×
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

Parapet Height & Leeward Step

Parapet Height & Leeward Step

Parapet Height & Leeward Step

(OP)
I have a one story building abutting a two story building.  The upwind distance on the two story building is 250'.  My surcharge on the low roof from the leeward drift is quite high (about 78 psf).  However, my parapet height is 7' above the high roof elevation and I feel like there is some logic to reducing the lower roof drift surcharge, I just don't see the justification in ASCE7-98.  Any thoughts?

RE: Parapet Height & Leeward Step

I don't know the parameters of your specific roof, but carefully read 7.7.1, 7.7.2 and 7.8 (ASCE 7-98)

RE: Parapet Height & Leeward Step

(OP)
Thanks. I am thoroughly familiar with those sections, and have been through the commentary many times over, as well.  This is a point that does not seem to be covered.


                    !
                    ! parapet =7'
    lu-250'        !
--------------------
                   !
                   ! roof elev difference =20'
                   !
                   !----------------

My windward drift on the upper parapet is about 3.2 feet (based on 250' upwind length). Base snow is 1.05', for a total of 4.25'.  My parapet is 2.75' higher than my snow drift and, in my mind acts as at least a partial impediment to blowing onto the low roof.  Commentary section 7.7 implies that the upper roof snow is "blown clear" as shown in Fig 7-8 (2nd to last paragraph in section).  I wonder if that is really the case for a really tall parapet.  How tall, I do not know.  Hence the post for opinions.  In the end, I put all the leeward drift on the low roof, as if the parapet was not there. Better safe than sorry.

RE: Parapet Height & Leeward Step

I do not believe that you can use a parapet to "block" snow from drifting onto a lower roof.  The actual dynamic behavior of the snow is not linear (i.e. the snow does not translate perfectly horizontally but rather is blown upward and deposited below due to the wind lines around the building shape.

RE: Parapet Height & Leeward Step

I have wondered this before too

Thread507-3990

RE: Parapet Height & Leeward Step

Until I added forensic engineering/failure analysis to my workload I also questioned what seemed to be overly conservative code requirements.  I now feel that the snow drift articles of ASCE/BOCA are far from conservative and actually may need to be made more demanding, particularly the density formula and the length of the drift.  

I have investigated the failure of far too many buildings and roofs, some of which were designed in accordance with these codes and still failed under what was nothing more than a "once a decade" snowfall.

The vast majority of my investigations unfortunately revealed that the roof failed as a result of engineers either ignoring or misunderstanding snow drifting at changes in rooflines, parapets, etc.  

A code article is a minimum requirement and that is what the State Board will hold you to upon their investigation into any roof collapse.  If the situation is as black and white as you describe it, you better do it by the book or pray for warm winters.

RE: Parapet Height & Leeward Step

The lower roof should be designed for full snow accumulation.  The effect of the parapet will reduce the lower roof load a bit, but it's not likely that this can be determined.

The wind velocity drop will happen irregardless of the parapet and snow will still be 'precipitated' as a result of the reduction in wind velocity.  In addition, there will be small 'vortex' that will form due to the 'suction' developed...

RE: Parapet Height & Leeward Step

The code provides that, for drifting snow wind can be from any direction. Wind from one direction (behind the parapet) may reduce drift in that direction due to aerodydamic shading, however, from the other direction the parapet acts as a collector. The worst case must be considered.

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! Already a Member? Login


Resources

eBook - Integrating the Engineering Ecosystem
Aras Innovator provides multiple options for integrating data between systems, depending on the scenario. Utilizing the right approach to meet specific business requirements is vital. These needs range from authoring tools, federating data from various and dissimilar databases, and triggering processes and workflows. Download Now
Research Report - Simulation-Driven Design for SOLIDWORKS Users
In this engineering.com research report, we discuss the rising role of simulation and the paradigm shift commonly called the democratization of simulation. In particular, we focus on how SOLIDWORKS users can take advantage of simulation-driven design through two analysis tools: SOLIDWORKS Simulation and 3DEXPERIENCE WORKS. Download Now
White Paper - Industry 4.0 and the Future of Engineering Education
With industries becoming more automated, more tech-driven and more complex, engineers need to keep their skills and knowledge up to date in order to stay on top of this wave—and to be prepared for the Industry 4.0 future. The University of Cincinnati offers two online Master of Engineering degree programs designed specifically for practicing engineers. Download Now
eBook - The Design Gridlock Manifesto
In this eBook, you’ll learn 6 ways old CAD technology slows your company down and hear how design teams have put those problems to rest. “The Design Gridlock Manifesto” shares first-hand modern CAD experiences from 15 companies around the world. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close