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Hi,
I'm getting some sag in the bench, the top is made of Curly Cherry and is 5/4, 60" long and 16" wide. The legs are 6/4,15" and 16" wide. I have two 1"x 2" rails connecting the legs, the are placed on a side, so the 2" side is vertical. All joints are Mortise and Tenon.
With a weight load of 200lb, the sags 1/8", I have used The Sagulator after constructing the unit and receiving dishearten information.
So, can I fix this problem?
Or will building a new top that 1.75 thick and will meet acceptable requirements be my only option.
Thanks

Gallery

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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.
 

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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.
 

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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.
 

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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.
 

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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.
 

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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.
 

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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
 

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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.
 

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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.
 

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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.
 

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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.
 

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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!
 

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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.
 

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You could brick up the middle portion.

- gargey
LOL. I was thinking that milling a solid rectangular prism out of a tree trunk would be structurally sound too.
 

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Actually the deflection is inversely proportional to bh^3, this assumes that the beam is a simple rectangle, since you have supports under the simple plank you could just bump them up a bit in width and it would make a huge difference.

From the picture I am going to assume that they are 2 inches, if say you increased them to 2.25 inches that would make a really big difference. To save this piece you could simply put a third support under the bench running lengthwise.

Are the lengthwise supports attached firmly to the top? Stronger wood really isn't going to do that much, it is the modulus of elasticity that matters for deflection and wood is all pretty close in that regard.

It is a very pretty bench btw.
 

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Wow, thanks for all the great suggestions!
The top is attached to the legs and rails wth figure 8 clips.
I don't mind making new legs and rails, keeping the top wld be nice.
A local hardwood supplier has a great selection of wood, I originally was going to use Hickory for this build.
The next build will have 3 rails that are thicker, the legs dimensions will remain the same if possible.
So, since I plan on rebuilding the legs & rails, any suggestions on how to avoid a similar situation?
 

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Here is a decent explanation that isn't too math intensive. https://www.quora.com/Structural-Engineering-Why-is-an-I-beam-shaped-the-way-it-is

This wikipedia page is a good primer on it, but sorry it has a lot of math, what you are looking to do is increase "E" the moment of inertia. https://en.wikipedia.org/wiki/Deflection_(engineering)

On the first page pay close attention to the C shape. One thing that is important is solid bonding of your top to the rails for stiffness, simply stacking planks on top of each other is not nearly as stiff as if they were glued or nailed together so they cant move laterally.

I do this on aircraft, wood is actually a lot harder material to deal with calculation wise since it isn't the same in any direction.
 

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Gargey, I hope DMC1903 finds as much humor in your comment as I did knowing it was made to lighten his load.
The bench really is a beautiful coffee table so why not just call it a day and do a new one with your new learned lessons. I also had no idea there was a site to calculate sag so your problem helped more than just you find ways to eliminate problems before they arise
 
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