Wood strength for spans in furniture?

[JeremyH.]

Hey everyone,

I'm thinking about making two different couch-ish things.

I don't want any support leg in the middle.

One will be around 6'8" across, and has to support up to 3 people. I'm not sure if wood or metal would be best, but if metal it'll have to be covered by wood. the trick here is that I don't want to no huge 2x6 below the cushion. I want below the cushion to be just wood, and minimal in size as it spans the length of the couch to different legs. The legs I was thinking may be solid laminated wood so they would look almost like a piece of thick plywood from back to front on the sides.

What kind of options do you think I'm looking at if I want to keep the surface that the cushion sits on around 2" in height, that spans the whole thing?

 

[Rob-GB]

Is the seat going to be contoured as in an Adirondack style chair?

 

[aloysius]

OK I get that you don't want a solid timber substructure.  That leaves space frames instead.  In theory, these can be made to the immense strength & racking resistance requirements needed with even very small timbers.  The key is in the engineering, whereby even lightweight structures made from individually weak members like matchsticks or ice cream sticks can be laminated & jointed into multilayered, crosslinked trusses.  Only the top & bottom chords of spaceframes need be constructed from continuous beams. 

A well designed, heavily triangulated planar spaceframe of 1" x 1" members, carefully constructed and especially well jointed with intersecting angles approaching the 45 & 90 degree ideals would work either in timber or steel.

 

[Michael Kellough]

You can use this deflection calculator for 80/20 aluminum and go from there.

 

[lwoirhaye]

You can make a frame for webbing.  That sort of thing has a nice feel.  The frame will need to be built quite strong and you'll want to consider where you put any cross-bars because sitters may be able to feel them through the cushions.  You can make a torsion box too but the cushions will "bottom out" on it.

 

[jimbo51]

A big issue, so to speak, is the size of the sitters and the force with which they sit. I have been told that I let myself fall the last few inches when sitting and thus subject the furniture to unnecessary trauma. Over time this leads to premature failure of the support system in the furniture. In the case of a long span design, the failure could be sudden and lead to breaking the material.

A few tests might be in order to avoid putting a lot of work into completing a piece that has fundamental design/construction flaws.

 

[Cheese]

In theory, these can be made to the immense strength & racking resistance requirements needed with even very small timbers.  The key is in the engineering, whereby even lightweight structures made from individually weak members like matchsticks or ice cream sticks can be laminated & jointed into multilayered, crosslinked trusses. 

Visions of the birdcage Maserati.  [scratch chin]

 

[Dane]

Just as a point of reference, we have one of these Twilight Sofas:http://www.dwr.com/living-sofa-and-sleepers/twilight-sleeper-sofa/880589.html?lang=en_US&mrkgcl=664&mrkgadid=3227555094&rkg_id=h-dacd6543887220b77539313042ccc2e7_t-1517593848&adpos=1o1&creative=96969740319&device=c&matchtype=&network=g&gclid=CjwKCAiAtdDTBRArEiwAPT4y-0NAr2rk2xcV8S_89vyAnRUHdFi9eynx9FHXSdgpk1Ebr--_qloLQRoCP-IQAvD_BwE

It spans probably 80" with just an aluminum square tube.  There is a foot in the middle of the seat that isn't visible unless you are lying on the ground looking under the couch.  I could measure the tube width, if you like.

 

[Michael Kellough]

That aluminum tube looks like a good 2" minimum but it's the post in the center of the back rail that makes it work.

 

[JeremyH.]

Is the seat going to be contoured as in an Adirondack style chair?

Nope.

You can make a frame for webbing.  That sort of thing has a nice feel.  The frame will need to be built quite strong and you'll want to consider where you put any cross-bars because sitters may be able to feel them through the cushions.  You can make a torsion box too but the cushions will "bottom out" on it.

Not actually a fan of webbing or soft undersides to cushions.

Just as a point of reference, we have one of these Twilight Sofas:http://www.dwr.com/living-sofa-and-sleepers/twilight-sleeper-sofa/880589.html?lang=en_US&mrkgcl=664&mrkgadid=3227555094&rkg_id=h-dacd6543887220b77539313042ccc2e7_t-1517593848&adpos=1o1&creative=96969740319&device=c&matchtype=&network=g&gclid=CjwKCAiAtdDTBRArEiwAPT4y-0NAr2rk2xcV8S_89vyAnRUHdFi9eynx9FHXSdgpk1Ebr--_qloLQRoCP-IQAvD_BwE

It spans probably 80" with just an aluminum square tube.  There is a foot in the middle of the seat that isn't visible unless you are lying on the ground looking under the couch.  I could measure the tube width, if you like.

I can see your point. Maybe a foot in the rear wouldn't be bad. But I think visibility of what I want, without such large cushions, would still make it a no go.

The aluminum does appear to be pretty strong if it can work for that.

Thank you for all the help so far everyone.