Engineering question...

I want to construct a simple cabinet for holding a 25'' TV set. I'm planning to construct the surfaces of MDF and make some structure of pine to give it the strength to hold the TV height. The structure will resemble a table: four legs joined by four rails to support the upper panel (table top). Three sides will be covered by panels (one will stay opened for shelves/drwawers) and also will be a bottom panel. Panels will be secured to legs so it will give the cabinet more strenght. The question is which width I need for the pine beans that will conform the structure? The cabinet will be about 80 inches long by 20 inches height and width. The holding "legs" will be 20 inches apart from each side. I wonder if I'll need another central rail. Any other idea for giving the cabinet strength? There are any existing tables on the net for estimating material withstanding?
Any suggestion will be appreciated Thanks in advance
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Faustino Dina
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You can find information on elasticity modulus on the net also. This will tell you how much sag you will get with a weight application. Lots of neat stuff out there. http://www.engineersedge.com/beam_defl_even.htm
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Great structural information site Greg, I for one thank you.
--
Chipper Wood

useours, yours won't work
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Wow, that is a fantastic site. Yes, thank you, Greg.
Dave Hinz
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Nice formulae BUT... - I dont know the Modulus of Elasticity (E) for 1/2 MDF edge banded by a pine beam - I dont know the Moment of Inertia (I) of the same arrangement
I suppose you guys when plans your furniture doesn't relies on such pretty formulae, but on experience. I was expecting some response like " 2 x 2 pine beam will be ok"....
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If you ask an engineering question, you get an engineering answer.
Mike
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I'm not an engineer but I think engineers doesn't recalculates structural ressistence for each project... I believe they REUSE some design patterns already calculated by someone else and tested an validated for many others in practice...
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Sometimes you can. More often you can't. In almost all of my designs, I had to do my own stress analysis because there wasn't anything else around similar enough with published data.
todd
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structural
patterns
others
of course nobody else has enough data to satisfy the todd.... not in politics OR woodworking.
randy
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Faustino Dina wrote:

Depends on what you're doing. For some things there are codes and standard analytical procedures, for others there aren't. But it's rare that a full stress analysis of a subassembly can be lifted from one project and dropped into another--there's usually enough difference in some aspect of the loading or attachment to merit recalculation even if the assembly itself is unchanged.
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--John
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You can find MOE, density and other properties of MDF here http://www.lungster.com/l/speakers/mdffaq/mdf.html#Q3

pine
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It's hard to see exactly what you mean without seeing a plan.
I have built entertainment centers before and the TV's are very heavy and must be accounted for carefully.
What I would do is cut your MDF to size, attach some pine as you say underneath it to give it extra strength and place the MDF on two sawhorses at the same distance as you plan to install your legs and put your TV on it and see if it sags at all. That will tell you how many legs you need. Be conservative. If in doubt, add strength.
I assume you are planning on painting it so if you have to add another leg, you can cover any mistakes with paint.
Or, you could just add another leg anyway. Not a bad idea in my mind.

planning
it
Three
shelves/drwawers)
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    Being a graduate structural engineer, I surely love the site referenced, but alas as you may of seen or pointed out, how do you find the values to plug into the deflection calculations. Instead of running the numbers, set up a experiment. Figure out the span that you want to start with, support it on the ends and place a like load + 20% (ye ole fudge factor, which is MOST important) with bricks, blocks, stones, etc in the middle of this span (creates the most deflection). Since the supports at the end of the span, for your test will be simple supports, then your deflection will be greater than the supports you will create (fixed or moment supports). Observe and measure the deflection. Remember the weight of the material will generate some deflection, though you may not see it with your eye. If you are uncomfortable with the deflection, then move the supports in and retest. If the span length is too short for your comfort, then add stifiners and retest. I suspect that you will quickly come up with a suitable design. How do you think the engineers/architects of old did this type of work ? Through testing and trial and error (though the errors some times resulted in loss of life).
HTH JAW
Faustino Dina wrote:

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