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

[Kurt Owens]

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?



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[andy bessette]

You'll need very substantial longitudinal structure.

[Kurt Owens]

You'll need very substantial longitudinal structure.

OK, so in what form? The aprons aren't enough then?

[andy bessette]

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.

[Kurt Owens]

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

[Brian Holcombe]

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.

[Rick Malakoff]

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

[Kurt Owens]

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

[Brian Holcombe]

You can rationalize it but why not build a stronger design?

[Pat Barry]

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.

[Rick Malakoff]

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

[Malcolm McLeod]

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?

[Alan Schwabacher]

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.

[Richard Line]

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.

[Kurt Owens]

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 (https://www.amazon.com/Rok-Hardware-Fasteners-Screws-ROKSTBFAST/dp/B01FGEZUSQ/ref=sr_1_3?ie=UTF8&qid=1497976913&sr=8-3&keywords=table+top+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?

[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. :p

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

[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.

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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

[Andrew Hughes]

I'm not sure if it was suggested have you considered two benches.I did the table and bench thing for someone last year we considered one long bench.
Did you mention white Ash that will be nice.
Heres my work in White pine.

[Brian Holcombe]

Nice work Andrew!

[Pat Barry]

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!
Better option would be a long center (longitudinal ) support. The center one could be taller as well for added strength and stiffness and it won't be readily seen.

[Kurt Owens]

Better option would be a long center (longitudinal ) support. The center one could be taller as well for added strength and stiffness and it won't be readily seen.


OK. One more question for you :) . How would you suggest joining it? My first thought was just another tenon into the short aprons on the side. For reference, the aprons and this longitudinal support are a full 1" thick).

Actually, do I need to somehow re-enforce all the tenons in this thing?

[Joe A Faulkner]

Why not make the short apron 1.5 or 2 inches thick so that the tennon on the center support can be longer? Also the center support could be thicker than 1" as well.

[Kurt Owens]

I was just contemplating this. I think I'll do just that.

[Dave Garrett]

This sounds like obvious advice but make sure you can extract your newly built behemoth from your workshop. A friend of mine Julius Dorsey at Chicago Fire Furniture was commissioned to make a loong table for a family of 9. On day of delivery the table didn't fit in our oversized industrial elevator, extra roomy stairwell or even the wide window-frames on the 4th floor. quite an investment to dangle without damage anyways. So don't forget the third measurement - the way out!

[Kurt Owens]

This sounds like obvious advice but make sure you can extract your newly built behemoth from your workshop. A friend of mine Julius Dorsey at Chicago Fire Furniture was commissioned to make a loong table for a family of 9. On day of delivery the table didn't fit in our oversized industrial elevator, extra roomy stairwell or even the wide window-frames on the 4th floor. quite an investment to dangle without damage anyways. So don't forget the third measurement - the way out!

It just has to fit through my garage door :). Good advice though. I'll have to check the width of my door!

[Ted Reischl]

Bench seating for anyone is a pain. Benches are miserable. They make criminals sit on benches while they await trial.

I can just picture 3 or 4 squirmy kids trying to figure out how to get seated. Then one standing up and pushing off on it.

It will make a nice plant stand for your wife though. . . .

[Mike Brady]

A bench is fine for dining seating, but I would suggest you have a bench on only one side and it should seat only two people. You could make it long enough for three but three adults will not be happy using it. Two is fine. I have an arts and crafts table with a bench on one side and it works just fine. It is a backless bench. By the way, if you make a long table, be sure you can get it out of the basement shop. I cant get a sofa down my basement stairs and my friend has a new house and he can't get furniture into his basement due to turns in the staircase.

[Andrey Kharitonkin]

I have build low assemble table 5' long that is tested to hold at least around 400 lbs without visible difficulty:

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Actually, I was able to assemble another workbench on it (250 lbs plus myself standing on it). It has middle legs built in, but in reality they were not needed and not used (they don't have any coasters on them and do not touch the ground).

Legs are 3"x3" and top rail is 1-1/2"x3". Low rail does not have much stress and can be smaller, like 1-1/2"x1", or it can have arc shape. Or something similar to help leg and rail to hold 90 degree.

I used construction grade spruce or pine for it. Bridle joint turns out to be very good for high load. I used similar construction to build bench for bench drill press plus moving platform and counterweight for it. Should also hold 400 lbs when it's finished.

Added later: briddle joint also has two draw pegs in each and top is 1-1/4 MDF. Your size is similar to bed size. Look at beds design to assess structural strength.