when to use 1/2" vs. 3/4" veneered plywood?

The usual Google searches didn't turn up much on this. Is there a rule of thumb on when to use one vs. the other?
It's specifically for a custom sized version of this:
http://www.staples.com/images/products/catalog/enlarged/430593_01_enl.jpg
There will be around 80 lbs of weight on the bottom piece (UPS and computers) and maybe 10lbs each on the shelves (printer/scanner), so I'm thinking only the bottom piece has to be 3/4" plywood and the rest can be 1/2" to save weight, although the desktop may look better w/ 3/4" plywood. I did find a reference that each 4'x8' piece of 3/4" plywood weights 75 lbs (vs. 50 for 1/2") and this project will require roughly 2.5 sheets, so it'll be a difference of 75 lbs...
Thanks in advance,
ken
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Not "may", it will look better. Alternatively, you can put a 3/4" or 1" strip of hardwood on the edge to give the illusion of thickness.
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wrote:

Also sturdier. I can bend a piece of 1/2", but have trouble with a 3/4 ". Consider "relative" thickness and strength. Although not exactly linear, you are talking about a 50% increase. I used high density 3/4" particle board with a good hard surface material [formica?] applied with contact cement, edged wih solid wood. That was a long time back, and it's still good.
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Sun, Dec 12, 2004, 2:45am (EST+5) snipped-for-privacy@yahoo.com.SPAMGUARD (Ken Yee) wants to know: <snip>
http://www.staples.com/images/products/catalog/enlarged/430593_01_enl.jpg
<snip>
I've found 1/2 plywood amazingly strong. I've got some knock-down bookshelves, with about an 18" span, the work well as chairs. They'll take around 300 lbs, no prob.
I'd support a piece, so it's as wide as your shelf would be, and put weights on it, and see how it does. Of course, it may look OK, but still bow a bit in time. If you're unsure, you can either go to 3/4", or run a brace or two on the bottom side. You could probably even add the brace later. Or, you could laminate two pieces of 1/2" - I do that at times, because that's all I've usually got on hand. One of my kids gave me a big chunk of that laminated stuff that's about 2" thick. No prob about that stuff bowing, but it's almighty heavy.
JOAT We cannot direct the wind, but we can adjust the sails. - unknown
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Ken Yee wrote:

If you're concerned about the ability of half inch ply to hold 80 pounds of UPS and computers then think about the ability of the desktop to hold whoever decides to sit on it. And somebody _will_ decide to sit (or lie, or do unspeakable but vastly enjoyable things) on it.

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--John
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And I supposed this is from personal experience? ;-)
Thanks. I think I'll go for 3/4" plywood on the bottom and desk horizontal pieces and 1/2" plywood everywhere else w/ all the parts hooked together w/ Kreg screws (except the front two posts which will have through screws)..
ken
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Ken Yee wrote:

I'd think about doubling up on the top, or putting a little bit of structure under it.

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I'm actually changing the depth of it from the 30" of the original down to 24" to get it to fit into a particular area, so hopefully using a 3/4" plywood top should be ok. Also shrinking the width to 48" to fit, so it should just take 5 pieces of 2'x4' plywood...
Still think I'd need to double the top's thickness to 1.5"?
thanks,
ken
p.s., no plans to do unmentionable things on top of it ;-)
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Ken Yee wrote:

Well, I've got a monitor stand here that's 2 feet wide with a 3/4" top and after several years with a 19" monitor sitting on it it's got a definite sag.

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A 19" monitor is somewhere around 80lbs. I had a 17" one that was enough of a bear.
Definitely look into an LCD. My 19" is maybe 20 lbs. Gobs easier on my eyes and one of the best computer purchases I made :-)
ken
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I've got a 19" CRT monitor: it's 62 lbs.
Eventually, someone WILL sit on your desk. :)
David
Ken Yee wrote:

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I designed and built one using a sheet and a half of 3/4" oak ply.and some oak and cherry scraps . There are a few pictures on and some dimensional info on my web page http://members.tripod.com/mikehide2 .
Actually as far as strength goes 1/2" ply is probably fine however the real consideration is probably deflection, in that case it is not so attractive . mjh
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Very impressive work! Love the chairs and that large desk w/ the elevators! And the carving. Wow. :-)
Does using splined wood for plywood edging make a difference in strength at all? I was planning on using iron-on edge veneering to keep it simple. How did you get the wood edging to curve around that keyboard cutout for the desk top? Interesting way to hook that left shelf up as well.
ken
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the splined edging allows you a reasonably strong method of joining two ply sections . As far as increasing he overall strength it may contribute but I would not imagine by much .
As far as the edging , I was referred to an article in FWW.by someone on this group. It is a simple method and consists of a thin section of oak in this case ironed on to the ply edge which has been previosly liberally coated with tightbond glue [or similar glue] and let dry . I must admit at the time I had very little faith in the procedure but apparently it works very well .....mjh
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