re: construction 2x4 vs. 2x6 SOLAR.KnowledgePublications.com

re: construction 2x4 vs. 2x6
12 may 1999
you know**j-a-f-o wrote:

> have finally decided to build a little brandy and cigar room within
> the steel building for after-dinner reflection.
 
hey, no snow load... 

> i plan to have a 20 foot by 15 foot room tied into the perlins running
> through the back of the building. i've already studded out the room
> with 2 by 4's on 16" centers. since i plan on storing various items
> atop the room, i intiially thought that i'd go with 2 by 6's on 18"
> centers... am i doing this with too much overkill?

mebbie.

> can i get by with two by fours on 18 inch centers for the ceiling...
> and still have a good degree of structural integrity for the
> prospective storage atop?

with 15' 2x4 rafters on 16" centers supporting a distributed load of w pounds
per square foot and total load w = 15x16/12w = 20w pounds and bending moment
m = wl/8 = 20wx15x12/8 = 450w in-lb and section modulus s = m/f = 0.45w with
f = 1000 psi and d = 3.5" = sqrt(6s/1.5") = sqrt(1.8w), w = 6.8 pounds per 
square foot, if i did that right. not much. 2x6s on 16" centers make w = 16.8
pounds per square foot (including their weight and that of the ceiling.)
still not much.

adding a beam and a few posts or an interesting kingpost arrangement at 7.5'
(supporting some bookshelves for your trophies?) makes m = 113w and s = 0.113
and 3.5 = sqrt(0.45w), so w = 27 psf, enough to support some 10 cent/ft^2 2x4"
mesh 4' welded wire fencing under some 5 cent/ft^2polyethylene film under a
two-deep layer of strawbales, good insulation :-)

you might make a sandwich to hold the fence up with 1x3s on the bottom,
held together with deck screws, which would further increase the rafter
strength, or forget the 2x4s and make the ceiling with double curved 16'
1x3s on 4' centers with 1x3 spacers every 2', with the fence stapled over
the lower 1x3 and a 2.8' vault in the middle.

the fence could be covered in burlap above, then stuccoed for a medieval
effect. just add gargoyles, soft gregorian chant and flickering candles.

nick