Bench seating supports

[YorkshireMartin]

Hi all,

I'm struggling to decide on the support form of my bench. It needs to seat two adults, possibly three.

My original idea was to use a single piece of 25mm thick 250mm wide walnut for the top, then have 50mm square legs inset from the ends and tenoned to the top. Joining the legs together would be some 20mm walnut, as a full leg height stretcher meaning the span of the legs is around 200mm. The leg stretchers would be set into a dado cut in each leg.

On top of that, I was to use a maple stretcher (20mm x 100mm) across the length of the bench, which would be tenoned through the full height stretcher in the legs and flush with the top of the legs, enabling the top to be glued to the width stretcher, leg stretcher and the leg tenons. Phew, I hope that makes sense, so hard to explain.

Now I'm thinking that a seating span of 1,500mm requires a central support or, if you'll pardon the expression, a third leg. :lol: #-o

The span of 1,500mm is full width, i expect the inside distance between the legs (if two are used) to be around 1,000mm. Is the maple stretcher alone enough to support this?

Racking forces are also a concern as with a 25mm thick top, I'm limited in tenon length on the legs. Thickness limited too, as the legs are only 50mm square, giving a tenon width of approximately 16mm. I want to avoid a through tenon, for aesthetic reasons.

Considering adding a second stretcher lower down to counter the racking force.

It would be great to hear opinions.

Regards,

Martin

 

[swb58]

I'm not clever enough to pass comment on much that you say but I don't think the third leg idea is the way to go. Three legs on a stool is good, it never wobbles. Three legs in straight a line like a bench is not going to work so well unless your floor is perfectly flat and your legs are perfectly in line.
Better to beef up the under seat support if needed. Test it, support it so you have a metre span in the centre and get some bodies on it.

 

[woodenstx]

I made a bench that is 1300mm long with 36mm MDF ontop and 69(ish)mm square legs and stretchers in PAR pine

It'll seat 2 adults and a 7yr old with ease, could probably do 3 adults to be fair. I ran some poor FEA on it and the estimate was it would take 500KG before hinting at breaking :lol:

I'd imagine a 100mm deep stretcher (almost any thickness) will take a massive load of weight before giving up, especially when stitched to a second one with the top.
As said, I'd avoid the extra legs for level and general moving issues

 

[ColytonJohn]

I had to make a sketch of this to understand your description and may still have it wrong - if so then I apologise!

Anyway...

A 20 mm x 100 mm length of walnut on end supporting the 25 mm top would probably add sufficient strength (assuming the adults aren't Russian weight lifters) but I think the rest of the design needs work.

You're quite right to be concerned about racking as tenoning the legs 16mm into a 25mm top isn't going to work with or without a central leg (which would bring the problem swb58 described).

I'd be more inclined to design it as a small table using aprons and skirts to join the legs together and then attach the top with cleats to take account of movement; gluing to the stretcher and legs as described would risk the top splitting as it 'moves' with moisture changes. If still concerned about racking connect the legs with a more conventional H stretcher.

Cheers,
John

 

[YorkshireMartin]

I had to make a sketch of this to understand your description and may still have it wrong - if so then I apologise!

Anyway...

A 20 mm x 100 mm length of walnut on end supporting the 25 mm top would probably add sufficient strength (assuming the adults aren't Russian weight lifters) but I think the rest of the design needs work.

You're quite right to be concerned about racking as tenoning the legs 16mm into a 25mm top isn't going to work with or without a central leg (which would bring the problem swb58 described).

I'd be more inclined to design it as a small table using aprons and skirts to join the legs together and then attach the top with cleats to take account of movement; gluing to the stretcher and legs as described would risk the top splitting as it 'moves' with moisture changes. If still concerned about racking connect the legs with a more conventional H stretcher.

Cheers,
John

I'm wasn't as concerned with movement on this one as I might otherwise be. The walnut has been well seasoned and acclimated and is a single piece rather than a lamination, so I *think*, as long as I dont glue it down across it's entire width, I'd mostly be OK for seasonal movement wouldn't you say? ABW supposedly exhibits little in the way of seasonal movement.

I'm thinking on the feedback give about the other issues.

Thanks gents.