Phillip Hallam-Baker wrote:

wrote in message
. ..
Which is better glass or plastic?
Mike

Depends. If you are building something really big you might
want to consider polycarbonate, its a lot lighter and much
stronger than glass. 'Bulletproof' 'glass' is actually
polycarbonate, often in a sandwich with safety glass.

My wife unit wants me to make some overlay frames for the
non safety glass windows on the ground floor of our house.
This is to stop children going through. The glass is
over a hundred years old and was made by blowing a cylinder
of glass, then opening it out to make a plate. These windows
are 50" square so replacement with the same stuff would be
expensive.

So I have been trying to calculate the thickness of
polycarbonate that stops a 40lb child thrown at 5mph.

I think you'll find that any commercial thickness will do that--the problem
is doing it without deflecting far enough to contact and break the glass.

Rather than trying to calculate it, make up a test rig the size of your
biggest window, put a piece of plywood behind it at the same spacing as the
windown, smear it with something that will transfer easily (peanut butter
if you don't have anything else), then toss a sandbag at it. If there is
no peanut-butter transfer then you're there. If there is, try the next
thickness up until you find the one that's stiff enough.

You might also want to mount it with some venting--if it's a tight seal then
even if the polycarbonate doesn't contact the glass, you might break it
from the air pressure increase at impact.

Rather than polycarbonate, consider acrylic--you're not trying to stop
bullets, you're trying to keep a kid accidentally walking through a window,
and acrylic is up to that just fine--further, it's a bit stiffer than
polycarbonate (as anybody who has tried to use both in insufficient
thickness for a router table insert will tell you) so you won't need quite
so much thickness to avoid deflecting it into the glass.

Lastly, if you are going with polycarbonate in the windows talk to the
building inspector first--the stuff is burglar-resistant which means that
it's also fireman-resistant and there are some special considerations with
regard to window latches and the like that may be required to work around
that issue.

Phill

--
--John
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