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a tensile roof?
14 apr 1996
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. he worked on the
>~~> manhattan project... published numerous scientific papers and to all
>~~> appearances, pursued a dignified... distinguished, scientific career...
> 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.
hey, i don't work at villanova... i'm always on sabbatical :-)
and i don't do "collectors."
ok, ok, ok, that bubble roof might not be very practical for the location
near the high school, because some students might find it amusing to pop
the roof with their knives in fits of pique.
so maybe we should cover it with some burlap and chicken wire and 1/2" of
cement, and replace the telephone pole posts and beams underneath with 2 x 4s
to hold up the plastic film ceiling over the shadecloth. but that lets out
daylighting thru the ceiling.... hmmm. let's take another more fundamental
look. we don't really want sun shining thru the ceiling in summertime, but
we do want light and heat coming in thru the south wall in the winter, and
we would still like to try 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...
which we'd interpret to mean that one can turn off the bubble pump and reduce
the r-value when it is snowing, thus melting the snow with stored solar heat,
which is similar to the practice of local commercial growers, who deflate
their poly film greenhouse pillows when it snows to reduce their r-value from
1.2 to 0.8.
section 1610.4 of the code goes on to say "the flat-roof snow load on
continuously heated greenhouses shall be calculated using the following
formula..." which comes out to be 12 psf where i live...
and it further drones on:
retail greenhouse: a greenhouse occupied for growing large numbers
of flowers and plants and having general public access for the purposes
of viewing and purchasing the various products. included in this category
are greenhouses occupied for educational purposes.
so how about looking again at something like that tensile structure with
the profile shown below, from the east:
.
. .
. . . ferrocement
. . .
.16'. poly film bubble ceiling . ...........
.-24'. . . .
. . . chicken .
pond . .<- 38' . ->. coop .
............................................p ~~~~~~~~~ p................
thermal storage pond-> ppppppppppp
this might look like a 16' x 66' transparent wall from the south. the roof
might be made with 4' wide chicken wire strips running north and south, wired
together and joined with pressure-treated 2x4s that sandwich the chicken wire,
like this:
.........
. ns 2x4.
......... -- 1/2" cement --
ccccccccc cccccccccc chicken wire
bbbbbbbbb bbbbbbbbbbbb burlap
pppppppppppppppppppppppp poly film or epdm
.........
. ns 2x4.
.........
..............................................................
2 x 4 on edge, running ew
..............................................................
. spacer.
. .
. .
. .
.........
steel
cable
let's see. how big does the steel cable have to be? if the roof has a
nice slump in the middle, and the distance from top to cable is, say, 6',
we need roughly 20 psf x 32 x 4'/2 = t 4'/32', where t is the cable tension,
so t = 10240 pounds. a 10k/50k diameter steel cable every 4'? that's not bad,
but there will be a lot of force trying to collapse the north and south walls
inwards... this would work better with a few posts in the middle, as before.
then we can reduce the height of the south roof peak to 16', vs 24'. we don't
need that much solar heat. recall that we only needed a roof r-value of 1.7
with bubbles in place to make this work.
so the roof might look like this in slightly 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 .
16' ppppppppppppppppppppppppp pppppppppppppppp.
p p p p p p p 8'
.....p.......p.......p.......p... ...p.......p.......p......
pssssssspsssssss.
p is the poly film ceiling here, . . .
made in 3 16' x 32' pillows, . . .
sloped slightly upwards from . . .
north to south, with the bubble pssssssspsssssss.
pipes along the north edge. . . .
. . .
the poly film would attach along . . .
struts s, which would be in pssssssspsssssss.
compression to keep the roof . . .
from collapsing. . . .
. . .
pssssssspsssssss.
there would be a layer of greenhouse shadecloth under the poly film and ropes
on 4' centers under that. struts s might look like this from the south:
poly film poly film
poly film poly film
ww ww aluminum extrusion clamps
30% greenhouse shadecloth
.........
. ns 2x4. (on 16' centers)
.........
. 2 .
<--ew rope . x . ew rope -->
...ns rope... . 4 . ... ns rope...
.....
| 4' |
there, is that better, paul?
nick
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