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a lightweight plywood/bubble daylighting roof? (attn: hayden cochran)
15 apr 1996
nick pine  wrote:
>paul milligan  wrote:
>>   nick@vu-vlsi.ee.vill.edu (nick pine) wrote:
>>~>a quote from _the unbelievable bubble book_, by john cassidy...
>>~>
>>~>  once the world's leading fizzicist, dr. aristid v. grosse, for many years
>>~>  the director of the research institute at temple university, spent his
>>~>  youth in the study of high-energy particle physics...

>>	nick, i think you should take a sabbatical, and do a long involved
>>study that features dr. grosse standing in front of one of your collectors.

well, maybe you don't like all that chicken wire. how about this?

still trying to take advantage of the spirit of section 1610.2 of the 1993
boca code, which defines a "continuously heated greenhouse" as
 
   a production or retail greenhouse with a constantly maintained interior
   temperature of 50 f or more during winter months... the greenhouse roof
   material shall have a thermal resistance (r) less than 2.0...
                   . 
                       .
                       .     .                               
                     . .             . <-- plywood or osb under epdm rubber
 		    .  .                       .           
 		   .20'. poly film bubble ceiling        .   ...........
 		  .    .                           .         .         .
 		 .     .                                     . chicken .
          pond  .      .<-              38'     .          ->.   coop  .
     ............................................p ~~~~~~~~~ p................
                          thermal storage pond->  ppppppppppp
   
in more detail, with different dimensions:
 
                     48'                  .          
          r.r.r.r.r.r.r.r.r.r.r.r.r           .      32'
          p  ppp  p  ppp  p  ppp  p           p       .
      20' ppppppppppppppppppppppppp     <--s  pppppppppppppppp.
          p       p       p       p           p       p       p 8'
     .....p.......p.......p.......p...   pond.p.......p.......p......
 
                                              pssssssspsssssssp
          p is the poly film ceiling here,    .   e   e   e   .
            made in 3 16' x 32' pillows,      .   w   w   w   .
            sloped slightly upwards from      .   b   b   b   .
            north to south, with the bubble   pssssssspsssssssp
            pipes along the north edge.       .   e   e   e   .
                                              .   w   w   w   .
          the poly film would attach along    .   b   b   b   .
            struts s...                       pssssssspsssssssp
                                              .   e   e   e   .
          ewb are the east west beams.        .   w   w   w   .
                                              .   b   b   b   .
          r might be 2x4 rafters.             pssssssspsssssssp
 
the roof might weigh about 15 psf, with 12 lb of snow load and 3 lb of
dead load. if the plywood or oriented strand board roof has rafters on 2'
centers with 8' spans we might have f=1200 psi, w=8x2x15=240 lb, m=wl/8
=240x8x12/8=2880 in-lb, s=m/f=2.4 in^3, b=1.5" and d=sqr(6s/b)=3.09",
so we might use 2x4s as rafters.

the ewbs need to hold up 32x48x15x15/5/3=1536 pounds over their 16' span,
so if m=1536x16x12/8=36864 in-lb, s=30.72 in^3, b=3" ==> d=7.8", so we
might use 2 2x10s for ewbs, or a couple of 2x6s with a downwards king post
and a wire.

suppose we use diagonal tension supports like this, as seen from the east:

  20'p
     p  \          ewb
     p              k              ewb
     p      t       k          /    p    \          ewb
     p        \     k       t       p       t        k          /   p
     p              k    /          p           \    k     t        p
   8'p..............k...............p................k/.............p
     p                              p                               p
     p                              p                               p
     p                              p                               p
pond.p..............................p...............................p...
     p                              p                               p
     |                             32'                              |

if the kingposts k were 6' long, the diagonals t might have an approximate
tension t such that 1536/2 = 6t/8 ==> t=1024 lb. some twangy wires with
good attachments...

each post p would have a load of about 32x48x15/12=1920 lb. suppose they are
eastern spruce 4x4s with cp=750 psi and e=1,200,000 psi. then l/d=8'x12"/3.5"
=27<50, c=1920/(3.5"x3.5")=157 psi  
...ns rope...            . 4 .             ... ns rope...
 			 .....
                           |          4'          |

the struts would only be holding up poly film pillows filled with 1-2' of
bubbles. the pillows would act as solar collectors for some of the
960x1.3x20x66'= 1.65 million btu or 483 kwh/day average sun that would fall
on the south wall of the perkiomen valley wetlands structure in december.
the peak solar power would be about 160 kw. some of that heat would be stored
in the thermal pond, with a little warm water pumped back up through the
collapsed pillows infrequently, to melt the snow off the roof. the roof
could be painted white underneath, or covered with 4' wide builder's foil
under the rafters, with an operable vent door to the rafter cavity, and
another to the outside.

nick




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