Again, pretty neat software. Thanks. It is in my bookmarks.
Only concern is it provides sag but not breaking strength. I have
used 1/4" glass panels for some 16"x12" curio cabinet shelves but no
weight to speak of. I also built a small A/V cabinet years ago that
had 16" x 20" 1/4" glass shelves holding some pretty beefy stereo amps
and tuners (20 pounds+); but load points were cloe to the ends of the
shelfs. In both cases, I talked to the local glass shop and they gave
me size and strength guidance.
IIRC, They do give you a "target sag" recommendation, which anything
over you would think would be bordering on failure for glass, which is
better than nothing. But it is the closest thing I could find that
answers Joe's question, if only partially.
I let my glass guy make the recommendation when I give him the shelf
template, or the dimensions I need and specify the use ... and most of
the time I end up with tempered glass in 1/4" thickness for china
cabinet shelf use.
Here's a South African glass industry standard
or http://tinyurl.com/yzpmlpz . Note that it is metric.
Just for hohos I ran a 1x1 meter by 4mm piece of glass through the sagulator
with the standard's maximum recommended 17kg/square meter load and it came
up with .068 in/ft sag, so if you go by the .02 recommendation that the
sagulator uses you may be safe, but I'd check both the sagulator and the
standard to make sure that it will both hold the load and not sag--glass
doesn't usually warn you before it lets go.
Most glass is a liquid and sags. In an old house you may see the
bottom of the windowpane is thicker than the top portion. Quartz
glass is a solid--probably less sag over time and transmits more
lightwaves than regular glass.
I believed that for a long time until it was pointed out to me that
there are intact glass windows in Roman and Egyptian buildings which
would belie the flowing glass myth. If the glass sagged enough to be
noticeable in a two hundred year old house, the sag in two or four
thousand year old glass would be very noticeable, and it's simply not.
I've seen lots of old, wavy glass - new wavy glass too, especially in
China - but have always attributed that to lousy glass making. Never seen
any thicker at the bottom though.
There appears to be no argument about glass being an amorphous
(non-crystalline) material. When I was a geology student more than 50 years
ago we were taught that there is no such thing as glass beyond a certain age
(some multi-millions of years) because it ultimately crystallizes. Now,
humans haven't been making glass long enough for it to crystallize but there
are naturally occuring glasses - obsidian, for example - and it was/is those
that were not found beyond a certain period in the past.
OTOH, the profs all po-pooed poor old Alfred Wegener and his silly
continental drift idea :)
On Thu, 10 Dec 2009 06:56:35 -0800 (PST), the infamous RicodJour
I scratched the windows in the new door I put in the utility room 7
years ago. It's a halflite and the scratch was right next to the door
knob. I was taking the label off with a scraper. Fingernails, water,
soap, and several chemical solvents proved that it wasn't just glue,
it was a scratch, and I fumed for weeks about it.
A few months later, I was cleaning that window and couldn't find the
scratch. To this day, there is no scratch on that lite. This leads me
to finally believe in that "liquid glass" thang, KWIM,V?
To know what you prefer instead of humbly saying Amen
to what the world tells you you ought to prefer,
is to have kept your soul alive.
-- Robert Louis Stevenson
Probably because public school "science" teachers are taught it and pass it
along and while kids don't get anything _useful_ out of those "science"
classes they do get the notion that glass is not a solid. I recognized that
as bullshit the first time I heard it--the teacher gave a definition of
"solid" which glass met in every particular and then said "glass is not a
solid", but could not explain why it was not a solid and could not explain
what it was other than "glass" like "glass" is some fifth state of matter.
The notion that it's not solid is really a philosophical debate about the
definition of "solid" and has little do to with the realities of structural
glass is amazingly flexible in long pieces. watch them unload 12'x4'x1/2"
glass sheets from the case sometimes. they'll flex back and forth by
multiple inches. i have some double pane windows in my house that are 10'x6'
of normal strength 1/8" glass, and i can press in the middle of the pane to
see the sheet flex.
Slightly OT, but of interest, I bet:
Long, long ago in this NG, there was a discussion about where to
locate lifting straps to hoist a flat panel. It was one of the
better discussions I can recall, and brought out some of the best
of the engineering types from even other NGs to get an answer. I
even passed along the discussion to a nephew, who worked for a
concrete panel company. What I remember most was to "lift on the
5ths." In other words, you divided the panel (or shelf or
retaining wall etc) into 5 parts, using 4 lines and then support
it at the first points in from the end. It assumes that the panel
is of uniform thickness and strength. It's counterintuitive in
many respects, but recognizes that the interior sections lend some
mutual support and also the outer panels "lift" as they pivot
around the point of attachment.
I assume that this would also apply to an UNLOADED glass, wood or
shelf of other material. Of course, with an uneven distribution
of load, such as a big weight in the center, this might not be
YMMV and also this is based upon my poor, addled brain thinking
back many years.
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