×
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

HEADER DESIGN FOR DORMER JACK RAFTER

HEADER DESIGN FOR DORMER JACK RAFTER

HEADER DESIGN FOR DORMER JACK RAFTER

(OP)
hello,
I am reviewing and stamping a set of plans for 44x28' A frame house with  12/12 pitch roof. the owner wants me to add a dormer for the upstairs bathroom. on the building section I noticed a skylight detail with a header above the skylight. the header is shown square to the rafters (rafters are 2x12 @ 16" SPF #2).
question: would this header be designed to act as a normal beam with strong axis and applied loads vertical, or is it the more complicated out of plane bending.
the dormer will be 8' wide so the header will have significant load on it. the header will be framed into tripled rafters on either side of the dormer. thanks in advance for any advice.

RE: HEADER DESIGN FOR DORMER JACK RAFTER

You should be pretty conservative if you design that dormer as if it had a flat roof.  I dont really follow your explanation about the skylight.  Where is the skylight?  Is it on the roof or on the exterior wall of the dormer?  you said that the dormer is 8' wide.  I dont understand how its possible to have a skylight 8' wide unless it is on the wall, not the roof.

RE: HEADER DESIGN FOR DORMER JACK RAFTER

(OP)
the skylight is already detailed on the plans on the other side of the roof - it is not in the dormer. when i saw how he had it sqaure to the rafters i started asking myself how i would design it being that it is not plumb.

RE: HEADER DESIGN FOR DORMER JACK RAFTER

i'd change that detail to have the header plumb.  A good framer will be fine with that detail and it simplifies your design.

RE: HEADER DESIGN FOR DORMER JACK RAFTER

If I understand you right, the header will be oriented like a "diamond" shape.  If so, you do have to design for biaxial bending.

DaveAtkins

RE: HEADER DESIGN FOR DORMER JACK RAFTER

There's an old rule-of-thumb that says a dormer can be as wide as 3 rafter spaces without becoming 'structural', whereby the double-rafters under the dorner walls provide the same amount of wood as if there was no dormer. This yields a 4' wide dormer.

In the above problem, the 8' wide dormer becomes structural. Shouldn't the designer be equally concerned about the triple 2x12 in-construction rafter beam on each side of an 8' wide dormer, which receives the point-load of the header in question? If the rafter-beam was spanning the long direction (44'), LVL might be required, especially if you're trying to hold l/360.

RE: HEADER DESIGN FOR DORMER JACK RAFTER

(OP)
the rafters are spaning the short direction so the projected span is 14'. the triple 2x12 are my starting point for the design check becasue that is what the dormer removes. if that does not work we will spec LVL's, thanks for that suggestion.

RE: HEADER DESIGN FOR DORMER JACK RAFTER

ETHICAL/LEGAL RESPONSE
From your posts ("reviewing and stamping a set of plans", "design check") it sounds like you are "plan stamping" this project; you are perhaps stamping a set of plans drafted by the owner/contractor/designer and not prepared in your office. If this is so, then I would respectfully suggest researching the ethics and legality of perform professional services in such a manner, as many licensing boards take a dim view of such a practice.

TECHNICAL RESPONSE
As another post suggests, I would simply detail the header as plumb, otherwise you should consider biaxial flexure in the header.  In reality, as the diaphragm/sheathing is far stiffer than the weak axis of your proposed header, you can also consider the weak axis as "braced" IF you detail the load path of this bending component into the sheathing (like a sub-diaphragm).  You would need to check the sub-diaphragm shear and develop the sub-diaphragm chord forces back into the main diaphragm.  You could also maybe put a collar tie in that location...

RE: HEADER DESIGN FOR DORMER JACK RAFTER

I do not see the blantant plan stamping, myself. The poster is designing the elements, just someone else is drafting it.

Don Phillips
http://worthingtonengineering.com

RE: HEADER DESIGN FOR DORMER JACK RAFTER

(OP)
the ethical question prompoted me to do some light reading in the NYS engineering manual. they don't mention plan reveiw and stamp as an unethical activity.
I think I am going to make the header plumb. thanks for all the input - its what makes this forum great.

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