Unsupported benchtop span

[RickCarpenter]

Is there a rule of thumb in how long an unsupported span of benchtop can be before there is (significant) flex? I am building a bench along the design of Jr Strasil's bench, ie, taller than normal and narrow, but with inspirations from the benchtop risers I have seen here. The legs will be 3"x7" pine laminations dovetailed into a 2.5" thick laminated poplar top, approximately 14" wide, at the very ends. There will be a deep apron in the front which will also help some with the top's flex, but it will not be attached to the top except by floating dowels. How long can I go with the top? I was hoping 7'. Would making one or more of the laminated top boards 6+" deep help any?

Thanks,
Rick

 

[PowerTool]

Hi Rick - 7' sounds like quite a span with no imtermediate support.
Here is the sagulator - which is designed for shelves,but should still give some idea of deflection.

Andrew

 

[Wanlock Dod]

Rick,

A 7 fiit top, with a foot of overhang at each end would only be a 5 foot span.

I would have thought that including some deeper boards in the top would make a tremendous difference to the vertical stiffness (a bit like a floor with in-built beams). I have no idea if this would affect the functionality of the top, although I wouldn't have thought so. I would expect more weight and more stiffness to be a good thing.

Cheers,

Dod

 

[RickCarpenter]

Thanks, Andrew. The Sagulator gives me a 0.04" deflection with a 100 lb center load on yellow poplar. I can imagine that with no load the natural deflection would be that or less, and final planing could take care of that. Anyhow, my heavy work will take place on a separate assembly table. Most of my pounding on the benchtop will take place near the ends... still, I'll probably include two 6" or 8" stiffener laminations. The center three laminations will be oak for dog holes, so I could easily make the two outer oak pieces the stiffeners.

Thanks,
Rick

 

[RickCarpenter]

Dod,

One leg will have to be flush since it will have a leg vise in line with the central row of dog holes. The other leg certainly could be inset. I was trying to stay with Strasil's overall theme, but that's a better idea.

Thanks,
Rick

 

[Wanlock Dod]

Rick,

Is it like this

Cheers,

Dod

 

[RickCarpenter]

Rick,

Is it like this

Cheers,

Dod

Yes, that's it, but mine will be taller and a little wider. I do think I'll use part of your suggestion and inset the leg opposite the leg vise end. I'll install a Roubo-style crochet on the end that overhangs.

Rick