Snow Drift Seismic Mass 1

[MTLoeb]

Hi, I was wondering about this the other day and am curious to know if any states/counties amend the code to require seismic mass consideration for snow drift. I know our ASCE 7-10 20% of uniform >30 accounts for the lower probability of having design snow and a seismic event, but it seems to me that in areas of high snow loading you often have drifts that last all season. And, drifts are denser and less likely to move in a seismic event. This wouldn't really be a consideration for the total seismic mass of a building, but when looking at something like canopy attachment I could see a value. Has anyone seen or done this?
Thanks

 

[KootK]

- I've thought about this but never done it or seen it done.

- Certainly, I see some sound logic in it.

- EQ design is rough stuff by nature. And, in North America, we're really designing for ductility more than we're designing for a particular load. Which makes the whole exercise much rougher still. As such, I'm inclined not to get too excited about situations where drifts aren't affecting large areas of the building simultaneously.

- Your canopy example is actually a pretty great instance where a higher standard would strike me as reasonable. At the same time though, I feel as though elements that would be dominated by drift like this will tend to also me elements that I would consider to be less mission critical than, say, whole building seismic resistance systems. So, in that respect, perhaps things sort of balance out naturally to a degree.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[skeletron]

BCBC 2018 specifies 25% of the snow load as calculated in the snow load subsection. So, I guess linguistically speaking, this leaves the designer some allowance to incorporate any irregularities that may occur in the snow load due to drift. I'm not sure if I would go through the precision activity of calculating the exact snow load distribution. Rather, I would take an average of some sorts if there was substantial drift or accumulations in a snowy part of the country.

 

[Dowels93]

The Ontario Building Code (OBC) also requires "25% of the design snow load specified in Subsection 4.1.6" to be included, which technically includes snow drifts. That being said, I would - and typically do - take a snow load for seismic that accounts for drifts through rounding up in broader areas (i.e. if an area increases from 2.2kPa to 3.0kPa from a 5m long drift in a 10m wide area, I might use 2.4kPa for the whole area) rather than determining every individual drift (especially with how often mech. units move on me).

 

[MTLoeb]

All good points, I do like the 25% of snow in the BCBC and OBC codes. Thanks for the input. KootK, your absolutely right with the ductility, so maybe a few extra pounds of seismic weight wont have too much effect in the big picture as long as you provide a continuous and ductile load path. I guess i'll just keep it as something to consider in the future, especially for canopies that cover doors in high snow regions.

 

[glass99]

ASCE07-10 12.7.2 tells you to use 20% of the uniform design load where the flat roof snow load exceeds 30psf for seismic mass.

Interesting about the OBC.

EQ design is rough stuff by nature

R factors drive me crazy. Just divide by four! oh wait, just divide by 6. Oh wait, its an anchor, just divide by one!

 

[dold]

R factors drive me crazy. Just divide by four! oh wait, just divide by 6. Oh wait, its an anchor, just divide by one!

I was listening to one of the AISC Steel Profiles podcasts where they interviewed Rafael Sabelli. He was recalling one of the council meetings where they were discussing what R values to assign to different systems. When he got to OCBF and how they came to the value of 3.25, he joked with something along the lines of: "...at some point after discussing the seemingly arbitrary values we were assigning, I suggested we just use R=Pi for OCBF..."

He also self-identifies as an eccentrically braced frame.

Its episode 24 for those interested.

 

[glass99]

dold: god, I suspected but never knew. Just use pi!

I think R factors are actually level of precision less than one significant figure. If you were using base three, they would be one sig fig. Meanwhile, all other seismic calcs are lovingly completed to 4 sig figs.