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It Seemed Like A Good Idea

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Blog entry by kshipp posted 04-20-2010 04:34 AM 1157 reads 1 time favorited 6 comments Add to Favorites Watch

I needed a bookshelf because I had books all over the place on shelves that weren’t specifically made for books. They worked okay but I wanted something a little more purpose built. I didn’t want it to cost alot but I also wanted it to be something kind of unique because that is just the way I am.

So I came up with this idea to make a simple frame out of 2×4’s and use wire rope for the shelves. I thought I had everything measured out perfectly and the build went exactly as planned. I’m working in an apartment right now so I’m mostly limited to hand tools. And my trusty Kreg jig of course.

I was lucky to have wonderful weather where I could work on out the patio. You can see my fancy portable workbench out there. I was pretty impressed with it’s holding ability though.
Outside

I used the string to line up the shelves. I wanted to make them on an angle so I wouldn’t need to use bookends. The thinking is that gravity would hold the books upright.
string

These are the support pieces for the wire rope.
supports

This is one end of the wire rope. The wire rope is looped through and eye bolt which allows me to tighten the wires. The other end is just through a hole in the 2×4 with a wire clamp on the end. I tuned them to about a ‘G’. That seemed to be the right tension. :)
wire end

This is some books on the bottom shelf. You will notice they are facing a different way than books normally do on a bookshelf. That is because this bookshelf is actually terrible at holding books. Oops.
oops
Also the spacing on the wires allows some of the books to slip through even though I thought I measured for that.

The wires still have some sag in them even if I tighten them more because tightening one wire will make the other more loose by drawing the sides in. I was kind of surprised by that since we are talking about 2×4’s on edge being pulled in.

So I’m looking for suggestions on how to salvage this. My thought was to add plexi-glass shelves over the wire so it would still maintain the look of kind of invisible shelves but the books would have something more solid to sit on.

What do you guys think?

-- Kyle Shipp, http://battleshipp.blogspot.com



6 comments so far

View David Craig's profile

David Craig

2135 posts in 1854 days


#1 posted 04-20-2010 06:00 AM

I think the concept you are attempting to put together is brilliant, but I think you have a slight flaw in the current design. Even with plexi-glass, I believe you are going to see shifts in gravity that will end up flipping the glass or tilting it enough to cause the books to go off balance and ultimately fall to the floor.

I am not an engineer by any means, but what I think you are attempting would be better accomplished if the ropes were tightened on the other ends of the 2×4s, by a winding mechanism of some kind. I don’t believe you can achieve the rigidity you require with the tension on the interior, there really isn’t any leverage you can work with that way. I believe the frame is salvageable, you just will have to put your thinking cap on and see if you can run the ropes through the holes to the exterior.

My two cents anyway,

David

-- There is little that is simple when it comes to making a simple box.

View Greg..the Cajun Wood Artist's profile

Greg..the Cajun Wood Artist

5261 posts in 2054 days


#2 posted 04-20-2010 06:43 AM

your design is unique and I am surprised about the sag in the 2×4’s, but it is wood… and over time the tension from the wires will cause the movement.

That reminds me of a bookcase I built years ago from a design requested by a client. I built the case out of teak and the adjustable shelves were made of 1/2” tempered glass. The glass shelves were mucch more expensive than the teak..

-- Each step of every Wood Art project I design and build is considered my masterpieceā€¦ because I want the finished product to reflect the quality and creativeness of my work

View jabber828's profile

jabber828

1 post in 1887 days


#3 posted 04-20-2010 04:12 PM

Love that design! I want to make one now.

You could run a single cable threaded through all of the eye-bolts (would probably require orienting them vertically) and put a tensioning device of some sort on it. That would relieve the issue of uneven tension on the cables while still letting you cinch it down tight and would stop the sag.

-- Joe B, Texas

View DragonLady's profile

DragonLady

298 posts in 1753 days


#4 posted 04-20-2010 05:03 PM

I’m sorry, I’m snickering because I’m evil, and it’s good to see that I’m not the only one whose ideas don’t always work out.

It’s a cool idea, though. If you used plexiglass, you’d have to anchor it to the wires.

-- A woman's work is never done-but power tools help!

View JJohnston's profile

JJohnston

1593 posts in 2037 days


#5 posted 04-20-2010 05:55 PM

Well, I am an engineer, and although I’m not well versed in the type of structural system you’re using here (the cable bridge), I can give you some thoughts based on fundamental principles.

First – I don’t quite follow what David Craig is saying, but it doesn’t matter how you put the tension in the wires; the force will be counteracted by the bending strength in the sides, and will ultimately be transferred into the horizontal rails, which will be in compression end-to-end. The total compression in the 4 rails will equal the total tension in the wires (technically the “horizontal component of the tension” because of the slope in the wires, but no need for the complication).

I’m not surprised that there’s still sag in the wires, even when the stiles are being pulled in. You can crank a tremendous amount of tension into a wire rope with a threaded adjustment (and you’ll need to, to hold books), and the amount the stiles are pulled in is proportional to the height of the case (the length of the stiles) to the fourth power – which means it doesn’t take much span before the deflection (bowing) becomes excessive, even if the stiles aren’t being overstressed.

First, you’ll need to make sure the connection between the cable anchorage pieces and the stiles can take the force. So far, this doesn’t sound like a problem.

Then, I think the most effective way to counter the stile bowing and allow you to put more tension in the wires is to add a rail, front and back, at mid height. This cuts your stile span in half, so the deflection will be 1/2 to the 4th power, or 1/16 as much. It will also reduce the bending stress in the stile quite a bit.

The next step is to reinforce the added rails against buckling due to the end-to-end compression. I’d recommend putting a horizontal piece from front to back at mid-length. This member does not need to be big, since technically there’s no force on it, it just serves as a brace so that the new pair of rails can act together. This is taken care of in your top and bottom rails, since you have the solid top and bottom.

Finally, you’ll need a way to tie the individual wires of each shelf together so they don’t deflect sideways and let the books fall through. How about a pair of little slats, one on top of the wires and one on the bottom, bolted through with machine screws and nuts. The friction between the slats and the wires will prevent each wire from moving sideways independently (same principle as the rail brace). Start with a pair at mid length, and if that’s not enough, go to the 1/4 points. That will give you a look like a cable bridge with most of the slats gone.

As a side note, you’re having the same problem piano builders had for centuries: how to get a frame strong enough to get enough tension in the wires to get good sound. Wood frames were never strong enough, and it was only when the Steinway company figured out a way to incorporate an iron frame that we got the “proper” piano sound we know today.

-- "Sometimes even now, when I'm feeling lonely and beat, I drift back in time, and I find my feet...Down on Main Street." - Bob Seger

View jockmike2's profile

jockmike2

10635 posts in 2992 days


#6 posted 04-21-2010 01:49 AM

Hey Shipp you don’t want my advice, you saw my breadboard didn’t you? However, if you could strengthen those 2X4s with say some angle iron maybe hidden in groves in the back it might strengthen up your sideways pull enough to get the strength you need. But what do I know?

-- (You just have to please the man in the Mirror) Mike from Michigan -

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