Snow Load and Effective Seismic Weight

[SteelPE]

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?

 

[EngineeringEric]

The True-Engineer in me says to use the number which will result in the largest lateral load (7 psf), but if you know that the structure will not actually see the loads then i would use less. I think since the roof will only see 27psf under true ASCE 7 calculation then using ASCE lateral the EQ Weight would be 0 psf. The county can provide the minimum load for gacity design but they are not specifying the required load for a seismic condition.

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.

 

[jittles]

That's an interesting question. I have run into a similar situation, but it wasn't an issue of that 30 psf threshold

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).

 

[SteelPE]

I'm leaning towards using the additional 7 psf. I don't think it is going to make much of a difference in the design as the roof is only 16,000 square feet or so.

 

[PicoStruc]

For your information, in Canada we use 25% of the snow load in seismic weight, no matter the snow weight.

 

[Archie264]

If it doesn't make a difference just use it. On projects where it might make a difference you might want to consider calling the county and getting them to go on record with their thoughts/intent regarding the issue. After all, at the end of the day they're the governing jurisdiction and it's their criteria that must be met. May as well find out what it is.

 

[Hokie93]

This situation has been clarified in ASCE 7-10. Using that standard, you would not need to include 20% of the snow load in the effective seismic weight in your situation because ASCE 7-10 makes a clear distinction between the calculated flat roof snow load (p_f) and the minimum snow load for low-slope roofs (p_m). For effective seismic weight calculation purposes, use the calculated flat roof snow load (p_f), rather than the minimum roof snow load (p_m), per ASCE 7-10 chapter 12.7.2.

 

[TDIengineer]

Steel,
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.

 

[msquared48]

Where I am, I would use the 7 psf additional, and with a normal dead load of 15 psf for a wood or metal roof it could very well make a diffence - up to 50% additional base shear for seismic. Wind may still control however.

Mike McCann
MMC Engineering