Minimum Roof Snow Load

[SteelPE]

I was recently awarded the design of a small metal building foundation (360 square feet.... IBC 2015). The jurisdiction requires the use of a minimum flat roof snow load (Pm) easy enough.

I was scanning through the loads the mfr placed on the building and I came across the following note:

Pm is based on the minimum roof snow load calculated per building code or the contract-specified roof snow load, whichever is greater. This value, Pm, is only applied in combination with Dead and Collateral Loads. Roof Sow in other loading conditions is determined per the specified Building Code.

Is this the correct way to apply minimum roof snow loads? It seems somewhat arbitrary, however, I am not sure of anything in ASCE or IBC that says how to properly uses this minimum Pm number.... I have always just used this number in all of my load combinations.

 

[WinelandV]

Hey SteelPE,

From ASCE 7-16, 7.3.4 Minimum Snow Load for Low-Slope Roofs

A minimum roof snow load, pm, shall only apply to monoslope, hip, and gable roofs with slopes less than 15° and to curved roofs where the vertical angle from the eaves to the crown is less than 10°. The minimum roof snow load for low-slope roofs shall be obtained using the following formula:
[formulas snipped]
This minimum roof snow load is a separate uniform load case. It need not be used in determining or in combination with drift, sliding, unbalanced, or partial loads.

Emphasis added by me.

Please note that is a "v" (as in Violin) not a "y".

 

[phamENG]

I agree it seems arbitrary. The only thing 7-10 really says about it is this:

This minimum roof snow load is a separate uniform load case. It need not be used in determining or in combination with drift, sliding, unbalanced, or partial loads.

It doesn't say you don't use it in combination with roof live load or wind load. I agree with (and also follow) your standard practice where you use the worst case snow load in all load combinations: drift, unbalanced, partial, or minimum uniform.

 

[SteelPE]

Maybe there is a disconnect with the code of the jurisdiction.

The metal building mfr uses the term Pm while the code uses the term Minimum Flat Roof Snow Load Pf... it goes on to say:

The design flat roof snow load shall be the larger of the calculated flat roof snow load using Pg or the value of Pf listed in this table

Somewhat something the local jurisdiction should be clearing up as they create the confusion.

 

[straub46]

Chapter 2 has the following exception/clarification for the snow load combos (The same exception is there for the LRFD cases as well)

I've always taken the minimum roof snow to be Dead + Minimum snow only. The roof snow, which is usually less than the minimum, is used in the combos with wind and live, and also used for drift, unbalanced snow, etc. Since the roof minimum snow is given a separate subscript from pf or ps, and has the wording quoted above by others as being a separate, uniform load case, I've always felt that is justified.

Go Bucks!

 

[phamENG]

Sounds like somebody just typed in the wrong subscript. Though you would think they would just modify ground snow loads so the P_m they want matches what ASCE 7 spits out. That would make it a little easier (for you and people locally - headache for anyone working from another state who doesn't know your jurisdiction has it's own map or ground snow load).

 

[phamENG]

Good point, straub46...I think that invalidates my last post. Perhaps the use of 'f' is intentional on the part of the jurisdiction. They want you to use the tabulated minimum in all load combinations since the exception still mandates either 'f' or 's' to be used.

 

[WinelandV]

Reading in the commentary, it says regarding the load combinations in 2.3.1:

Exception 2 permits the companion load S that appears in combinations 2 and 4 to be the balanced snow load defined in Sections 7.3 for flat roofs and 7.4 for sloped roofs. Drifting and unbalanced snows loads, as principal loads are covered by combination 3.

I find that to be unhelpful... Reading the commentary for chapter 7:

These minimums account for a number of situations that
develop on low-slope roofs. They are particularly important
considerations for regions where pg is 20 lb/ft2 (0.96 kN/m2)
or less. In such areas, single storm events can result in loading for
which the basic ground-to-roof conversion factor of 0.7, as well
as the Ce and Ct factors, are not applicable.
The unbalanced load for hip and gable roofs, with an eave to
ridge distance W of 20 ft (6.1 m) or less that have simply
supported prismatic members spanning from ridge to eave, is
greater than or equal to the minimum roof snow load, pm. Hence,
if such a hip and gable roof has a slope that requires unbalanced
loading, the minimum snow load would not control and need not
be checked for the roof.

What's the ground snow load that you're working with, and what is the local AHJ minimum?

Please note that is a "v" (as in Violin) not a "y".

 

[SteelPE]

So how does this all work with IBC 2015?

Can say that I have ever ventured much into Chapter 2 of ASCE-7 as the load combinations are pretty specific in IBC and seem to be independent of ASCE 7. I may be mistaken, but the exceptions outlined by straub46 are not in IBC.

Pg = 40 psf Pf min = 35 psf

 

[phamENG]

Hmm...no, it doesn't appear to be there.

So per IBC, there's no grounds to not use the higher value. But even if they were using ASCE 7, the AHJ's use of 'p_f' and calling it the "Minimum Flat Roof Snow Load" rather than "Minimum snow load for low slope roofs" to describe it would still mean that you have to use it in all load combinations.

So either way, the MBS manufacturer should be using it in all combinations.

 

[WinelandV]

I think, per the AHJ's use of a different minimum pf, that you should alert the manufacturer about the required pf (because that will affect their unbalanced loading and drifting).

It may send your owner's building to the bottom of the work stack.

I wonder if it may be possible to ask for an approval for the loading as-is from the AHJ?

Please note that is a "v" (as in Violin) not a "y".

 

[SteelPE]

Just a small bit of history I may offer.

The AHJ has always used a flat roof snow load of 35 psf. These loads are typically more than what you would find using the ASCE 7 ground snow load maps and calculated Pf in ASCE 7. About 10 years ago there were several roof collapses due to snow loads.... so it seems like the AHJ has some reasoning behind their requirements.