×
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 7-10 Fig. 28.4-1 Sidewalls and Flat Roofs

ASCE 7-10 Fig. 28.4-1 Sidewalls and Flat Roofs

ASCE 7-10 Fig. 28.4-1 Sidewalls and Flat Roofs

(OP)
I have a 2 part question about ASCE 7-10 Fig. 28.4-1. I've mostly used Ch. 27 for MWFRS wind, so I'm less familiar with Ch. 28.

First, why does load case 'A' not have side wall pressures?

Second, when you have a flat roof (zero degrees), you no longer have a definitive ridge, so which load case applies in which direction? Could you consider one load case for both directions?

If you could use one load case for flat roofs I would think 'A' is more applicable. The negative wind pressure on the roof is higher toward the windward edge and lower toward the leeward edge, which is very similar to how Ch. 27 would have you do it. But then it still makes me wonder why load case 'A' doesn't have sidewall pressures; if I were using Ch. 27 it would give me sidewall pressures.

Side note: I'm aware of footnote 7 regarding flat roofs, but that still doesn't seem to address my question.

Thanks for any help.

Edit 3/26/17 12:04pm: fixed one of the chapter references.

RE: ASCE 7-10 Fig. 28.4-1 Sidewalls and Flat Roofs

We work with this chart all the time, or at least the computer programs we coded to match the chart do. Our take on your first question is that the two sidewall suctions are equal and opposite so don't have any particular impact in regard to a MWFRS solution. The magnitudes of Load Case B are in the same range and will accommodate the net forces in that direction. C&C values will be higher and control all of the individual wall components for all walls.
We don't tend to do purely flat roofs but do single slopes a lot. Notes 7 and 8 tend to cover that condition. Load Case A was originally created to deal with the primary framing orientation, i.e., frames spanning from eave to eave with a ridge in the "center". Load Case B is used in the direction where bracing as opposed to primary framing is used. At least that is our take on the process.
IF you think of wind at the corner of the building as the illustrations show, you will have a zone on the roof of higher pressure regardless of whether you are using A or B. Also Zone 2 pressures are higher than Zone 3 pressures, although not to the same extent that 2 and 2E or 3 and 3E differ.
Obviously if you have a small enough building with a flat roof and joists only spanning the full length being walls, i.e., no internal primary framing menbers, you get to pick and choose which direction is which although I would probably still run both load cases (one way or the other) just to be sure of coverage.

RE: ASCE 7-10 Fig. 28.4-1 Sidewalls and Flat Roofs

(OP)
Thank you for the detailed response, that makes sense.

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