Snow Load and Effective Seismic Weight
Snow Load and Effective Seismic Weight
(OP)
I am doing a project in a jurisdiction where they have a ground snow load of 35psf and a minimum flat roof snow load of 35psf. Now, when I run the calculation I am getting a required flat roof snow load of 27psf or so (I=1.1) which is less than the required minimum of 35psf, so I am using Pf=35psf. Now, when it comes to calculate the effective seismic weight which value do I use. In accordance with ASCE7-05 if the Pf>30psf I must use 20% of the snow load. So there is a difference of either using 0 psf or 7 psf. What would others use?






RE: Snow Load and Effective Seismic Weight
IMHO: Will there ever be an EQ when the structure sees 35psf snow(7psf), No. Will it ever see an EQ with snow at 27psf, maybe so use 5.4psf.
But... when in doubt make it stout, 7 psf aught to do.
RE: Snow Load and Effective Seismic Weight
The minimum flat roof snow load where I am is 40 psf. Ground snow is 50, so with certain factors the calculated roof snow load can be less than 40. Our standard procedure is to include the 8 psf seismic weight (.2 * 40) and not reduce it, mainly because it's usually pretty close anyway.
However, we have had a project where we were designing the foundation for a PEMB. They calculated the seismic snow based off of the calculated roof snow load (that was less than the 40 minimum) and the powers that be bought off on it. Again, this value did not cross the 30 psf threshold, so it wasn't as big an issue (maybe a 1-1.5 psf difference) as your situation (7 psf difference).
RE: Snow Load and Effective Seismic Weight
RE: Snow Load and Effective Seismic Weight
RE: Snow Load and Effective Seismic Weight
RE: Snow Load and Effective Seismic Weight
RE: Snow Load and Effective Seismic Weight
I would include the snow mass assuming you are in a relatively mild seismic zone. Better safe than sorry. Earthquakes can happen with snow on the structure and as you said it probably won't change the design anyway.
RE: Snow Load and Effective Seismic Weight
Mike McCann
MMC Engineering