I did check this (continuous joist at three supports) and you are correct.
The rafter does work but the bending moment at the second support increases
the member to be stressed to 94% of code value. This is still within the
allowable limits and I've added a 10-psf live load to the member to allow
for some short term load. I have also warned the owner in the report not to
use the attic space for storage (the access is less than 24-inch square so
it is limited as to what can be placed up there).
Believe it or not, I am charging the owner less than $750.00 for the report,
calculations and sketches. The number of hours debating the design reduced
my profit considerably, but it was, in my opinion, the best time spent as I
am comfortable with my design and repair and have written the report as
concisely as possible to insure that the conclusions drawn are reasonable
based on the information provided. I have also added additional notes to
have the contractor verify the connection between the rafter and ties.
In a closing thought consider how little time a contractor would spend
evaluating the problem in a conventionally constructed home how, without the
help of an engineer, he might resolve the problem. In my experience, I have
seen few total collapses of structures due to negligence. There are some
unsafe conditions and shoddy work, but rarely do we have wood frame home
fail and take lives. Sometimes I think we need to have a little more faith
in materials we work with and in our intuition as to how the materials will
perform as a system. The numbers, as we all know, don't always tell the
truth because they can't always identify the interaction of materials,
connections and external unknowns. Our job is to make the best decision we
can, without intimidation of the consequences of making a mistake, but being
diligent enough to have attempted to look at as many possibilities as
I think we did that here and I thank each and every one of you who took the
time to enter the evaluation process with me - I truly appreciate the
enthusiasm, concern for my ultimate decision, and experience that each of
you have brought to the discussion.
Dennis S. Wish, PE (Proud Engineer)
> -----Original Message-----
> From: r nester [mailto:rnester(--nospam--at)juno.com]
> Sent: Friday, June 08, 2001 12:37 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Use of Collar Ties in Light Frame Wo
> Dennis ---
> The idea of suspending the middle of the ceiling joists will certainly
> fix the ceiling sag, but at a price. The rafter/joist "truss" now
> carries an additional load - that of the ceiling DL + LL. At this point,
> the rafter-to-joist connections may now be overstressed, trading one
> problem for another. Proceed with due caution. When you deviate from
> "conventional framing", even a little, you may be on soggy ground.
> Otherwise your analyses appear reasonable.
> --- Russ
> On Wed, 6 Jun 2001 23:17:49 -0700 "Structuralist" <dennis.wish(--nospam--at)gte.net>
> > Steve and Russ,
> > Thanks for your comments. Let me put a few things into some
> > perspective
> > here. Actually what was stated in the first post was that it
> > appeared that
> > the ridge droped "a few inches" which would suggest three or more.
> > However,
> > I failed to state that upon inspection I did not verify any
> > measurable
> > deflection in the ridge, but noted that the ridge was not "straight"
> > across
> > the garage.
> > I think that I let this go a bit too far and failed to correct some
> > things
> > clearly. The two inch deflection figure came from one of the
> > e-mails where
> > I did some load calcs on the rafter ties - 2x6 DF #1 using current
> > 97 UBC
> > criteria. I considered a 6.0 psf dead load on the 2x6's which were
> > 25-feet
> > long and spaced at 24-inch on center. Neglecting live load, the dead
> > load
> > deflection was nearly 2-inches.
> > Now, what I am leaning to believe that nothing actually happened
> > along the
> > ridge. One of the things I think we all missed asking was "how much
> > space
> > was there between the bottom ends of the rafters which were flush
> > framed to
> > the ridgeboard?" Surely, if the ridgeline dropped a few inches the
> > angle
> > between the cut at the original slope and cut at the end of the
> > rafter after
> > a three inch drop would show some space or nail withdrawl from the
> > toe-nails
> > into the ridgeboard. However, when looking at the picture where the
> > crack
> > occurs, there is no space at all. The fit is tight to the ridgeboard
> > and
> > there is no indication that a change in slope caused the rafter to
> > rotate
> > around the top corner crushing the face grain of the 2x ridgeboard.
> > In other
> > words, a consistant tight fit suggests no drop in the ridge.
> > The picture also shows that the inside edge of the wood where the
> > split
> > occurs is the same color as the face and outter edges of the
> > ridgeboard.
> > This is possible as there is no ventilation in this part of the
> > attic
> > (another reason to believe that the ceiling was added later) and
> > therefore
> > sunlight could not reach the wood to discolor it. However, I would
> > be
> > inclined to say that if the ceiling and insulation was installed
> > some years
> > later, the ridge would have been subject to some outside light if
> > only
> > diffused through an open garage. There is a patina to the wood which
> > probably occured in the first couple of years of the home. The crack
> > has the
> > same patina as the exposed surfaces which makes me believe the crack
> > has
> > been there since the home was constructed.
> > All of this aside, there is a problem and that is the ceiling. I've
> > designed
> > the fix by hanging the ceiling joists (aka rafter ties) from a new
> > 3.5x14"
> > Parallam to replace a site built double 2x8 beam spaning 22'-6" and
> > carrying
> > a 12'-6" tributary ceiling load. The double 2x8 probably kept the
> > existing
> > ceiling from failing completely, but it did nothing to control the
> > deflection.
> > As per the collar ties, I am going to install them, but I will
> > install them
> > flush to the top of the new Parallam and use the collar ties to
> > laterally
> > brace the compression edge of the beam at 4' on center. This will
> > give me
> > more than 60% more capacity in bending to the beam should the owner
> > decide
> > to throw some boxes up in the attic.
> > FWIW - I calculated the beam for no lateral bracing to see how it
> > affected
> > the bending stress and at 22-feet unsupported, the load used reached
> > about
> > 90% of the beams bending capacity. With the compression edge
> > supported at 4'
> > the beam had reached nearer to 40% of it bending limit.
> > Finally, as to the ridge deflection the most reasonable explanation
> > is that
> > the 22-foot long board may have been installed crown side down.
> > Considering
> > the flush, tight fit of the rafters, it would be a workmanship
> > problem
> > rather than one of defect or failure.
> > I thank you all again. If I have convinced one of you I think that I
> > will
> > have done my job on this one. I certainly convinced myself having to
> > justify
> > this to my peers. Shows the powers of a peer-2-peer services such as
> > time -
> > a tremendous tool.
> > Thanks again everyone!
> > Dennis
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