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Re: Wood beam splits

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Jim:

The current NDS assumes that you have checks and splits in the wood.  In
others words, you are NOT permitted an adjustment (for the better) if you
have NO checks or splits.  So, the base (and only permitted) shear stress
values in the 2001 NDS (and I would assume the forthcoming 2004/5? NDS)
assumes that you have a worst case of checks/splits that would have
resulted in a CsubH value of 1.0 in the 1997 NDS.

A question...does you split/check travel all the way through your member?
In otherwords, is it really a check (i.e. a radial "crack" due to
tangential shrinkage likely due to seasoning which more than likely does
not go through the whole member), a shake (i.e. a tangential "crack" that
likely formed while the tree was still standing) or a split (i.e. a
"crack" that originally was a shake or check that has now gone completely
through the member)?  Checks are common, especially large cross section of
wood.  I work for a timber framing company and many, if not most, timber
members will develop checks...some that look distressingly large at times.
This is due to most timbers (5"x5" or larger and we definitely always deal
with larger) coming to the shop in a "green" condition.  Kiln dried for
timbers are fairly rare.  The end result are checks in most of the members
that we produce.  The checks that we encounter rarely travel very deep
into the member, and as a result should have little impact on the shear
capacity of the member (especially since the default assumption is that
they WILL check/split).

So, I would say that you may not need to do a repair at all.  What I would
suggest is take a look at how close your theoretical shear stress is to
the 1997 NDS values.  If it is close, then it might be prudent to repair.
But, if it is not close, then it may not be necessary.  You can even
reduce the shear area for determining your theoretical shear stress (you
mention the "crack" at midheight and is "deep"...you could assume that the
crack goes from the surface to the center [unless you can measure how far
it goes in], thus you would still have about 1.5" by 12" of good
material...use that as your shear area).

You could also look at the notch provisions in the NDS.  Since the "crack"
that you mentioned is located near the support (what I assumed when you
mentioned "bearing"), it should have a similar effect as a notch in the
tension side at the support (assuming that the length of the "crack" is
not too long).

If you do reach a repair stage, then at first blush (i.e. without thinking
about it much) you repair sounds reasonable in its approach.  If you do
this repair, I would definitely suggest that the hole be
pre-drilled...otherwise, you may just split the beam (in the other
direction) again when driving the lags.

HTH,

Scott
Adrian, MI

On Sun, 27 Jun 2004, Jim Wilson wrote:

> SEAINT,
>
> I am reviewing a 3"x12" doug fir beam with wood splits along the length of the member.  I've searched the archives and AWC and only come up with the elimination of the shear stress adjustment factor CH in the 2001 NDS.  NDS 1991 Appendix E which provided Kv as a shear coefficient for horizontal shear has also been removed in the current NDS.
>
> What is the current recommended design approach if there are no prescribed adjustments for splits?
>
> AITC Tech Note 18 has a somewhat elaborate procedure for splits in Glu-lam beams, but does the theory transfer over to regular cut wood?
>
> To repair an in-service beam with a deep mid-height longitudinal crack at bearing, my thoughts are to drive lag bolts vertically from the bottom of the beam up through the crack and almost to the top of the beam.  I would start with say 4 bolts 3-6" inches on center, starting at the bearing point.  The bolts would then act as shear pins to transfer horizontal shear between the top and bottom of the beam.  Do wood beams perform in such a way that this would be a logical and appropriate fix?  The beam is exposed, so plating the outside is far less desirable.
>
> TIA,
>
> Jim Wilson, PE
> wilsonengineering(--nospam--at)yahoo.com
> Stroudsburg, PA
>
>
>
> ---------------------------------
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> Yahoo! Mail Address AutoComplete - You start. We finish.

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