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RE: Use of Collar Ties in Light Frame Wo

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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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