Advice on torsion box entertainment center

[John Milanski]

Hi all,
I'm trying to build a 12ft wide entertainment center using torsion-box shelves (see attached .pdf). The current design is made of 4, 3ft bookcase sections screwed together. Each bookcase section has 3ft span torsion-box shelves within it. It's all done with 3/4" ply for the vertical pieces.

What I'd like to do is reduce the number of vertical pieces, essentially making a 12ft wide, 8ft high wall of torsion box shelves, supported by a 3/4" support every 3 feet. The question is, will these 3/4" plywood supports be strong enough? Or should I double them up?

Thanks again!

-- John Milanski

[Rob Russell]

May be a dumb question, but do you have an 8' door to get this into your house?

[John Milanski]

No, not a dumb question. With the orignial design, I was going to build the 3' wide x 8 to 10ft high bookcases separately, and then bring them inside. With the new design (super long torsion shelves with 3/4" ply vertical support every 3ft), I planned on building it in place...

[Jamie Buxton]

John, I'm not understanding your two designs. In both cases, the shelves are 4' long, and 3/4" material runs vertically. Despite my confusion, I'd say that 3/4" verticals are just fine.

Well... one more thought. You need something to prevent the whole shebang from collapsing sideways. Put a back on one of your 4'-wide sections, or fasten the cabinetry to the wall. Whatever means you use to prevent sideways collapse will help prevent one mode of failure of the vertical pieces. If you push on the end of a thin long stick, the middle may bow out, which leads to collapse. In preventing the middles of your cabinet verticals from bowing, you prevent that failure.

Jamie

[Jamie Buxton]

John ---
Here's another thought....

Your picture looks fine in the front view, but it conceals the fact that you're storing objects of several different depths. CDs are only 6" deep. Bookshelves are typically 10-12" deep. Electronic components are usually 17" deep, and need 20" or so after you add cables and connectors. TVs in your size range may be 25" deep if they are CRT-based, and 5" deep if they are plasma. If you make the whole unit deep enough to handle the TV, everything else will be way too deep.

Usually I break up a big entertainment wall. I make some areas deep, to handle TVs and electronics, and some areas shallow, to better serve books and art and such. You can break up the wall vertically, so that one 4'-wide section is deep. Or you can break up the wall horizontally, so that the lower parts are deep and the upper parts are shallow. Or you can do some combination of both approaches. A good thing about different depths is that it helps break up something which would be a monolith looming over the room.

Jamie

[John Milanski]

Thanks for your thoughts Jamie! Very helpful!

It's hard to describe the design in words. Originally, each of the 4, 3ft wide, 3/4" ply bookcases was a different depth. That would make the pictured, original design that much easier to build.

But what that design also ends up with is two, 3/4" vertical pieces of plywood screwed together at 3', 6', and 9' (one 3/4" vertical is the right side of one bookcase, the other the left side of the next bookcase). I was hoping I could essentially remove one of the two vertical 3/4" pieces of adjoining bookcases, attaching shelves to a common vertical support. (damn I wish I had a picture)

I'm just afraid that one, 3/4" vertical side of plywood every 3 feet is just not enough to support the weight of the shelves. Not afraid it will collapse sideways (the whole unit is sandwiched between two walls), but that the vertical will splinter under the compressive weight of the shelves.

Not sure that explains, but i tried. Any more thought from anyone? Thanks!

-- John

[Steve Jenkins]

John how thick are your shelves. It looks like they are just 1-1/2. If they are thicker you can make them from 1/2" and all the ribbing inside from 1/2" and it will cut down the weight a lot.
I don't think you will have a problem though, with all the shelving the dividers will be braced from bowing since it is between two walls. I assume that at least some of the shelves are fixed? Steve

[Jamie Buxton]

...snip...
I'm just afraid that one, 3/4" vertical side of plywood every 3 feet is just not enough to support the weight of the shelves.

-- John

In a word, "naw"! It takes an enormous force to crush a piece of 3/4" ply standing on end. As I tried to explain in an earlier post, if the 3/4" piece gets real tall, and you put a huge force on it, the failure mode is that it buckles; the middle bows sideways and then the enormous end-loading can collapse it. However, you're going to have shelving which prevents buckling, and backing or ties to the wall, which also prevents buckling.

(As a matter of esthetics, I find that 3/4" wide walls on something that big looks too small. I like to make them 1 1/2" wide, generally by doing something like a face frame -- lumber applied to the front edge of the plywood. The wall is still 3/4" ply, but the front face is 1 1/2" wide. This lumber also takes care of the issue of covering those nasty plywood edges. But my taste probably isn't yours. :) )

Here's a link (http://www.jamiebuxton.com/cabinet%208.html) to a cabinet I built. As you can see, it is very heavily loaded, but hasn't collapsed. The walls are made from 3/4" MDF, with 1 1/2" wide lumber on the front edge. There is a 1/4" back which helps to prevent the walls from buckling. (You can also see me playing with different cabinet depths vertically and horizontally, just to break up the bulk of this huge piece.)

Jamie

[Todd Burch]

John, I get an error trying to open the PDF with Acro. 4.0. Perhaps 6.0 is required...

Anyways, I've built a few of these.

Question - why did you change the design? The first design you had - qty=4, 3' cases, is a good approach. What's driving the change? It certainly won't be "easier" to build as one big piece, and the material savings, if that is what you are going for, isn't worth the time (in my book).

If I made this in 4 pieces, I would provide for a 3" worth of stiles on each of the 5 verticals. If a change in depth occurred at two adjacent cases, I would would split the 3" to 1.5" and 1.5, or change to 4" and split it to 2" and 2".

I would allow 1" air gap between each case, and 2" at each end. I would have each stile overlap each side, in the inside of the case, 1/4".

A < 3' shelf really doesn't require torsion box construction. A 2" wide front edging is good enough. If you are really worried about sag, put an additional 2" edge at the back. I prefer wide (wider than 3/4") front edges on shelves.

Todd

If you go with the 4 cases, you can make them narrower than 3'. If your wall is exactly 12' wide, I would make the two middle cases 33" wide and the two outer cases of them them, for instance, 3' minus 3", or 45" wide each.

[Jamie Buxton]

One small point of disagreement with Todd.... In John's sketch (which Todd hasn't seen), he's got a biggish TV (36" wide) sitting on a shelf which has two doors directly below. The doors look like they are inset, and they have no center stile between them. If the TV shelf sags, it will run into the doors. If I were going to use that style of doors, I'd probably make the TV shelf from two layers of 3/4 ply, and maybe even put a beam underneath it, hidden behind the doors. I get real embarassed when doors don't fit.
Of course, the doors could be changed to some other design which would better accomodate a little sag in the shelf, which would be the more practical approach.

[Jamie Buxton]

John ---
Your pdf is a little bit of a problem. Todd can't even get it to load. I can, but can't zoom out far enough to see the whole image; Acrobat is zoomed out to the max.
What are you using for your drawing software? Maybe somebody can suggest a way to make a drawing which is a little more reader-friendly.

Jamie

[Todd Burch]

Jamie, perhaps there is a vertical partition behind the doors, even more inset. I do this a lot in my designs: cause structural elements to not affect aesthetic features.

[Jamie Buxton]

Jamie, perhaps there is a vertical partition behind the doors, even more inset. I do this a lot in my designs: cause structural elements to not affect aesthetic features.


Good approach, Todd.

This does raise the question of what's behind those doors. For some reason I was presuming there are things which are wide enough to collide with a center style. If instead, what is intended to go in there is electronics components like receivers and such, the hidden stile would be great. However, I'd make this cabinet module a little wider. Most components are now 17" wide, and they like a few inches on each side for air circulation. 17+17+2+2+2 = 40" internal width.