×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

ASCE 7-05 Sliding Snow Provisions

ASCE 7-05 Sliding Snow Provisions

ASCE 7-05 Sliding Snow Provisions

(OP)
I need to calculate the snow load for a gable roof with a 3.5:12 pitch and 4'-0" high perimeter parapet. How do you calculate the amount and distribution of sliding snow that accumulates up against the parapet?

The "Cs" factor in section 7.4 reduces the the magnitude of "Pf" to account for the snow that slides off. However, the only specification section that specifically addresses sliding snow is section 7.4, however, it is specifically for calculation the accumulation of sliding snow on lower roof and therefore doesn't appear to be applicable.

Has anyone else run into this? Also, should the sliding component be superimposed on the the balance and unbalanced snow loads? Any help with this would be greatly appreciated.

RE: ASCE 7-05 Sliding Snow Provisions

I can't say that I've ever hear of this situation before and I am not away of any code provisions for it.  I would approach it in one of two ways.

1.  Equation 7-3 in ASCE 7-05 provides an equation for the density of snow.  You would have to come up with some way to estimate how high the snow would pile up behind the wall, and maybe look at impact, but design the parapet as a retaining wall.

or

2.  Prevent sliding from occuring in the first place by placing stops set behind the parapet.  Similar to when angle like pieces are put above doorways on the roof with metal roofing to prevent the snow from falling on people traffic in and out of the building.  (I found a nice website demonstrating this here http://www.snojax.com/ but there are other available products or solutions).  This would prevent you from designing for this situation in the first place, but you might want to check that your snow loads are appropiate for that situation.

RE: ASCE 7-05 Sliding Snow Provisions

I am not AWARE of any code provisions for it, sorry for the typo.

RE: ASCE 7-05 Sliding Snow Provisions

Drift provisions in the Canadian Code, Cf, take this case into account.

RE: ASCE 7-05 Sliding Snow Provisions

If it is not able to slide off of the roof because of the obstruction, then you should not use any reductions that consider that the snow is able to shed off of the roof.

As far as additional load because of the parapet blocking sliding, I am not sure about that one. It is essentially like a snow guard. The commentary on page 329 just says the following:  "When snow guards are added to a sloping roof, snow loads on the roof can be expected to increase".

Here is some useful information on your situation.
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd228_e.html

RE: ASCE 7-05 Sliding Snow Provisions

According to ASCE7 the Cs for a 3.5:12 slope is 1.0 unless you have a "unobstructed slippery" roof surface.  Use Cs=1.0 for your snow load as sliding snow probably is not applicable.

Use the drift provisions from section 7.8 - Roof Projections (ASCE7-05) for the drift loading at the parapet wall.  The drift at the parapet is 0.75Hd and you use Lu equal to the upwind roof length.

RE: ASCE 7-05 Sliding Snow Provisions

I think there can be additional load on the parapet when it is acting as an obstruction to prevent sliding. Adding the standard parapet drift plus the roof snow without reducing for sliding would be a minimum I would design the roof for in this case.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources