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Posted by RicodJour on February 20, 2008, 9:37 pm
>
> > It seems to me that you're overanalyzing this thing.
>
> Well, I think I narrowly framed my original question (double 2x6s or
> notched 2x8s), and we pretty quickly got to the practical answer (use
> tapered 2x8s). However, people were interested in the whole story, so
> I've been happy to provide the extra details. That has led to some
> interesting side questions, which are perhaps less practical.
The 'extra' details are usually the things that determine the best
solution. There seems to be a bell curve to newsgroup questions.
Newbies leave out all the pertinent information, then have to fill it
in when people point the necessity of the omitted information.
Posters with a bit more posting experience, but maybe not more
technical information/experience, post more information up front.
Posters with more technical experience often try to simplify the
question to the one issue that concerns them. This leads back to the
'insufficient information for meaningful response' replies.
> > Your house is from 1908 - prime balloon framing period. Is that how
> > your floor joists are connected?
>
> No, it's platform framed. The only weird detail is the 1x4 plate
> between the ceiling joists and the rafters.
>
> > The only small wrinkle was the 1x4 plate resting on top of the CJs
> > requiring the notch. You'd already established that shear and
> > bearing area weren't issues as a single 2x6 was adequate. Since you
> > will have double the sectional area there's really no question of
> > the notch being a problem.
>
> Just to be clear, I won't have double the sectional area. I'll have
> the sectional area of a 2x6 after I do the tapering. If I want to
> sister the existing 2x4s and consider its sectional area, then I have
> to be concerned with the sistering details. But since even the single
> 2x6 is deflection controlled, there is "reserve" sectional area there,
> so I'm not concerned about the possiblity of stress concentration from
> the tapering.
That was the low blood sugar talking. Sorry about that. Should have
said _almost_ double the cross section. I don't any practical
advantage to the tapering in your particular situation. This is what
BobK wrote in his first reply:
"You might consider a taper cut rather than a sharp cut out.
Timber is happier if you avoid abrupt section changes but in this
case the local stress state appears to "compress" :notch rather than
wanting to split it open."
I agree with the second part of the last sentence, and the
_generality_ of the first part. Your situation won't benefit from
tapering. It won't change any stress state. The same way doctors are
supposed to "first, do no harm", carpenters are supposed to "not
remove any wood without a reason." In your specific situation there
is not reason to do it, so why do you want to do it? You should
listen to BobK. ;)
> > My concern with the deflection is due to the fact that I don't
> > consider 1/360 adequate for a floor.
>
> So what do you consider adequate? Will my skim coat plaster ceiling
> crack under a live load deflection of L/360?
Eventually. They all do. I don't think you're taking a big risk, and
frankly if you're not storing anything up there you're not taking any
risk at all. You could do me a favor though. After the supplemental
CJs are in place, take an elevation of the bottom of a central joist
at midspan. After you have rocked and plastered, do it again. I'm
curious to see what sort of deflection you get from just the dead
load.
> > It wasn't clear from your earlier posts exactly how much room (height
> > and area) is in the attic, and it wasn't clear what would be stored
> > there.
>
> The roof is hipped at an 8:12 slope, so over the 8' region, the
> clearance at center span varies from 0' to 5'4" or so. I'm not
> actually storing anything up there, my real concern is about the
> deflection from a person standing (squatting) on a single joist. As
> for others storing stuff up there, I guess that's why the building
> code requires a 20psf live load where the clearance exceeds 3'6". I
> think that's adequate in this situation.
For you, yes, I agree. Do not doubt that someone will do something
stupid in that attic in years to come.
> > As an aside, we could have gotten to the meat of the matter a lot
> > more quickly if you'd posted a picture of the situation and/or given
> > more complete information upfront.
>
> I don't know about that, the meat of the matter from my point of view
> was in my original question, that's why I specifically made it as
> short and constrained as possible.
And therein lies the rub. You were able to inspect the situation
firsthand and have way more information to help make a decision.
Playing detective requires me to ask questions - some of which will be
dead ends. I still have to ask them. The more experience I get the
more finely tuned my analytical skills become. These newsgroups help
me develop those skills and hopefully save somebody a 'learning'
experience.
> Above all, I appreciate the time and attention shown in your
> responses, they reflect the practical experience that I lack.
Hey, don't mention it. It's fun batting these things back and forth.
R
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