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

[Kurt Owens]

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.
...
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.

Yes the apron is 2" right now. I think my wife would rather have a smaller table than 4" aprons. I'm sure I could get 2.75 without her even noticing. That might help a bit.

I didn't even notice I had the wrong option selected for weight!

[Kurt Owens]

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.

Thanks, Pat.

Would you suggest removing those smaller cross supports and replacing them with another full length long one? Or would it be better to put one cross support in the middle and a shorter long support on ether end? (Basically making an --|-- shape in the the middle) In either option, keeping the two outer aprons of course!

[Malcolm McLeod]

...do you mean something like a tongue along the top of the apron and a groove in the bottom of the seat?

Yes (plus minimum character limit)

[Roger Nair]

The concern I would have with the bench seat design other than beam capacity of the seat is the connective capacity of the tenons to the legs, which, I fear is the weakest link in the system. The connections are highly leveraged, small in size and number and must resist the considerable mass and momentum of the seated people.

[Kurt Owens]

The concern I would have with the bench seat design other than beam capacity of the seat is the connective capacity of the tenons to the legs, which, I fear is the weakest link in the system. The connections are highly leveraged, small in size and number and must resist the considerable mass and momentum of the seated people.

So how would you improve it? Wider tenons or something else?

[Simon MacGowen]

7' long. The design is not going to be good even if we assume two persons sitting on the ends are the heaviest. Either major change is needed, such as making wide sides/boards as legs and add a pair of legs in the middle. The alternative is to use two shorter benches (eg 3.25' long each instead of one long one), which would be my preferred option if I was to do this project. Four shorter benches (two on each side of the table) are also easier to move around or transport.

Simon

[Mel Fulks]

I look at that design as a late child of the old "hall chairs" for servants. Designed to be uncomfortable and take as little space as possible. No one will sit there longer than it takes to boot up or out boot.

[John Lanciani]

From a design perspective I agree with those above that say no way, not strong enough. At a minimum I would add lower stretchers. From a practical perspective I have two observations; first, four 200# adults won't fit and be able to eat on a 7' bench. Second, even if they could fit, picture in your head four adults (or even three) sitting on that bench and trying to get themselves pulled in to the table to be able to eat.

IMHO backless benches are for kids and ballgames, not for adult dining.

[Kurt Owens]

four 200# adults won't fit and be able to eat on a 7' bench. Second, even if they could fit, picture in your head four adults (or even three) sitting on that bench and trying to get themselves pulled in to the table to be able to eat.

I am trying to make sure it could withstand 4 adults; I am not really expecting to ever have that many using it at once. In reality this bench is going to be used by 2 kids with the occasional extras on holidays. The other side of the table will have chairs for the adults.

[Kees Heiden]

Another point to think about, people are not static, they are moving up and down and back and forth. If 4 adults with more alcohol in their body then absolutely neccessary happen to plunk down on your bench all at the same time, then the load could easilly be doubled to tripled for a short while.

[Simon MacGowen]

I am trying to make sure it could withstand 4 adults; I am not really expecting to ever have that many using it at once. In reality this bench is going to be used by 2 kids with the occasional extras on holidays. The other side of the table will have chairs for the adults.

If something is designed and made just for kids, the consideration will be different. But, will the kids grow up and still be able to enjoy the heirloom piece? If just for two kids, why that long, taking up space while limiting its use because of structural concerns. An adult guest may unknowingly sit on it unless verbal warnings are handed out beforehand. When it comes to seating, though looks is important, safety would be my first priority.

Simon

[Kurt Owens]

If something is designed and made just for kids, the consideration will be different. But, will the kids grow up and still be able to enjoy the heirloom piece? If just for two kids, why that long, taking up space while limiting its use because of structural concerns. An adult guest may unknowingly sit on it unless verbal warnings are handed out beforehand. When it comes to seating, though looks is important, safety would be my first priority.

Simon

which is why I want it to be able to hold up to 4 adults.

[glenn bradley]

I guess I am not even sure what my options are

My knee-jerk suggestion.

table stiffener.JPG

Reasonably effective and reasonably invisible.

[Ted Reischl]

Right off the bat, I am not expert with "sagulator".

But, I think there is a lot more to this calculation than meets the eye using that tool.

1. The seat itself will support X amount of weight.
2. There are 2 aprons under the seat.
3. How those aprons are attached will affect any "sag"
4. People are a "live" load, books are not.
5. You could easily have a 300+ person sit in the middle, or even two.

The sagulator probably works well for the intended purpose, calculating sag shelf.

BTW, other than looks, there is nothing wrong with adding another pair of legs in the middle.

What I would do, all that said, is to build it as you have drawn it. If you can use 2.75 inch aprons, do it. But think about possibly having to double them up or even adding another longitudinal support down the middle.

Those cross pieces do zippity doo dah when it comes to sag.

BTW, your table is going to have a similar problem, it is going to be bouncy in the middle. Think about some additional longitudinal stiffeners for that while you are at it.

What I am saying is that the sagulator may or may not be supplying sufficient information for you to make a decision, so be prepared.

[Doug Hepler]

Kurt,

I ran my interpretation of your drawing through Sagulator and got an acceptable result.(Uniformly distributed load set at 100 lb per foot) Are the aprons and the center longitudinal piece 2 x 4 white ash?

First, I recommend that you repeat the calculation. If you still are not satisfied, I recommend that you use the principle of a torsion box. (Read up on this if you are not familiar with them.) You could actually make a torsion box if you wanted to, but I think that adding pieces of 2x4 running lengthwise between the crossmembers (fitted snugly and glued in place) would probably make the seat stiff enough. If you still were not sure, add some more of them. If you still were not sure, add a piece of plywood to the underside.

Doug