Structural Design Question

Does the sag go away when the load is removed? Is it going to be loaded to that level as a rule? If it doesn’t maintain the sag, and isn’t intended for constant load, it shouldn’t matter.

Maybe you could just find skinnier friends.

-- Half of what we read or hear about finishing is right. We just don’t know which half! — Bob Flexner

Are you sure it’s sagging under load? The second picture (side view) looks like the bench is already sagging. You could add something like a 1X3 support under the center of the bench without it even showing. I really think I would just call it a coffee table and build a new bench with different specs.

Thanks for quick response and suggestions.
I did not notice the sagging in the photo, adding a center support between the rails is an option.
I have considered making a thicker top out of a stronger species, maybe Hard Maple or Hickory.
Yes, the bench does sag when it’s sat on and rebounds when EL Gordo stands up.
It’s obvious, I made a mistake in the design, I have a few months to fix it.

A center support running front to back will probably not help at all. You could add a stretcher running parallel to the aprons. Or you could attach angle iron to the inside of the aprons.

-- Rick M, http://thewoodknack.blogspot.com/

I’ll try to answer the design question.

The deflection is proportional to 1/h^2. If you use 1×3 (h=3) instead of 1×2 (h=2) the 1/8” deflection becomes slightly less than 1/16”. If you use 1×4 instead of 1×2 the 1/8” deflection becomes 1/32.” The 1/32 would be barely noticeable. That does seem to ruin the look. So I would go with the angle iron idea.

If you sister up and add stretchers the deflection is proportional to 1/t. To get the same 1/32” effect using 1×2 (t=1) without adding iron you would need a total of 8 stretchers (add 6). That’s silly? Again, iron wins.

-- Dave, New England - “We are made to persist. that's how we find out who we are.” ― Tobias Wolff

If you think the legs are staying vertical and it’s just the top that’s bending, screw a 2” angle iron the length and in the center. It should help and possibly alleviate the problem. You also could glue a piece of hardwood like maple (oak bends) that is 2” thick and maybe 4” wide.

-- The smell of wood, coffee in the cup, the wife let's me do my thing, the lake is peaceful.

I would like to add my two cents on this issue. Your rails could have been 3 inches instead of 2 inches, or if you like the 2 inches for the rails then put a small curve on the ends of the rails where they are 1 and 7/8 inches but the middle is still 2 inches. This would put a small bow in the middle of the long rails and keep the top (seat) flat.

By the way it looks nice and you should be very proud / happy with the way it turn out.

For me I found out that making a full size mock up with very cheep low end wood helps a lot and shows any areas that need to be adjusted before I start to build with high end wood.

keep making sawdust

-- Don't only practice your art, but force your way in to its secrets... Ludwig Von Beethoven.

You could remake the stretchers slightly taller and cut a dado in from the top of both that would allow you include a rectangular steel bar of ~3/8” x 2” adding significant beam strength while changing the appearance very little.

-- “I never in my life thought I would have to say this, but the proper role of government is not to fund the distribution of crack pipes,” Lauren Boebert

First of all I chose to look at this project because it is really nice. Second I am making a desk for my grand kid and I did not even know there is a site to engineer sag like that. So you have been invaluable to me as well as making a great piece.

You have many options. Don’t worry about it, put some angle iron spanning the length nestled up against the rails and screwed on nicely (What I would do), add more wood underneath (also what I would do). These are all great ideas.

What you need to remember is this is a beautiful piece that has been a great learning experience. I love your work.

nice bench

Your bench looks great, be proud of your work. I would double up the stretchers or add the angle iron, which ever is easier. Good luck.

-- Bob in Montana. Kindness is the Language the blind can see and deaf can hear. - Mark Twain

To add strength you could add torsion support, google torsion box. I agree with Papadan, the proportions look more like a coffee table. You could also add a third set of legs in the center to shorten the span. Yet another option could be taller apron pieces to help the top carry the load or a thicker top panel.

Thank you very much for the generous response and great suggestions. Implementing a piece of iron in the rails is very appealing.
Would making a thicker top resolve the problem or is the rails the main problem?
Was my choice of using Cherry one of the problems?
If I have to build another, it’s not a problem….just another lesson in woodworking.
Thanks again!

I would have used taller rails but now, since the project is complete, I suggest adding angle iron or aluminum extrusion along the lenght to reduce the sag.

A 10 series extrusion, 1”×1” is lightweight and stong enough for this application.

You can get a lot of information on loading of aluminum extrusions from T-Slots or the 80/20 sites or educational sites.

-- "Facts do not cease to exist because they are ignored." -- Aldous Huxley

You could brick up the middle portion.