Best wood for long shelves

Page 1 of 2  

I am building a 6 foot long shelf that is going to be supported at the ends (no option of support in the middle). The shelf needs to support about 50- 70 pounds of weight (stereo equipment), so the "sag" (or deflection) in the center is going to be a problem. I plan on building the shelf as a hollow box, and using steel angle iron on the inside as a stiffener.
Just out of curiosity -- what commonly available wood product is best for resisting deflection under a continuous load (i.e. shelving)? The easily available ones at my local lumber store are: laminated pine, mdf, playwood and particle board, I am assuming that the laminated pine would be best, since all the grain is running longitudinally. Is my assumption correct?
--
Murray Peterson

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Check out the sagulator shelf deflection calculator. Might be useful. http://www.woodbin.com/calcs/sagulator.htm
--
Stoutman
http://home.triad.rr.com/brianmelissa/woodworking_frames.htm
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Murray Peterson wrote:

You could make the shelves a torsion box. Think hollow core door. Lightweight and very strong. 1/4" ply top and bottom, hardwood edging and pine innards. No steel needed.
Joe Barta
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
207.115.17.102:

That's pretty much what I had in mind, but with some 1/2 inch steel angle- iron screwed to the inside of the front and back edges for increased stiffness. Sounds like my design may be overkill for the application.
--
Murray Peterson


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Murray Peterson wrote:

Over 6 feet, 1/2" steel angle ain't gonna do squat. You'd get far more bang for your buck increasing the thickness of the torsion box.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Serious overkill Murray. The 1/2 inch would weigh a ton and you won't need the strength that it will bring.
--

-Mike-
snipped-for-privacy@alltel.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mike Marlow wrote:

I can't imagine 1/2" angle iron bringing much strength at all over 6 ft... and not that much weight either. Or maybe we're thinking different things. I'm thinking about 1/8" thick steel angle with each leg being 1/2" wide. Am I missing something??
Joe Barta
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yeah. other folks thought you meant 1/2" _thick_ steel -- a continuous piece running the length of the shelf.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Robert Bonomi wrote:

1/2" thick angle iron... in a shelf? I can't visualize that. What are the length of the legs and how would it be oriented?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

D'OH!!!! Color me the same color as a brain fart! I'm thinking 1/2" thick steel - and wondering why anyone would want to do such a thing. Some days are just really bad...
--

-Mike-
snipped-for-privacy@alltel.net
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Barta wrote:

The angle iron has a modulus of elasticity that is about 15 times that of wood.
That 1/2" angle iron gives about as much strength as a 1/2"x2" lip of oak under the shelf.
Of course, if you put the 2" dimention vertically under the shelf, the lip is about 16x stronger.
For those who care about the math, the deflection of a beam is inversely proportional to the width of the rectangular cross section. However, it is inversely proportional to the *cube* of the height of the cross section.
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks -- those numbers help me a lot. I wanted to create a shelf with quite a thin cross-section (1.2 inches at most), but then ran into problems with deflection.
Sounds like the interior piece of iron could be enough to get me what I need.
I'll put the thing together sometime in the next week (if work permits). I'll post back and let everyone know how it worked (or didn't).
Thanks to all for their helpful responses. Learning new things is always fun, especially if I can see the results hanging on my wall :-)
--
Murray Peterson


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Unfortunately, it has a moment of inertia (even assuming there are 2 of them as the poster suggested) about 350 times less than the shelf. In the end, unless 1/2" angle iron is a lot thicker than I think, it doesn't end up contributing much to the combined moment of inertia, which means it doesn't reduce deflection in a significant way.

i.e. not very much.

Of course, if the lip is made of tungsten carbide, it would be 30x stronger.

It's also directly proportional to the cube of the length. If the OP could reduce the width just 6", the deflection would drop about 25%.

todd
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
todd wrote:

Oh, certainly. I should have maybe qualified that statement.

Yep.
True. But I think my suggestion is more practical. <grin>

Yep.
It was kind of fun digging up my old statics references--I trained as an engineer, but I've been doing software for the past 6 years.
Chris
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Murray Peterson wrote:

Yep :)
-- dadiOH ____________________________
dadiOH's dandies v3.06... ...a help file of info about MP3s, recording from LP/cassette and tips & tricks on this and that. Get it at http://mysite.verizon.net/xico
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

For the sheet goods, on average, plywood would be the best, then particle board, then MDF for initial sag.
I'm a bit unclear on how you're going to build the shelf as a hollow box. If you're going to go to that much trouble, and then include a steel angle, I'd suggest just doubling up on the wood for the shelf. Two of the 3/4" glued-up pine boards would deflect only about 3/32 at the middle under a distributed 70 pound load (assuming a shelf depth of 12").
You can get an estimate of sag using the Sagulator at http://www.woodworkersweb.com/sagulator.htm .
todd
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Are the sagulator numbers good for long-term loading, or just initial deflection? My experience with purchased bookshelves has been poor -- shelves that progressively sag more as the years go by.
BTW, the loading is not distributed -- almost all of the weight is pretty close to centered.
--
Murray Peterson


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

From the sagulator: "The Sagulator computes initial sag only. As an engineering rule of thumb, wood beams/shelves will sag an additional 50% over time beyond the initial deflection induced by the load."
--
Stoutman
http://home.triad.rr.com/brianmelissa/woodworking_frames.htm
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Murray Peterson wrote:

I'm no engineer... more of a shoot from the hip from experience type. I think a torsion box with 1/4" glued on ply top and bottom with a total thickness of 1-3/4" should be strong enough for even the heaviest stereo equipment.
I think the "sagulator" is just a guide and it has no use in calculating the strength of engineered assemblies... which is what a torsion box would be.
Joe Barta
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

We're in luck. I'm a mechanical engineer who specialized in materials engineering. I wasn't attempting to calculate the strength of a torsion box. I'm sure it would work. But for the original poster, I'm just imagining that a couple of 3/4" thick boards that he can buy off the shelf and laminate together would be easier to deal with and would also be very effective.
Of course the sagulator is just a guide. For one, it doesn't take into account end conditions, which can have an effect on deflection. It also makes assumptions regarding the bulk properties of the material that may or may not be true. And it can't do stuff like figure the deflection if a thicker piece is applied to the front of the shelf to add rigidity.
todd
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.