On 10/16/2015 5:16 PM, dpb wrote:
On 10/16/2015 1:10 PM, dpb wrote:
...[snip for brevity]...

...

The half-inch isn't at all a design standard, no, but it comes about as
a result of the building code 1/360 allowable deflection rule for joists
and beams. 15-ft/360*12"/ft -- 0.5" exactly, in fact. So, 1/2" in a
16-ft span is less than that. Being within that pretty much assures that
stresses won't be excessive for structural timbers.

If you really want to understand how this is done and the criteria used,
the following link is very thorough on the details but readable in
defining terms for the non-engineer--

http://www.wwpa.org/TECHGUIDE/DesignValues/tabid/855/Default.aspx

There's a link to properties/design values at

http://www.wwpa.org/Portals/9/docs/pdf/beam.pdf

Since this is from the Western Woods Assoc, it's mostly for the western
woods but there's an entry for "Pine...(South)" that'll be good enough
although you could go double check from the treated products group
website, too.

The most critical item in that table is that while the modulus doesn't
change much for graded structural to #1 and #2, the bending stress limit
reduction is almost a factor of 2X reduction for #1 vis a vis select
structural.

Does your other tool mention anything about the actual materials in its
stress limitation computation?

Oh, one last point...the pricing for PT you've gotten (cheaper by quite
a lot than I figured would have been, but confirmed here that's not out
of line of what is available pricing locally) is for #2 so the above
derating would be in play if you were to try to design to those
criteria. Your choices are, of course, either go w/ the higher grade for
more confidence w/ smaller members or instead of two use three (or four
instead of three, obviously). Can choose whatever is the most
cost-effective solution at whatever level of comfort you wish as to
conservatism in sizing for an anticipated load/usage.

Oh, and I just saw a typo I missed first time...

... the bending stress limit reduction is almost a factor of 2X
reduction for #1 vis a vis select structural.

should be for #2, not #1.

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