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RE: Use of Collar Ties in Light Frame Wo

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Roger,
Your assessments are correct. Would you believe that this is a home in one
of the more promanent gated communities and golf courses in the desert? The
home, I would guess, is around 2500 square feet and probably sells today for
around $400,000.00 with monthly assessments at least another $700.00 a
month. Of course this does not include golf course use:o)

I don't want to give out the name of the development as it is very well
known. It is my experience that the majority of these exclusive developments
are simply cheap tract homes constructed by developers who never thought the
development would bloom the way it has. There is an exclusive area to this
place with individual custom home lots and the homes are $1.5 million and
up. I've designed a few beautiful homes in this developement.

as to the collar ties. I don't see as I have much choice. I think I can
respond better to Samir's comments:
" I think collar ties will do the job as they have been installed in
conventional framing, meets code, for many years.  I would venture to say
that if you try to calculate the thrust force in the collar tie using 3- 16d
nail you might just get by.  I know that the prescriptive nailing schedule
in the code, table23-II-B while accepted by code provisions hardly calc's
out.  The thing that is puzzling in your case is that the ridge board should
be a compression member only that supports the rafters and when considering
a 2 foot span of the ridge itself, it should not have a bending failure.  It
seems to me, may be the owner or prior occupants must
have hanged a heavy load, like a car engine, car top or simply a boxing bag
to have cuased this failure."

Samir actually put his finger on the key issue here. There is a problem
(which I think I did mention) with the ceiling joists. Let me explain a
little more clearly:

The garage is 25-feet deep by 22'-6" wide. The ridge board runs parallel to
the 22'-6" and the rafters are supported above the garage header and rear
stud wall of the garage. 25-foot-long 2x6's are used as rafter ties -
certainly not sufficient to support a gypsum ceiling and insulation (and
usually some light storage such as decorations for holidays.

The original builder spliced together (2)2x8 boards to create a 3x8 beam and
ran it below the ridgeline - setting it above the rafter ties. He then
connected the rafter ties to the 3x8 using Simpson Hurricane Clips. It is
most likely that the ceiling and interior finish in the garage was not part
of the original design and the only thing being supported on the 2x6 ties
was the garage door opener.  Now it supports a gypsum ceiling and insulation
(as the garage is used as a laundry room as well). Sill, the ceiling is
seriously deflected as the 3x8 was not sufficent to support the 2x6 C.J.'s.

So now we have a condition where the weight of the ceiling is pulling the
roof rafters inward at the plate line. In my opinion, this still would not
have placed the ridgeboard into bending, but would have increased the
compression of the rafters against the board. I would expect this to be
especially true if the rafters were strapped over the ridge which was fairly
common. As the tails come in from the ceiling deflection, the rafter will
want to rotate around the bottom corner of the rafter at the ridge board.
This will either force the ridge up or, it the straps are in place, increase
the compression against the ridge. BTW, face mounted Joist hangers were used
at the 2x ridgeboard.

The break must have been due to bending as there is no other reason this
could have happened but the only way I can see this happen is if the rafter
tails were unrestrained and allowed to move outward. The failure was kicked
off by the natural defect in the wood - a change of direction of the grain.
Rather than parallel to the face of the ridgeboard, the grain ran at a
diagonal from top to bottom over a 48" span of the ridgeboard. Still,
something would have had to cause the grain to separate and I have not found
a solution to this.

This goes back to why I think the collar ties will work. First let me point
out that the 2000 IBC Table 2304.9.1 lists "Collar Tie to Rafter" and
specifies (3) 10d nails OR (4)3"x0.131" nail (I assume pneumatic) OR (4) 3"
x 14 gauge staples face nailed. This is a difference over the 97 UBC
Conventional framing that I think you are refering to. The 97 code Table
23-II-B-1 that you refer to does not specify Collar Tie nailing. It does
provide for a (3) 16d face nail between ceiling joists and roof rafters at
the double plate line.

With this said, it is common knowledge (and mentioned on this list more than
once) that many of the prescriptive methods do not calculate properly which
only means to me that there are actions in play that we are not considering
or have chosen to ignore. I don't believe that we can adequately justify
this type of roof system by calculations - as was said by most, the numbers
aren't going to balance. Does it mean that it is wrong or will fail. In my
opinion, no - it only means that we have not explored what works
conventionally in enough depth to find out what we are missing in our
analysis.

At this point, I am going to use the collar ties for a couple of reasons.
The failure was not, in my opinion, the result of a bending failure but a
defect in the ridgeboard. While I am surprised that it appeared to drop, I
am now not so sure that other areas of the roof did not rise because of the
excessive deflection in the ceiling. In other words, we are thinking the
failure is downward, but in most liklihood, the rafter tails were pulled in
forceing the ridge upward. I don't think this justifies the ridge board
failure but suggests a potential solution by relieving the deflection in the
ceiling.

I designed a ceiling beam to replace the 3x8 spliced beam and an appropriate
members works out to be a 3.5" x 14" Parallam PSL 2.0E beam. Once the
deflection is relieved, the ridge may drop to the level with the lowest
point currently in place.

Rogers issue with Creep is equally important as I would not expect the ridge
to drop any more since the lowest point represents the maximum creep already
occuring in the wood.

Therefore, adding the collar ties provides some additional protection -
albeit redundant from what the ceiling joists are to provide. Still, the
can't harm the system (unless the joist is split in the process of nailing.

Inasmuch as I have to have this done tonight, unless I get some critical
information to contrary, I am comfortable with progressing based on the 2000
IBC's recognition of collar ties as lateral supporting members. The final
word is with the building offical who may still send it back - I'll see what
happens.

Thanks to all for your comments and suggestions both public and private.

Dennis S. Wish, PE



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