long span alcove shelf design

[janner89]

Hi all, have an alcove unit project coming up and the design has particularly long spanning shelves (approx 2m). They are 270mm deep. (drawing attached). Is likely to be loaded with books and potentially ornaments.

Am slightly concerned about sagging over this distance…the current proposed construction would be a torsion box - 2 skins of 9mm MDF glued and pinned to a batten frame of 32mm redwood, giving a shelf thickness of 50mm. On the front edge I’d fix a 50mm poplar lipping.

The shelf will be supported on dominos and screws either end and screwed through the back panel (potentially may integrate supporting batten along back panels).

Do you think this will resist sagging or would it require integrating a steel box section or something more robust - am reluctant to go down this route mind as it adds a lot of cost and time!

Cheers

 

[Phil Pascoe]

https://woodbin.com/calcs/sagulator/

 

[rogxwhit]

I 'd just say that for a torsion box to work the skins need to be continuous. The top one's a tension member.

 

[janner89]

Thanks for replies, have seen sagulator but not sure if it helps with predicting how a torsion box might react?

 

[recipio]

A torsion box is the answer but they are labour intensive to make. It is the glue line that gives strength. I have posted before about using flush doors as torsion boxes. When you cut into them the 'eggbox' interior is exposed and you glue in a batten between the outer skins to restore strength. If you don't like the look you can obviously veneer or even paint them. A lightweight and strong solution.

 

[blackteaonesugar]

I did some fairly recently at similar dimensions though not quite 2m. Probably more like 1700.
I used 18mm mdf with a 50mm ply lip mitre-jointed on the front and it's all fine.
I suspect you'll be ok, given the depth and it sounds like you're going about it in a methodical way.

I would perhaps use your torsion box approach and potentially look to add further support by including floating shelf style brackets to give a bit more outward support.

 

[scholar]

I use this ’torsion box“ construction a lot and I think it will be fine for your design.

I always use 6mm MR MDF over a softwood batten frame and I don’t think you need 9mm - this will make it much lighter - just use a reasonable spacing of stretchers in the frame.

I glue my assemblies up in my vacuum bag (I use a fire door as the base inside the bag and usually a 22mm MDF top platen to avoid the vacuum crushing the 6mm MDF into the voids. You can equally successfully glue up on a flat surface (eg I use my 8’ long workbench, but you need to plan your cramping which needs cauls across the piece etc.

Whilst on the subject, the process I have developed is to glue up the inner frame of battens first (I use Dominos) - this isn’t really essential for the strength of the final piece but it makes the final glue-up much easier. Also, to stop the skins sliding about on the frame, I hot glue in some little locating blocks on the inside of each skin that will hold the frame in position on the final glue-up.

I have done long shelves about 300mm deep (40mm thick) in this fashion and they are very rigid - I have used these fixings Concealed Shelf Support, for Installation into Woodwork or Masonry Walls - Häfele U.K. Shop - these require some careful fitting (including ensuring solid blocking for the pins) but they work really well (you need a thin spanner to get to adjust the pin - I had to make one).

Hope that helps

Cheers

 

[janner89]

I use this ’torsion box“ construction a lot and I think it will be fine for your design.

I always use 6mm MR MDF over a softwood batten frame and I don’t think you need 9mm - this will make it much lighter - just use a reasonable spacing of stretchers in the frame.

I glue my assemblies up in my vacuum bag (I use a fire door as the base inside the bag and usually a 22mm MDF top platen to avoid the vacuum crushing the 6mm MDF into the voids. You can equally successfully glue up on a flat surface (eg I use my 8’ long workbench, but you need to plan your cramping which needs cauls across the piece etc.

Whilst on the subject, the process I have developed is to glue up the inner frame of battens first (I use Dominos) - this isn’t really essential for the strength of the final piece but it makes the final glue-up much easier. Also, to stop the skins sliding about on the frame, I hot glue in some little locating blocks on the inside of each skin that will hold the frame in position on the final glue-up.

I have done long shelves about 300mm deep (40mm thick) in this fashion and they are very rigid - I have used these fixings Concealed Shelf Support, for Installation into Woodwork or Masonry Walls - Häfele U.K. Shop - these require some careful fitting (including ensuring solid blocking for the pins) but they work really well (you need a thin spanner to get to adjust the pin - I had to make one).

Hope that helps

Cheers

Thanks for this, briilliant reply. As it happens i've got some 4mm mr mdf in stock, do you reckon this might be too thin or should be ok if frame is adequately spaced?

great tips with hot glue, was thinking of glueing up 5 to 10mm oversize then finishing to exact dim on the table saw.

 

[scholar]

Thanks for this, briilliant reply. As it happens i've got some 4mm mr mdf in stock, do you reckon this might be too thin or should be ok if frame is adequately spaced?

great tips with hot glue, was thinking of glueing up 5 to 10mm oversize then finishing to exact dim on the table saw.

Well my technique is probably conventional enough (ie I don’t claim any particular inventiveness) but I developed it from my Dad whose approach was to use 1/8” hardboard skins over a softwood frame - he glued these up using Cascamite and they served fine for years, but I did find a few cases where the glue bond on the rough rear face of the hardboard was a bit weak.

Anyway, to address your point, I guess 4mm MDF would be as good as 1/8” hardboard. And yes, you would clean up the whole sandwich to a clean edge for lipping (I use some mahogany type hardwood - 10 or 12mm, as it is reasonably stable (this needs to be a bit oversize for gluing and I find it helps to leave the lipping a while once glued before trimming flush as you can get a visible line if the lipping shrinks after trimming).

This process is probably too laborious for anyone doing this commercially btw - I just plod on happy that I don’t need to make money from these jobs

Cheers

 

[Jameshow]

I 'd just say that for a torsion box to work the skins need to be continuous. The top one's a tension member.

Opposite - bottom ones in tension tops in compression.