HEADER DESIGN FOR DORMER JACK RAFTER

[rnorth]

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.

 

[COEngineeer]

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.

 

[rnorth]

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.

 

[ctcray]

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

 

[DaveAtkins]

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

 

[planitarch]

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.

 

[rnorth]

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.

 

[sundale]

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...

 

[DonPhillips]

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

 

[rnorth]

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.