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Wood beam splits

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Jordan Truesdell wrote:

"The use of fasteners should be intended to prevent the crack from
extending 
into areas where bending controls, as the loss of shear continuity will 
result in a loss of bending strength & stiffness via reduction in MOI.

Perhaps a related question...would anyone recommend the application of 
epoxy/adhesive in the split to help prevent further damage, i.e.
extension 
of the crack due to cyclic loading? What about drilling the end of the 
crack? Are there any flaw propagation (fracture) analysis tools for 
wood?  This sort of discussion reminds me why I like steel so much ;-)"


Jim Wilson wrote:
>
>Thanks for the help, this beam did turn out okay with checks along its
>length.  It is split mid-height for about 8" at one end so I am 
>recommending couple of lag bolts pre-drilled and driven through the 
>crack.  Now it will be evident that the beam has been "fixed" even
though 
>it doesn't need it.



I'm chiming in a little late on this thread, and I'm not an expert on
wood design, but my two cents worth:

I'd be a little hesitant to install lag bolts up through the member as
Jim describes.  The vast majority of wood checks and splits aren't due
to stress or cyclic loading but to differential drying shrinkage.  Wood
timbers are nearly always installed in a green or semi-green condition,
because there's no economical way to dry large sections in a reasonable
time period.  Most of the drying takes place out the end grain of the
wood; very little moisture enters or leaves through the sides of a
piece.  Thus, the ends dry and shrink laterally, but are restrained by
the as-yet-green center portions of the timber.  That's why the ends
often split.  That's also why woodworkers paint the ends of large pieces
of wood with paraffin; it slows down and equalizes the drying throughout
the length of the piece, and reduces end checks.

In many areas of the country, the humidity goes up and down
significantly with the seasons.  The timber will expand and contract
with these changes.  A 12 inch member may gain and lose more than 1/4
inch in these cycles.  When you put a lag bolt through most of the depth
of a timber, you restrain this movement.  If you install them at the end
of a dry cycle, you may find crushing under the lag head.  If you
install them at the end of a wet cycle, and the stem of the lag binds in
the pilot hole, you will actually exacerbate the checking you're trying
to control.  Now, these seasonal swings are nowhere near as severe as
the first drying cycle, where the wood goes from living, 20 percent
moisture material to whatever constitutes dry in your climate.  That's
where most of the damage (checking) will occur.  But the wood will
continue to move with seasonal changes in moisture.

The idea of injecting epoxy into the crack seems to me to be less
potentially damaging, but the basic idea is to let the wood do what it
wants without tearing itself apart.  It wanted to check where it did.
It may again.  You're probably better off smearing the epoxy across the
end grain to slow down moisture migration.

The idea of drilling a hole at the end of the crack is completely wrong.
This technique applies to crystalline materials, where the tip of the
crack represents a huge stress riser under cyclic loading.  It would be
appropriate to steel or aluminum, and maybe to concrete (not sure how
practical it would be in concrete).  In fibrous materials such as wood
and fiberglass, this is either completely ineffective or
counterproductive.  In wood, the hole would expose the inside of the
timber to air, locally accelerating drying by an order of magnitude or
more.  It will doubtless cause further checking.

I don't fully understand the base assumptions behind the establishment
of allowable shear stresses in timber; but I seem to remember that they
pretty much assume a split.  You may not have the problem you think.

HTH

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

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