Return to index: [Subject] [Thread] [Date] [Author]

Re: Seismic and snow

[Subject Prev][Subject Next][Thread Prev][Thread Next]
That makes no sense.

If you have a high snow load, it is reasonable to assume that a seismic event is likely to occur while snow is on the roof (20% is what is assumed to be the intersection of the probability curves).

It is not reasonable to assume that seismic events may only happen while snow is on the building. I have always assumed that the 0.6D number was there because we (SEs) traditionally overestimate the dead loads in order to account for the worst case loads (sprinklers, MEP, finishes, two layers of asphalt shingles, etc.). Since the conservative approach is to UNDERestimate the dead loads in the case of uplift, the code makes it easy on us and lets us take a factor. Adding a seasonal snow load to offset a seismic uplift seems non-conservative.

Jason Christensen wrote:

Food for thought.
I just designed a building that had a 100psf flat snow load, with a high
seismic area (SDC = D).  The building was of light frame construction with
wood shear walls.  I added the twenty percent of the snow into the design
base shear as prescribed by code.  Then when I designed my shear walls the
controlling load combo for uplift was 0.6*D + 0.7*E.  Here is my question,
another engineer I talked to mentioned using twenty percent of the snow load
in addition to the dead load for the uplift design.  His reasoning was 'if
the snow has been added to the seismic base shear then it will be there for
the uplift also'.  This makes sense to me, however I cannot find anything in
the code that would say this is ok.

Your comments would be helpful.

Jason




******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********


******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********