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Re: Use of Collar Ties in Light Frame Wo

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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>
writes:
> 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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