Unbalanced Snow Load on Hip-Gable-Hip Roof

[trackman417]

Long time lurker.

All questions I have had had been found in these forums. Now at a loss for checking on the unbalanced snow load(per ASCE 7-10) for the following roof.

I already performed everything required for the unbalanced snow load on the gable that runs the length of the building - but my question is what do I do with the unbalanced snow load at the valleys that are made where the two roof types intersect(when wind is running along the length of the building)?

My best guess(now that I am writing this post) is to stick with chapter 7.6.1 of ASCE 7-10 and just add the surcharge on top of the unbalanced load already applied to the gable rafters, and be extremely conservative? Or use one or the other?

The two hips that run perpendicular to the length of the building are both 8/12 slope(architect may make them steeper - have an RFI out). The gable roof that runs the length of the building is less steep at 5.08/12.

Let me know what you think

 

[DaveAtkins]

Personally--and I expect some disagreement from others--I think you have done enough. Design for unbalanced snow on the gable, which is the predominant roof shape, and don't worry about the small amount of snow which will build up in the valleys. There just isn't enough engineering fee for that type of building to finesse the design.

DaveAtkins

 

[ajh1]

You might consider the provisions that were in the old UBC 97 code. I think that is the most recent US code that really went into a snow buildup due to intersecting roofs like you have. As I recall those provisions didn't apply until the roof pitch was at least 2:12.

 

[trackman417]

Ended up running a model in Tekla Tedds(trial) to see what they think it should be.

The attached ends up being a section where the two ridges intersect along the length of the building. Have a 1 degree slope because it would not allow a slope equal to 0. Ran the same calc for different lengths.

Will just design for I*Pg for the whole roof. Seems reasonable since the architect is specifying 2X12's minimum - Mine as well put them to work!

Have an email out to them and see where this is stated in ASCE 7-10, or if this is just Tekla/Trumble being conservative. Will post what they say when I receive a response from them.

Thanks for the help

 

[XR250]

Personally--and I expect some disagreement from others--I think you have done enough.

I think I have checked that once and it did not seem to control anything so I have never checked it since.
Def. not enough fee to model it.

 

[TehMightyEngineer]

Agreed with all the above, not worth the trouble. If you seriously need to consider it I'd just come up with a rough additional uniform snow load over an area I felt was subject to drifts.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[trackman417]

Just confirming the program was erring on the side of caution with the roof, so it kept with the 20psf for the whole area outlined above.

Thanks for all the help!