seismic 'base')?

Do you design for transfer forces only (ie the forces transferring between the above grade SFRS and bsmt walls)?

Or do you also assign some inertial load to it (and if so, how much)?



-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think

 

[wikidcool]

Great info, thank you all. I'm usually doing wood-framed structures (R=6.5), 1-2 stories.