Which is best way to strengthen plywood?

[Simon_M]

I searched for a HOWTO for strengthening plywood e.g. adding an edge brace, without success on UKWorkshop. I want to brace the long edge of a piece of 1/2" (12.3mm) birch B/BB plywood to prevent it twisting or bowing (in a workbench).

My first thought was to add a brace e.g. 3/4" x 1 1/4" to the edge, so the total thickness on the edge will be 1 3/4", using some offcuts of hardwood I have. Second was to switch from an L shaped support to a T shaped support e.g. thinner material on both sides but still the same total thickness, perhaps glued or glued and screwed. I thought the second idea would also "clamp" the plywood (using the screws) together or will screws be unnecessary or cause more trouble than they are worth?

I could also glue strips of plywood offcuts on both sides of the edge to triple the thickness - but this might not increase the stiffness by much for the same effort? Would there be an advantage in moving the bracing away from the edge e.g. back 1"? The reason I suggest a small brace is because a bigger one would obviously "trump" a smaller one, but I wanted to maximise the strength for the extra material used, but using bigger offcuts isn't a problem.

 

[MikeG.]

What are you trying to achieve? Why do you need stronger ply? I mean, if you were a boat-builder, you'd glass it. Context here is everything.

 

[sunnybob]

If its a shelf, making a thicker front edge wont help, you'll need a middle support.
More info required.

 

[Simon_M]

What are you trying to achieve? Why do you need stronger ply? I mean, if you were a boat-builder, you'd glass it. Context here is everything.

The workbench https://www.axminster.co.uk/axminster-p ... nch-508223 was originally paid for by my insurance company and has just been replaced (so, it's a replacement for a replacement). Here’s a sketch of the side of the existing legs, with the addition of a plywood back and shelf to limit future "racking" problems and add some storage. The plywood is 1/2” (12.3mm) thick, which I have cut to make a back and shelf.

View attachment plywood back and shelf.pdf
The plywood back fits across the rear legs and between two rails. Not shown is the worktop that sits over the legs - the only support across the legs are the two (or three) stretchers. I decided not to continue the plywood back up to the worktop because I wanted to leave a space for clamps and cables etc. and because the tool well is very thin material.

If it’s a shelf, making a thicker front edge won’t help, you'll need a middle support.

The shelf will be supported on stretchers so supported across the whole width and I will put front-back supports as well - sufficient for some extra weight towards the corners (if required).

There are two horizontal stretchers across the workbench. They can be mounted as shown or put lower down so that a cabinet (not purchased) can be put between the legs. I’m also considering adding a third stretcher at the front. Overall, I think this will greatly add to the rigidity and help to prevent racking.

I could have simply bought thicker plywood but I think it will already offer a lot of stability especially compared to a cabinet th’s veneered chipboard. The cabinet is both expensive to buy and not as useful to me as having a shelf and an open space e.g. under the vices.With the plywood back, there doesn’t seem to be much point adding a fourth stretcher e.g. it would be better to add strength rigidity to the long edges of the plywood back. The original question was about using the extra support as shown or having it either side e.g. for the same amount of material is an L or T shape offering the same support?