On 10/15/2015 6:07 PM, Dean Hoffman wrote:
On Thu, 15 Oct 2015 15:41:37 -0500, dpb wrote:


I did a quick beam deflection calculation for SYP 4x6 on 16-ft spacing
with a dead center load of 150 lb plus distributed 3.5 lb/ft. Since
that's one beam, the actual load for two would be 300 lb which ought
to cover the kind of traffic I'd think this would bear given what it
looks like would be from the picture...

Anyway, center max deflection for that loading is roughly 0.5" which
indicates it would be more than adequate for the purpose.

As for using something other than 20-footers which undoubtedly will be
pricey, a couple 10's with a (say) 3/8" flat plate of 3-ft length
bolted thru with 1/2" bolts would work and be reasonably easy to do
with only a couple of folks and a temporary bracing to hold the ends
up while making the connections. Pre-drill the plates and then bore
the holes in the beams in place. Could get a little fancy and have
somebody fabricate them with a little arch in 'em and have some rise
to the center for some aesthetics, too...

Gets as involved from there as may care to be including adding a
center riser with a tension rod/cable from each end to stiffen 'er up,
etc., etc., etc., ...

Would some sort of pre made floor truss be at all practical?

Standard sizes would likely be quite a bit taller and sorta' show their
provenance which might not be desirable. I've no idea cost--have
never actually used one meself, only bought the one house in TN already
in final close-in that used 'em so never had any reason to...

Of course, one could substitute height for thickness on the 4x in a cube
power ratio of additional height to reduction in thickness and maintain
the same vertical stiffness. That is, I ~ bh^3 so to keep same
deflection one needs to raise the height much less than decrease the
distance to keep same overall in the vertical direction.

Well, let's just see numerically

4x6 -- 3.5*5.5^3 = 582
2x8 -- 1.5*7.5^3 = 632

so, in fact, the 2x8 would be ~10% stiffer to the vertical load than the
4x6 but more "wobbly" side-to-side w/o any cross bracing.

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