Timber Framers Guild Forums Tools Timber Engineering kingpost truss

Anyone have any two or four cents about a kingpost truss that would span 32 ft., 12/12 pitch? As the gable is in plane with the porch, I'm planning on having the bottom chord extend beyond the rafters as a part of the porch plate to prevent endgrain blowout and eliminate the need for inner rafters (upper chords). I'm thinking 8x10 bottom chord, rafters and kingpost, with 6x10 struts from rafter midspan to kingpost. 8x10 (rather than 8x8) rafters to avoid compressive buckling. Any comments (beyond 'get a structural engineer')?

thanks in advance,
Matt

Buckling will occur along the least axis so using 8×10 instead of 8×8 won't help.

Is it a failure of intuition saying it seems easier to buckle a 2x4 than a 2x12 therefore... Or is it simply that the gain is minimal therefore 8x8 is substantially the same as 8x10 as far as buckling goes. Just curious.

Thanks

Both would tend to buckle on the 2" axis ... however:
My time on the Internet is limited, I tend to "speed read", and I should have read your post a more carefully. Your intuition hasn't failed. The greater the depth of the beam or rafter the more it resists bending and there is an advantage to using an 8×10.

thanks Joe

8x10s (with the wide face plumb) would be less prone to buckling in a roof truss, since chord buckling is restrained in the plane of the truss. But buckling is not likely to be an issue since the compressive elements are either short (struts) or restrained in both directions (rafters aka upper chords).

Maybe it wouldn’t be such a bad idea to consult a structural engineer. Certainly I would recommend learning more about how trusses work. One good place to start is with the historic truss series in Timber Framing, soon to hit the stores in book form as Historic American Roof Trusses.

Thanks Ed.

Actually my question was prompted by the truss series. re: issue 73 and the kingpost truss engineering addendum: in the Lynnfield truss model you site steep bending stresses in the braces (meaning inner rafters). Here I'm talking about a kingpost truss with similar span, but no inner rafters. I'm sure that a steeper pitch will reduce bending stress in the rafters, but on a 12/12 pitch the struts would not be that short (as per your post), so hence my concern...

Thanks again for your reply