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RE: Slightly off-topic question

To: "Shop-Talk list" <shop-talk@autox.team.net>
Subject: RE: Slightly off-topic question
From: "Randall Young" <ryoung@navcomtech.com>
Date: Sun, 23 Jun 2002 12:00:21 -0700
Thanks for the thoughts, Peter.

> The cracked appears, from your post, to be 6 feet from the wall.

My apologies, the break is actually near the center.  It's probably 2' long
and runs at an angle.  The new 'sister's would overlap it by several feet,
although I'm really expecting the new wood to carry the whole load.  It's
just easier to leave the broken one in place than to remove it (and have to
support the floor above it).  I'll make them longer, if Home Depot has them.

>  Two 16
> foot joists would not overlap the break so you face the same failure
> again. Each joist would end 8 feet from the wall.

It's true they would end short of the wall, but both would overlap the
break.

> I would really try to get a full length joist up there.  You should be
> able to bow a 24 foot joist enough to get it up there.  Lay the joist
> flat, not on edge, on one wall.  Have a big friend hang on the center of
> the beam while you raise the other end into place.  Once up there twist
> the joist to put it on edge.

Good thought, but there are shelves and cabinetry in the way.  I'm really
hoping to avoid having to tear them out and rehang them.  Also, if my
envelope hasn't slipped a digit, you're talking about deflecting the center
of the beam by roughly 20" (to shorten a 24' beam by 3"), which strikes me
as enough to possibly break it, and more deflection than I would expect from
my heaviest friend <g>

> Either method will not produce a stronger joist, so it would likely fail
> again, either exactly at the old break or at the bolt holes.

With twice as much wood through the break area, I would expect it to be
stronger.  The original beams weren't exactly high quality either, I'm
pretty sure there's an edge knothole where the crack started.

>  In fact,
> you will stiffen the joist concentrating more force at the break.
>  Consider adding new joists in between the current joists.  I would also
> consider adding some more joists to in between the current joists.

I plan to.  I've actually already sistered one of the other beams, with a
single 16' 2x6 and bolts, it appears to be quite strong.

> Rather than bolt which would compromize the strength of the joists, I
> would use wood glue to laminate the sister joist in place.  Wood glue is
> stronger than wood and bolt holes would compromize the strength of the
> joist.

I'll agree glue is stronger than wood, but I'm not convinced that in this
case it would result in a stronger joint.  Glue will concentrate the force
at the face of the beam, while the bolts will distribute it through the
beam.  Wood is weakest in tension, so it will start breaking right at the
glue surface and continue across the joint.  The clamping force of the bolts
will also result in significant friction at the face, likely nearly as
strong as the glue.

Thanks
Randall

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