When I built my large modified-post-and-beam barn/garage I went thru
the same gyrations of tables and codes, all of which came up with
different numbers.
All approaches use a 'safety factor' between the point of probable
failure (read 'broken') and 'working loads'. The factor varies from 4
to 7, and other 'basic' numbers like Modulus Of Rupture (MOR) has
different 'opinions' in different publications. So the results vary...
I got the Wood Handbook from the US Forest Service: Huge file at:
http://www.fpl.fs.fed.us/documnts/fp.../fplgtr113.pdf
and spent hours understanding the formulas etc. It may be worth it to
you to do this... as you seem to want to dig in and do your own work
based on your own understanding, rather than all the conflicting
'experts'...
I don't have any neat design package, but I'll send you my .123
spreadsheet if you want. I think Excel will open it... Basically you
just need the values for:
MOR (Modulus of rupture) you want to use, with some safety factor,
and the acceptable PSI for the end bearing area (end of beams on
supports)...
Here's a look at the raw numbers for hemlock. Get any other wood out of
the handbook.
---------------------
Hemlock Density: 50.00 Lb/ft3 Grn 28.00 Lb/ft3 dry
Hemlock Weight (Lb. Per Board Foot) 4.20 green 2.30 dry
Hemlock Modulus Of Rupture (psi) 6400.00 green 8900.00 dry
Safety factor 4.00 green 6.00 dry
HEMLOCK: Safe Working Loads: GREEN DRY
Hemlock Working Stress-Beam 1600.00 psi 1483.33 psi
Hemlock Parallel (Post end) 950.00 psi 1050.00 psi
Hemlock Perpendicular (Beam Bearing) 360.00 psi 650.00 psi
----------------------------------------------------------------------------------------------------------
(Need a fixed font to see these)
As you can see I used a safety factor of 4 for the green hemlock I was
using, and that naturally increases to a factor of 7 when dry. So I
was OK with my 6400 pound Yukon driving on the first floor, and it'll
have a better safety factor now, 2 years later. And I haven't put much
load on the second floor yet....
The nice thing about a spreadsheet is you can try out different size
beams and see the resulting MOR numbers and decide on your safety
factor.
I just designed and built a 12by12 deck on our Cabin that is about 14
feet above ground. Most codes require 100 Pounds Per Square Foot for a
'Party Deck Load"... California requires 150 PSF in a few towns, as I
understand it, after several people were killed in a deck collapse. I
used 150 PSF, and had to go to an 8by8 beam on the outer edge because
of a 12 foot span. That beam costs $50 here in Vermont. I was able to
make tradeoffs on joist size, span and so on. I added a center support
beam to keep the joist span to 6 feet, which made a huge difference and
allowed 2 by 6 PT joists.
ANYWAY, you might decide to do all this, or not.... and use some
tables,
I checked all my barn calculations against tables and they were
close....
Have fun!!
Regards, Terry King ...On The Mediterranean in Carthage
(Back ...In The Woods In Vermont for the Summer)
a few barn photos at:
http://www.terryking.us/photos/barn/2www-barn1-2004/