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RE: Use of Collar Ties in Light Frame Wo

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Charles,
Yes, the compression side of the beam is designed to be braced at 2'-8"
maximum along the entire length of the beam. I assumed I could brace it back
up to the roof rafters at alternating sides with "kickers".
FWIW, the design was done on Trus-Joist's TJBeam program which takes into
consideration the location of the uniform load (above or below the
centerline of the beam) and calculates out the maximum distance between
bracing of the upper (and lower if existing) compression edges.
With that said, I was still very conservative on the load consideration -
10-psf live load and 10-psf dead load (actual dead load is closer to 5-psf)
and attic area is not expected to be used).
My concern was hanging the rafters as they had been. I have never been
comfortable with this. The load at each rafter is conservatively 500-lbs
(24" spacing). The H2.5 currently used is insufficient or my loads are much
too high. I specified the New Simpson HS24 as they are good for about 604
pounds in tension (normally uplift). The only other choice of wrapping a
strap below a joist and up the face of the Parallam was not possible here
without removing the ceiling.
Any other thoughts. I've shut my phone off because the real estate agent,
the buyer, the designer and everyone else is yelling for the work as they
want to complete the work in the next two weeks before escrow closes. How
can a guy get anything done correctly with all this pressure???????

Dennis

> -----Original Message-----
> From: Charles Espenlaub [mailto:Cespenlaub(--nospam--at)martinaia.com]
> Sent: Tuesday, June 05, 2001 5:06 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Use of Collar Ties in Light Frame Wo
>
>
> Dennis,
>
> Since the ceiling joists are only connected to the bottom of the
> beam, your
> 3 1/2" x 14" PSL seems kind of narrow for that span.  Have you
> checked your
> beam stability factor, CsubL, or braced the top of the beam at one or more
> locations?  If it is only  braced at the ends, you can loose about 30% of
> the allowable bending stress.
>
>
> Charles F. Espenlaub, III, P.E.
> Martin-Espenlaub Engineering
> Tel  215-665-8570
> Fax 215-561-5064
>
>
>
> -----Original Message-----
> From: Structuralist [mailto:dennis.wish(--nospam--at)gte.net]
> Sent: Tuesday, June 05, 2001 6:37 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Use of Collar Ties in Light Frame Wo
>
>
> 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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