I have a project coming up where I'll be sawing
white ash logs into 5" X 8" x 14' to 16' timbers for
use in timber frame trusses of sorts - the loading
will be the same as roof rafters. The unsupported
span is in the neighborhood of 10 feet.

My question has to do with pith located in
the timber. I have a number of straight tree stems
that could easily produce one 5 X 8, and some that
could produce two 5 X 8 timbers, and I'm looking
for input on how the location of the pith effects
the strength/grade of the member.

I'm familiar with the basic grading rules for
structural timbers (from FPL tech notes), and it
appears that pith centered in the timber does not
lower the grade .... defects(knots) on the faces
are what influence grade the most.

In a situation where two timbers (5X8's) can be
sawn from the log/stem, is there a preferred
or recommended approach? My first instinct would
be to saw either timber a bit away from the pith
to avoid having the associated pith defects on
the face of the timber - in essence, sawing a
thin plank that contains the pith, from the center
of the log.

As and aside, I believe I've read in one of the
FPL documents that anything 5" or wider can be/is
considered a timber ...

Thoughts appreciated

Carl

Boxed heart will give you the best result for straight beams. After that it depends on the tree. No exact science that I know of.

Carl, I think if it were up to me, and I may be doing the same for a structure for myself before long, I would put the pith side up. I would also be tempted to do a rip cut down the length aligned with the pith. This might relieve the stress to check on the other 3 visible sides, and wouldn’t have any effect on the strength in that orientation.

Is that 10’ the full length of the rafter, or just between braces?

We know that the wood around the pith, sometimes called juvenile wood, shrinks as it dries. If the pith is off center, then you will have a warped beam. So, do everything you can to center the pith exactly, end to end. If the log is crooked or sweepy, discard it.

Juvenile wood shrinks lengthwise when it dries, while regular wood has close to zero lengthwise shrinkage.

Juvenile wood is also weak, so putting it in the center assures small effect on the overall strength.

OK - I think I've got a handle on this now,
and thanks to all who replied.

My take away is that if sawing timbers for the
highest strength/grade is the goal, then choosing
straight logs and accurately centering the pith
will produce the highest rated timbers.

I was aware that the juvenile wood that surrounds
the pith shrank considerably more then non-juvenile
wood, but wasn't aware that is was also weaker.
Centering the pith would make sense, since it would
place the weaker juvenile wood at the neutral axis
of the beam, where stresses are the lowest. And
since timbers can't realistically be dried to
inservice MC before installing them, centering the
pith sure seems like it would go a long ways
towards equalizing the shrinkage around the timber.

Rather then try and squeeze two timbers out of the
larger logs, I'm going to first try and use up any
smaller diameter logs, and then "saw around" the
larger ones until I've got a cant to saw the beam
from.

also - as to the question: "Is that 10’ the full length
of the rafter, or just between braces?" the unsupported
span of the rafter is 10' - the overall length will
be 14 to 16 feet.

Carl