Torsion box shelf-all wood or reinforced with steel

[tiralie]

I have had a request by a client to build the following cabinet in Oak. He wants me to build it in solid Oak but that is another discussion.
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The second shelf from the top is 60" long by 14 1/2" deep and 1" think. He doesn't want to have a divider or support in it. Below it is a cabins for a flat screen TV. The client has told me he will be putting encyclopedias on this shelf. At over 30lbs. a foot solid oak, and veneered (mdf, chipboard) will sag a considerable amount.

I believe a torsion box is the best solution, however I am not certain a wooden core torsion box will be sufficiently rigid enough and have designed a steel core (see exploded view drawing below) to support this weight.

Any comments, questions or suggestions?
Thanks.
Tim

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[GhostFist]

Looks pretty bloody solid to me. Would like to see some pics of you building this shelf. If it aint wood I aint no good so I've an interest in working with metal and in particular how it could enhance my woodwork.

 

[Alan m]

i dont know how strong the torsion box would be (only one way to find out) but the steel one would work perfectly.

 

[Guy Ashley]

Tim

Just thinking completely out of the box here, and I am no engineer, but what about 5/8ths stainless steel rod routed into a groove front and back and then lipped to hide it.

Would a stainless rod of that diameter sag???

 

[windmill man]

Think you may be better with a bit of box section or angle if you have the depth of section to hide it.

Its a big ask 150 pounds of books on any shelf of that length and that thickness

 

[Theshipstore]

Tim  I like the steel core idea better than the wooden torsion box alone.. What's your thinking on the thickness of the steel. I would think something like 3/8" might be good encased in a wooden box frame going from edge to edge. I think this would handle the weight just fine.

 

[jmbfestool]

Just an idea!   RSJ's are used to hold alot of weight.  The upright section of the RSJ is the weight bearing part of the steel the top and the bottom are just to stop the steel from twisting. So you can have your wood as the top and bottom part of the metal and just have some metal uprights inside your wood.

It wont be alot of work as you would only need to router some dados (channels) and then PU glue some 18mm by 4 mm 60'' long steel plates.  As your idea is alot of work and I dont think would be any stronger as a torsion box wouldnt be affective so narrow you might aswell just have thin long plates upright routered into the wood. This will allow you to keep your wood thicker increasing the strength and still stay with in your 1 inch.  Your torsion box you basically making your wood almost as thin as veneer to cover your torsion box.

I have done a sketch up to scale ish.    The timber is 18mm thick so thats just above your 1inch require meant and the steel is like I said 18mm by 4mm  so leaving plenty of wood.    I suppose you could make it it less so its 13mm thick for your to planks of wood so you get your exact 1inch and make the steel 12mm by 4 ish but that might just be to thin suppose you could keep it 18mm just your gettin close to the edge of the 18mm wood which could allow it to cup but! using alot of glue should prevent this any way!  I think it will still be stronger than a torsion box covered in oak veneer  [tongue]

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[pugilato]

You might want to consider a deep dado along the backboard holding that long shelf without sagging.

 

[jmbfestool]

You might want to consider a deep dado along the backboard holding that long shelf without sagging.

Or just glue and a alot of screws?

If your saying a deep dado  it would have to be atleast an inch or more for the depth to actually affect the front of the shelf from sagging  if you was just do do a 10 mm or more then just glue and screws or Dominos would just do the job cus I dont think hes going to put a backboard thicker than 1inch or more

jmb

 

[tallgrass]

well, i am an ME and that is a very long long shelf indeed. my advice is to build one, load it and measure deflection. then check with your client. i think you will be surprised. a shelf loaded with books is extremely heavy, this is all about cross sectional area of your shelf.

 

[davee]

Tim, I am an engineer as well. I agree with Tallgrass that building a test would be best to demonstrate the issue to your customer.  If you use steel, you'll want the steel on the outside surfaces, not the inside.  Where you show the steel doesn't offer much benefit.  JMB's approach is better where the steel more in the vertical position.  The concept on having I-beams or c-shaped cross sections is ideal but doesn't fit into the customers expectations.  A torsion box works well, but you need thickness to make it effective.  Good luck.

 

[jmbfestool]

Tim, I am an engineer as well. I agree with Tallgrass that building a test would be best to demonstrate the issue to your customer.  If you use steel, you'll want the steel on the outside surfaces, not the inside.  Where you show the steel doesn't offer much benefit.  JMB's approach is better where the steel more in the vertical position.  The concept on having I-beams or c-shaped cross sections is ideal but doesn't fit into the customers expectations.  A torsion box works well, but you need thickness to make it effective.  Good luck.

Oh yeah! +1 for me! Jokes

I thought of this (sketchup below) but its getting something like this made and I thought my idea above was the easiest and quickest cheapest but still give alot of strength to the shelve.

This box section would be far stronger than a torsion box for something so thin I believe you should have your shelve to be a minimum of 32mm for this type of design cus your bottom and top would be about 4mm so you would be losing 8 mm having this box section  like an I beam but like I said the upright is the part which is the main support and if possible you want that as wide as possible an by having a top and bottom to create an I beam your loosing the depth and something so narrow every mm counts.  So my idea above just having upright inside your wood allows you to keep your shelve as thin as possible but allowing you to have your upright as wide as possible.
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[upscale]

The second shelf from the top is 60" long by 14 1/2" deep and 1" think. He doesn't want to have a divider or support in it.

Without a support of some type, I'd be wary of it sagging. It if was mine to build, I'd have the second self with a face frame of two inches and a piece of angle iron behind it. Of course, to make the entire unit look proportional, you'd then have to make the sides, top and bottom appear to be 2" thick also and quite possibly the wood surrounding the TV as well.

 

[pugilato]

I dont know, but this client seems to be presenting you with a losing proposition.  Unlimited funds to test the various options would be nice, but not in the nature of most clients.  Putting the heaviest item on the weakest shelf is non-sensical.  You are going to have to design the whole thing around that one shelf.  I still think that a deep dado on the backboard, in conjuction with dados on the vertical supports on either side of the shelf, and a support on the front of the shelf will do you right.

Another warning sign is the client's insistence on using wood (not plywood).  I dont know if I would personally take on this job without covering my arse with a significant amount of paper.

 

[tiralie]

Thanks for all your comments. I sure hope I get this job after all this.

Would like to see some pics of you building this shelf.

If the client agrees to the price I will definitely post. It won't be cheap.

Guy, that was my original thought. I will talk to my metal supplier and see what they say. My gut says it would flex too much.

Think you may be better with a bit of box section or angle if you have the depth of section to hide it.

Not sure I understand "a bit of box section or angle" do you mean box steel and/or angle bracket?

What's your thinking on the thickness of the steel. I would think something like 3/8" might be good encased in a wooden box frame going from edge to edge.

Yes,  3/8" would be maximum width I think. I have not discussed this with the supplier who would weld the frame .

my advice is to build one, load it and measure deflection.  this is all about cross sectional area of your shelf.

I believe that is how I will approach this. If the client agrees the the price I will build a prototype all wooden and then stand on it. I am 225 lbs. Should be good if it can support my weight.

If you use steel, you'll want the steel on the outside surfaces, not the inside. 

The client doesn't want to see the steel but I could put a thin veneer on it if it's the outside edges.

JMB's approach is better where the steel more in the vertical position. 

Really, you are going to give JMB way too much confidence here! [tongue] One of my first thoughts was a version of JMB solution. I was only thinking of two bars vs. his 4-5. I was worried about lateral torsion but because the shelf is fixed to both sides of the cabinet it shouldn't twist.

A torsion box works well, but you need thickness to make it effective. 

Isn't thickness relative or is that what you mean...the shelf is too thin to the length to be effective as a torsion box. Aren't airplane wings large tapered torsion boxes?

I thought of this (sketchup below) but its getting something like this made and I thought my idea above was the easiest and quickest cheapest but still give alot of strength to the shelve.
So my idea above just having upright inside your wood allows you to keep your shelve as thin as possible but allowing you to have your upright as wide as possible.

Thanks JMB, I think your concept would be easier to fabricate (cheaper for the client) than my original concept. Based on tallgrass and davees vote of confidence, I should probably mock this up and test. I promise not to post any video of my jumping up and down on it though! [wink]

Without a support of some type, I'd be wary of it sagging. It if was mine to build, I'd have the second self with a face frame of two inches and a piece of angle iron behind it. Of course, to make the entire unit look proportional, you'd then have to make the sides, top and bottom appear to be 2" thick also and quite possibly the wood surrounding the TV as well.

I agree completely. The client insists on 1" thick shelves, and sides. The client is copying a design he currently has but he is moving to a new house and wants the same thing.  I think the problem presented (shelf) is quite interesting and if he is willing to pay to achieve his goal we both win.

Putting the heaviest item on the weakest shelf is non-sensical. 

I know you mean this in the kindest way  [smile]. It's different I'll say that.

I still think that a deep dado on the backboard, in conjuction with dados on the vertical supports on either side of the shelf, and a support on the front of the shelf will do you right.

The back is going to be a maximum of 1/2" probably 1/4". So no dado there. I would have to use blind dadoes on the vertical supports as he doesn't want to see a break in the vertical line. That would be ok, as long it was the only thing I had to do. As I stated earlier a support on the front would have to be hidden, but it is doable and not something I had considered.
 

Another warning sign is the client's insistence on using wood (not plywood).  I dont know if I would personally take on this job without covering my arse with a significant amount of paper.

Yes, noted. Lots of Green paper too.

Tim

 

[Kevin Stricker]

Tim,
  Build a false back for the Flatscreen opening.  You don't want it on the back of the wall 1'  back from the front of the built-in anyways.  Building out that panel will give you a location for all your cable and make mounting a breeze.  It will also make your long shelf very strong.

 

[tallgrass]

this may be a little lateral but i had a similar project a while back( not bookshelves but similar engineering problem) wish i could share it with you guys...it was cool.

the solution was to make a lamination using fiberglass and wood with a slight arch to it. this yielded a very strong beam with a very light weight and the arch took up the sagging look and transferred some of the load to the outside of the case. this could be easily made with a vacuum bag fiberglass and that thin bendable plywood layered up in layers leaving you the only task of veneering for the salutation of a solid "wood" shelf that would be resistant to the evil drooping of fatigue..thoughts?

 

[tiralie]

Tim,
  Build a false back for the Flatscreen opening.  You don't want it on the back of the wall 1'  back from the front of the built-in anyways.  Building out that panel will give you a location for all your cable and make mounting a breeze.  It will also make your long shelf very strong.

Kevin:
Thanks. The client wanted to have some flexibility if he bought a bigger TV so I really didn't think too much about the opening for the TV. I will discuss your option with him. He is quite set in what he wants, but he does listen.

the solution was to make a lamination using fiberglass and wood with a slight arch to it. this yielded a very strong beam with a very light weight and the arch took up the sagging look and transferred some of the load to the outside of the case. this could be easily made with a vacuum bag fiberglass and that thin bendable plywood layered up in layers leaving you the only task of veneering for the salutation of a solid "wood" shelf that would be resistant to the evil drooping of fatigue..thoughts?

I really want to stay away from veneering, but if it's the right solution I will do it. I am not fond of laying up fiberglass (all that epoxy) and I am not sure how this will save any more time and or material costs in fabrication than say JMB's idea.
On this type of project I tend to just want to get it done, and with this shelf I really want enough it to have enough strength with the minimum effort.
Tim

 

[ajones]

Hi you could always pre stress the shelf there would need to be some allowance for movement at the ends

 

[upscale]

On this type of project I tend to just want to get it done, and with this shelf I really want enough it to have enough strength with the minimum effort.

Tim, the only other option I can think of at this point is a bunch of blind shelf supports. Certainly, more than one would need for just supporting a shelf. In any event, I'm most interested to see what solution you choose to use.
http://www.leevalley.com/en/hardware/page.aspx?p=51933&cat=3,43648,43649

Dave