Snow Drift From Silos

[waytsh]

Hi All,

I would like to get some opinions on the following condition. We have a number of silos that are planned to go up near an existing building. The silos are going to be higher than the existing buildings low eave by about 11', they are about 12'-6" in diameter, and only off the low eave by about 1'. Here is a sketch to help visualize the layout.

The tops of the silos are slightly domed, rising about 24" at the center. I am trying to determine a reasonable approach to determining the snow drift this would create on the existing building. Ground snow for this area is 35 psf and I am designing to IBC 2018 (ASCE 7-16). Conservatively we could assume that the silos are a single structure 12'-6" x 53' and calculate the leeward and windward snow drifts off that. Do you think this would be too conservative? Is there a better approach?

Thanks in advance.

 

[dik]

I would design them for a regular snow accumulation like an 11' wall, and reinforce the existing roof accordingly.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[HTURKAK]

My points ;

-ASCE Clause 7.7.2 specify leeward drift of Section 7.7.1..

- Your approach could be overkill if the adjacent bldg roof is sloped roof without parapet..

My opinion..









Don't underestimate a nail. A nail saves a horseshoe, a horseshoe saves a horse, a horse saves a commander, a commander saves an army, an army saves a whole country.. GENGHIS KHAN

 

[waytsh]

I appreciate the input. There is no parapet on the lower roof and it is sloped but only at 1/2:12.

I had toyed with the idea of using an average separation distance rather than 1' for everything. Also perhaps reducing the leeward fetch by some amount since they are circular and not rectangular. The existing building is not going to be able to handle much, if any, snow drift from the silos. I was going to recommend providing them with a distance to move them away from the building so that they could avoid having to deal with the reinforcing. On one hand I would like to make sure we are covered with our assumptions, on the other hand I don't want to go too crazy on the conservatism.

I was also looking at the roof projection provision that does not require snow drift consideration if the length is less than 15'. Maybe if I had them move the silos a little further apart we could argue that we are meeting that provision.

 

[dik]

Average is not so good... due to the curved surface, the bulk of the 11' height is in proximity to the existing roof.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[ProgrammingPE]

As HTURKAK mentioned, you'll be using Section 7.7.2 for Adjacent Structures to determine the snowdrift load. Your snowdrift load will then vary based on the separation distance. The separation distance is 1 foot where the silos are closest to the existing roof, and it is about 3.6 feet between the silos. (3.6 feet is the diagonal dimension shown below.)

I don't know what your upwind length is, but there may not be that much of a reduction in load if you account for the increased separation distance between the silos. (For your case the leeward drift is the same load regardless of which separation distance is used since (6h - s)/6 does not have an impact.) Also, since you will have some areas of the roof that will need to be designed for the 1 foot separation distance, you'll likely just use the same reinforcing for all locations.

I was also looking at the roof projection provision that does not require snow drift consideration if the length is less than 15'. Maybe if I had them move the silos a little further apart we could argue that we are meeting that provision.

You may be able to make that argument for the windward snow drift since that is all that Section 7.8, Roof Projections and Parapets, considers. That section, though, does not take into account any leeward drift, which is the snow that comes off of the roof of the silos, so I don't think you can ignore it based on that provision.

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