Return to index: [Subject] [Thread] [Date] [Author]

Re: Residential construction-hip beams

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Domenic,

While I believe that the hip rafter is a beam and should be designed that 
way, doesn't mean that is the way the roof behaves!

What is probably actually happening is that the sheathing on the hip and on 
the pitched roof is acting in tension as it would be if it were a shell.  
This is one of the things that we ignore in engineering and also one of the 
things that prevents a lot of roofs from failing!  However, if we analyze 
down to the gnat's eyebrow, we are going to get a lot more failures, plus, we 
probably will price the design out of the market.  Thank God that structures 
behave differently than we consider!

Remember, that with gravity loads, the horizontal span, not the sloping span, 
is used to determine moments, and even if you used the sloping dimension, 
when you adjusted your gravity loads to be perpendicular to the beam, and 
reduced them for the slope effect, the answer comes out the same.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Domenic DeAngelo wrote:

>>I would like to thank everyone for their response so far. I have been doing
residential design for about 12 years in New England. I know that when a
builder submits a "stock" set of plans for a hip roofed colonial, the roof
is framed with 2 x 10 rafters (depending on the span) and a single 2 x 12
hip. For a typical 28 foot deep colonial, the hip length would be
approximately 20 feet. With the triangular loading pattern and a snow load
of 30 psf (and a dead load of 10 psf) the single 2 x 12 would be grossly
overstressed (an approximate moment of 14370 psf). Even under the design
dead load of 10 psf the hip would be overstressed (3592 psf). However,
experience has shown that there have been no failures of roofs on a massive
scale (I have actually not heard of a single hip failing).
A long time ago I analyzed the hip section of the roof as a truss with the
bottom chord (the ceiling joists running from front to back) as the tension
chord. The compression chord consisted of the jack rafter which ran from the
front wall to its intersection with the hip, the plywood sheathing (which
was unbraced between the jack rafters (which are now running perpendicular
to the force), and the opposite jack rafter which ran from the opposite hip
down to the rear wall.
The situation has come to light when a building official (who is also a
registered PE) stated that the hips are structural members and need to be
designed as such. He will allow the building permit if I give him a stamped
letter noting that I have read his material and that I am still confident
that the hip is not structural in nature.
I do not necessarily have a problem with redesigning the hips and making
them structural members. Before I do and add what may be significant cost to
the homeowner (the hips as well as any mechanism required to chase the loads
down to the foundation), I would like to make sure that the hips are, in
fact, structural especially when the homeowner (and builder) will point to
countless other houses in the region which do not have structural hips and
appear to have no deficiencies in them. This will also impact any future
houses that I review, since, if I change my design philosophy I will upset a
bunch of applecarts. Again, if everyone is in agreement that they should be
structural, I don't have a problem with it. I just don't want some young
"whippersnapper" coming along and showing me wrong and my potentially having
to eat the cost of the engineered building.
Again, I appreciate any input into this subject from the group.<<

******* ****** ******* ******** ******* ******* ******* *** 
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) to the list, send email to 
*   admin(--nospam--at)seaint.org and in the body of the message type 
*   "join seaint" (no quotes). To Unsubscribe, send email 
*   to admin(--nospam--at)seaint.org and in the body of the message 
*   type "leave seaint" (no quotes). For questions, send 
*   email to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********