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. The legs will be 3"x7" pine lams dovetailed into a 2.5" thick poplar lam top, approx 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 top lams 6+" deep help any?

Thanks,
Rick

[scorrpio]

According to the sagulator, a 7' long x 14" wide x 2.5" thick poplar will have a sag of 0.11" under 250lbs centerload, or 0.07" sag under 250 lbs uniform load. Sag becomes visible at roughly 0.03" per foot of length - so for your benchtop, a sag under 0.21" will not be visible.

[mschrank]

I was going to point you to the Sagulator myself, but couldn't make it open where I used to have it bookmarked.

I did find it here on, of all places, a needlepointing website.

[bigsteel15]

I was going to point you to the Sagulator myself, but couldn't make it open where I used to have it bookmarked.

I did find it here on, of all places, a needlepointing website.

Here's a link that won't make anyone think you've lost your marbles.
No offense to any needlepointers out there.
http://www.woodbin.com/calcs/sagulator.htm

[LCHIEN]

if you are using the table for assembly then the sag of .11 inches might be objectionable even if visually invisible.

However, a 7' bench the sagulator calulates I'll bet on the 7' dimension.
Normally you would set the leg supports in about a foot from each end, reducing the span to 5' and the deflection will reduce to 0.04".
The calulation, if set to a uniform load rather than a center load will reduce the sag to .02"
A piece of furniture set upn this table with two slab legs would place the load closer to the bench legs than the unsuppported center, reducing the sag even more.
Depending upon what you build on this bench, I would think 2.5" thick poplar would be way strong enough for most wood projects.

[Russianwolf]

While the Sagulator is pretty cute, I don't know how much stock I would place in it. It doesn't take all variables into account.

The way it's set up looks to me like you would need to buy a piece of 10/4 Poplar that is 14 Wide and 7 foot long. It doesn't take into account that this is going to be a laminate top. I can't prove it, but I bet that an edgegrain laminate top will flex less than a plank top of equal size.

Looks like this it is more for use in shelves than benchtops.

[scorrpio]

Yes, a laminate is likely to flex a lot less. And reinforced with front apron, will flex even less - so the sag I posted is the 'worst case' IMO. And note that I used 250lbs load - I highly doubt you'll be assembling anything THAT heavy.

[RickCarpenter]

After some reconsideration, only one end will be flush since it will have a leg vise for an end vise. I will inset the other leg in a foot. Plus, I'll make two top lams about 6 inches deep, spaced about 1/3 way in from each side. And/or I could use some angle iron underneath, which would also add good weight.

Thanks y'all,
Rick