Calculating Seismic Loads Due to Snow. 1

[SteelPE]

I asked this question a long time ago (maybe 8 -10 year ago). At the time I didn’t get much response.

In regards to calculating seismic loads in accordance with ASCE (specifically 7-10). ASCE 7-10 requires you to include 20% of the seismic loads for flat roof snow loads greater than 30psf.

In many jurisdictions I work in, the code will provide a Pg and a minimum Pf. Often Pf is greater than what you would calculate using Pg. For example, a structure with Ce=Ct=Is=1.0 with a code required Pg=40psf would yield a Pf= 28 psf snow load (in accordance with ASCE 7). However, the local code requires a minimum Pf = 35 psf for this area.

So, the question is, how would you handle this with regards to calculating seismic loads. Would you use the 28 psf (therefore no additional seismic load) or 35 pf (7psf additional load)? From what I can tell, the local code gives no guidance as to what should be used.

I can see an argument both ways.

 

[Rabbit12]

My initial thought would be to use the 28 psf for seismic calculations with the reasoning being the minimum snow load is likely intended to ensure you don't under-design gravity members.

This is similar to how I calculate drift loading. I use the calculated flat roof snow load not the code mandated minimum.

Ultimately, this is sort of up to the AHJ. I could see them requiring you to use the larger and thus adding 7 psf to your seismic load.

 

[r13]

Be consistent. The local raised snow load for a reason, thus they are not likely let it squeeze through with deviation from local intent.

 

[phamENG]

I think it would be worth a conversation with the AHJ, or perhaps a little research into the history of the ordinance requiring the higher snow load. If there was a run of collapsed or damaged buildings do to under sized gravity members, the increase probably occurred as Rabbit12 suggests. The question, then, is whether or not it was an arbitrary number or one based on actual data. If arbitrary, has ASCE 7 snow load calculation improved since the local change? If it has, then you'd probably be okay using the ASCE 7 snow load for the ASCE 7 seismic load - it's written so the probabilities of the various environmental loading effects align. If the local ordinance is based on a statistical study of local weather, then you'd be better off using the more accurate, local data.

Then again, does it really matter in practice? 7psf isn't going to add a lot of seismic force, is it? I can see it adding up on large single story warehouses or PEMBs with super low dead loads, but most conventional construction should be able to take that in stride without significant impacts on the final seismic loads or LFRS design. In Boston, with R=3, you're looking at Cs = .077. So you're getting an extra 0.54psf in lateral load on your snow covered surfaces. A 2.7k increase for a 5000sf footprint doesn't seem all that bad to me.

 

[HTURKAK]

Pls look ASCE 7 ,C12.7.2 Effective Seismic Weight :......3. 20% of significant snow load, pf > 30 lb∕ft2 (pf >
1.4 kN∕m2....

....The full snow load need not be considered because maximum snow load and ,maximum earthquake load are unlikely to occur simultaneously and

loose snow does not move with the roof.

 

[SteelPE]

I am not a code buff, but my guess as to the minimum snow load as follows:

20 years ago, the jurisdiction used their own version of a national code (BOCA I believe). Snow loads were given for every city withing the jurisdiction. Snow loads used were higher than those recommended in ASCE 7. 12 years ago there was a code change to come into line with the IBC. Snow loads actually went down slightly when the code change happened. A few years ago there was another code change. This introduced minimum flat roof snow loads which seem to be in line with those used by the code 20+ years ago.

There were some roof collapses in-between, but I am not directly sure if they were the result of using the lower loads of the code put into effect 12 years ago.

Is 7 psf going to make a difference.... well, it's a considerable portion of the dead load we use, so, yes, using the 7 psf will make a difference on some structures. Especially on the 100,000 square foot building we design from time to time (phamENG already suggested that). That's an increase in seismic weight of 700,000 lbs. It is rare for us to design something in the 5,000 square foot range, and if we do, usually wind will control.

My copy of ASCE 7-10 does not have a section C12.7.2. What does that section say?

 

[HTURKAK]

....My copy of ASCE 7-10 does not have a section C12.7.2. What does that section say?

ASCE 7-16 ( C12.7.2 Effective Seismic Weight. During an earthquake, the structure accelerates laterally, and these accelerations of the structural mass produce inertial forces. These inertial forces, accumulated over the height of the structure, produce the
seismic base shear.When a building vibrates during an earthquake, only that portion of the mass or weight that is physically tied to the structure needs to be considered as effective. Hence, live loads (e.g., loose furniture, loose equipment, and human occupants)
need not be included. However, certain types of live loads, such as storage loads, may develop inertial forces, particularly where
they are densely packed.
Also considered as contributing to effective seismic weight are the following:
1. All permanent equipment (e.g., air conditioners, elevator equipment, and mechanical systems);
2. Partitions to be erected or rearranged as specified in Section 4.3.2 (greater of actual partition weight and
10 lb∕ft2 (0.5 kN∕m2) of floor area);
3. 20% of significant snow load, pf > 30 lb∕ft2 (pf > 1.4 kN∕m2) and
4. The weight of landscaping and similar materials.
The full snow load need not be considered because maximum snow load and maximum earthquake load are unlikely to occur simultaneously and loose snow does not move with the roof.)

I just want to remind again,loose snow does not move with the roof ..that is , loose snow should not be added to seismic weight calculation.

 

[Rabbit12]

HTURKAK, I don't understand what you mean by "loose snow".

It says right there in the code you reference that we must consider 20% of the snow load for pf>30 psf in our effective seismic weight calculation.

 

[phamENG]

I think the idea is that, by reducing the considered weight of snow by 80%, you are accounting for the fact that you probably won't have the snow storm of the millennium the same day you have the earthquake that sends California out to say aloha to Hawaii, and also the fact that some portion of the snow near the top of the heap will not be sufficiently consolidated by the weight above it to move with the building.

I wonder how that works in areas that require the rain on snow surcharge loading?

 

[HTURKAK]

HTURKAK, I don't understand what you mean by "loose snow"....

We are at different time zones and i see ur comment just at the morning.

Significant snow load is assumed to be pf > 30 lb∕ft2 .That is, approximately 40 in. snow depth . The friction force between loose snow crystals and between the roof is not enough to move with the roof for the shallow snow depths during seismic event. If the snow depth reaches to significant level (which the code suggests 30 psf ) lower most portion will be confined and will start to move with the roof during seismic event.
The code suggests consideration of snow loads is not required in the effective seismic weight calculation if the design snow load does not exceed 30 psf . If exceeds, consider 20% of the snow load.

....I wonder how that works in areas that require the rain on snow surcharge loading?

If it rains on snow, the snow will start melting and the maximum local design snow load will not occur. In case of short duration rain on snow build up and then freezing temperatures, the snow will get frozen and stick to roof. I did not experience such a case but,i saw the heat transfer from the roof , melted the lower portion and got frozen . I think 20% of significant snow load covers this situation also.

If you are in doubt, add the full snow load to seismic weight. The codes says (is not required, not necessary etc..) rather then a must.

 

[Rabbit12]

HTURKAK thanks for the explanation but, I'm not sure why you're putting so much effort into explaining why you use 20% vs the full snow load for effective seismic weight calculations. I think everyone here understands the code and how it pertains to calculating effective seismic weight.

However, that's not the OP question. He's asking if he should take 20% of the (potentially arbitrary) minimum p_f or the p_f calculated using the code prescribed p_g.