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Thread: What precautions should I take when building a bench that is 7 feet long?

  1. #1

    What precautions should I take when building a bench that is 7 feet long?

    I am going to be building a new dining table and bench for our house. It is a simple design - 8' long with slightly tapered legs and a bench to match. I am a little worried about sag in the bench. I punch the numbers in to the sagulator, and if 4 adults were sitting on the bench it said there would be excessive force.

    That doesn't seem right to me. I have sat on benches 7' but maybe there were supporting elements I didn't notice. The entire structure is white ash with 1" tenons into the legs. There are cross support as well (see image below). Should I be worried? Is there some other support I should add for the table or bench?



    Screen Shot 2017-06-20 at 6.45.50 AM.jpg

  2. #2
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    You'll need very substantial longitudinal structure.
    "Anything seems possible when you don't know what you're doing."

  3. #3
    Quote Originally Posted by andy bessette View Post
    You'll need very substantial longitudinal structure.
    OK, so in what form? The aprons aren't enough then?

  4. #4
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    This is your design, so you'll have to experiment to find a solution you can live with. Try supporting a 2x4 at its ends to get a feel for how much deflection to expect.
    "Anything seems possible when you don't know what you're doing."

  5. #5
    Quote Originally Posted by andy bessette View Post
    This is your design, so you'll have to experiment to find a solution you can live with. Try supporting a 2x4 at its ends to get a feel for how much deflection to expect.

    I guess I am not even sure what my options are

  6. #6
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    I think you should revisit this design from the premise on forward. This design asks a lot of its structure, if you can do something instead which breaks It in the center you will be doing yourself and future sitters a service.
    Bumbling forward into the unknown.

  7. #7
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    Hi Kurt,
    Remember that two people will be sitting at the ends so that should take off some of the load and the bench top will be come a diaphragm and stiffen it up quite well.
    Rick

  8. #8
    Quote Originally Posted by Rick Malakoff View Post
    Hi Kurt,
    Remember that two people will be sitting at the ends so that should take off some of the load and the bench top will be come a diaphragm and stiffen it up quite well.
    Rick

    This is what my gut tells me. I am trying to assume the bench could have 800 pounds on it, but in reality it probably won't have but 500 at one time in the center. that 500 should be distributed across the aprons and cross supports, right?

    I'm no engineer though

  9. #9
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    You can rationalize it but why not build a stronger design?
    Bumbling forward into the unknown.

  10. #10
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    How will you fix it once you have it built and then realize the seating platform is sagging excessively due to the folks sitting on it? Basically, trust the sagulator as a reference point - improve the design by making a more structurally sound support structure. One thing that would help would be to add another longitudinal support, maybe two are needed. You can mock the whole thing up and test it by loading it with sandbags before you actually build it.

  11. #11
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    Also there is the option of adding a center pair of legs if it starts to sag.

  12. #12
    Think about trying to walk on a 2x6 plank spanning a hole - - its much easier with the 2x laid 'flat' since you can balance better, but much stronger (less deflection) if you place it on 'edge'. Still the same board and span, but much different results. Your wide seat gives you ease and comfort, but the apron gives you 'edge' strength. ....I'll spare you the rest of the moment of inertia class.

    I don't know your material selection or dimensions, so won't tell you it will or won't work 'as-is', but will offer that it is fairly easy to test - - build the bench top and place it on the two longitudinal aprons, with the aprons supported only at the extreme ends. Get a couple of volunteers to sit, and measure the deflection. If it is enough deflection that you or your volunteers are uncomfortable, increase the height of the aprons (...aesthetics may take a back seat to strength requirements??).

    Another thing to plan for: You are building a composite beam. If the long aprons are firmly and continuously attached to the seat, the beam will be MUCH stronger. Think torsion box. The seat and the aprons will have to bend together, and so will be stronger. If the joint can slip at all, you'll get much more deflection. So maybe look at a continuous M&T joint, a long spline, or biscuits/dowels placed as close as you can tolerate?
    Last edited by Malcolm McLeod; 06-20-2017 at 12:11 PM. Reason: typo

  13. #13
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    To my eye it would look better if the aprons were deeper (vertical dimension) at least 4" for this span. That would stiffen the structure considerably, as would Malcolm McLeod's suggestion of attaching them to the seat continuously.

    To estimate the difference, put each part separately into the sagulator, find out how much weight they can bear at the sag you can accept, and add up the weights. To figure out the joined structure, treat the aprons as if their depth had the thickness of the seat added. You could then subtract from the width of the seat twice the thickness of the aprons before adding in its contribution, but it would probably be good enough to simply ignore the rest of the seat. Watch what happens to the stiffness as you increase the depth of the stretchers.

    Note that the Woodbin sagulator button that chooses whether the load you apply is total or per foot looks a little funny, at least on my screen. If you select the left hand radio button that says "total" just under it, that actually selects the option of loading per foot, so at 500 lb load on an 8' structure, you would actually be calculating the sag for a 2 ton load. You want the right button.

    Note that sitters will more resemble the "uniform load" condition of the sagulator than the "center load" unless everyone is sitting on one person's lap.

  14. #14
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    The sagulator is a good calculator for shelves and simple beam structures, but it does have limitations. Your design is a more complex structure, and may not be adequately modeled by the sagulator.

    Here are a few comments that should help with your bench (I'm not commenting on the table): 1) make the aprons as deep as you can reasonably do (I'm guessing they are about 2 inches deep, think about going more like 4 inches); 2) fasten the aprons to the top with screws about very 4 inches, maybe even consider gluing them to the top (this will ensure they work efficiently in transferring the bending loads); 3) add a stringer running down the middle of the bench's length and fasten it to the top as noted for the aprons; 4) consider making the stringer deeper than the aprons, it can be tapered to join neatly with the end aprons and be nearly invisible from view; the lateral; 5) the lateral supports do not help with the sag so you can eliminate them if you add the center stringer.


  15. #15
    Quote Originally Posted by Malcolm McLeod View Post
    I don't know your material selection or dimensions, so won't tell you it will or won't work 'as-is', but will offer that it is fairly easy to test - - build the bench top and place it on the two longitudinal aprons, with the aprons supported only at the extreme ends. Get a couple of volunteers to sit, and measure the deflection. If it is enough deflection that you or your volunteers are uncomfortable, increase the height of the aprons (...aesthetics may take a back seat to strength requirements??).

    Another thing to plan for: You are building a composite beam. If the long aprons are firmly and continuously attached to the seat, the beam will be MUCH stronger. Think torsion box. The seat and the aprons will have to bend together, and so will be stronger. If the joint can slip at all, you'll get much more deflection. So maybe look at a continuous M&T joint, a long spline, or biscuits/dowels placed as close as you can tolerate?

    As far as material goes - it is going to be White Ash. The top of the bench (and table for that matter) will be joined every 8 inches to the apron using these Z fasteners I have had good luck with them in the past pulling the top down tight but still allowing movement. When you say a continuous M&T joint, do you mean something like a tongue along the top of the apron and a groove in the bottom of the seat?

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