Replies

1. 
   ian | Jun 06, 2004 02:22am | #1

   Derek,

   what will be the load in the box?
   how well will the load be distributed between the top and bottom stretcher?
   the long dimension of the box?
   the long dimension of the stretcher?

   I suspect that it is all academic as the box will stiffen the stretcher and wilst you may be able to measure the deflection with a straight edge, you wont be able to see it by eye.

   Ian

2. 
   UncleDunc | Jun 06, 2004 02:31am | #2

   For the part of the span where the top and bottom stretchers are rigidly attached to the box, you can think of them as flanges on an I beam. The calculation for that portion has to be done with the box. You can't just calculate the stiffness of the stretchers and add them to the stiffness of the box. Well, you could, but it would understate the stiffness of the whole structure.

   Between the box and the legs, where the stretchers are attached only at their ends, you can't add their thickness together to compute the stiffness, because they're not acting together as a beam. You have to calculate the stiffness for each one and then add the stiffnesses together.

   That being said, why are you computing deflections? Unless you're storing your emergency uranium supply in the box, there won't be any deflection. Members that are sized to be visually appealing are almost always much, much stronger and stiffer than they need to be. If you size the stretchers just big enough to carry the load with a maximum acceptable deflection, they will look like they're about to break.

3. 
   be2003 | Jun 07, 2004 07:57pm | #3

   If the box is rigidly attached to the stretchers then the stretchers attached to the top of the box will be in compression, where as the stretchers on the bottom of the box will be in tension, so I don't think the two members can be added to each other to form an aggregate member.  I agree with the previous post that the assembly can effectively be treated as an "I" beam member because all of the forces will be concentrated on the outside faces of the stretchers with the suspended box acting as the web of the beam.  What are the dimensions of the stretchers and box, what kind of wood will you be using?

1. 
   DBishop | Jun 07, 2004 08:06pm | #4

   The box will be about 13" by 18" by 18" and probably made from Leopardwood (?? Any info about this wood would be helpful).  The box will probably hold a phone, answering machine, phone books magazines etc.

   The stretchers and legs will be made from Jatoba (which as I understand is super strong) and I was planning on using 1.25" square stretchers.

1. 
   chatobob | Jun 07, 2004 08:19pm | #5

   Sounds like it will hold a few dozen 12" thick cement blocks, in addition. 

2. 
   be2003 | Jun 07, 2004 08:33pm | #6

   Here is some info I found on Leopardwood:

   http://www.woodworkerssource.net/Merchant3/merchant.mv?Screen=CTGY&Store_Code=WS&Category_Code=Leopardwood

   It sound like your design will be pretty stout, I wouldn't worry about deflection over that short of a span.

   Sounds like it will be a great looking piece, be sure to post some pictures when it is done!

1. 
   DBishop | Jun 07, 2004 08:37pm | #7

   It sound like your design will be pretty stout, I wouldn't worry about deflection over that short of a span.

   Thanks for that.  I hope not.  FTR the run is about 32.5 inches the box will be suspended in the middle of the stretchers.  I am going for kind of a box within a table look, as seperate pieces rather than a "cabinet/table" hybrid.

   Sounds like it will be a great looking piece, be sure to post some pictures when it is done!

   Thanks, I'm really excited about the vision I have in my head.  I just hope my hands will allow me that vision to come about...

4. 
   whward | Jun 09, 2004 05:18am | #8

   the three elements act in parallel (are addative) as separate elements unless the joint between them is strong enough to carry shear loading.

   If it is strong enough, it is a somewhat complicated calculation and the combination is much stronger.

   Bill Ward  BSME '68 Univ of Mich.

1. 
   DBishop | Jun 09, 2004 07:25am | #9

   the three elements act in parallel (are addative) as separate elements unless the joint between them is strong enough to carry shear loading.

   I was planning on gluing them (long grain to long grain) in a patch near the middle (so as to not inhibit wood movement)  Would this joint be strong enough?