Advice on dining table supports for bamboo 5-ply on steel legs?

Question

I am planning on using a 1.5" deep, 5-ply bamboo board (sample pictured) as a dining tabletop attached to a pair of steel legs (pictured, width 20", height 29", length 2").

The board dimensions will be around 80" by 36" and the span between the two leg pairs will be around 62", leaving overhangs of 7" lengthways and 8" widthways. I'd like some advice on the necessary sub-structure (if any) to support the table.

I'm assuming normal C channels running widthways won't be necessary as it's a stable board not planks with grain. Options I've considered are

1. 2 steel L profiles either recessed into the tabletop or just screwed to the underside, running lengthways
2. similarly, a recessed C channel running lengthways
3. an apron made from additional plyboard pieces end-on.

I'd be interested in opinions on these options or anything else I've not thought of.

If using L profiles or C channels, should these be attached to the legs or would it be better to attach the tabletop to the legs directly? Would recessing profiles have any benefit over just screwing them to the underside face? Any benefit to C over L?

If using an apron, ideally this would be no wider than the legs and not visible when the table is in use. I don't think this could connect into the legs as I wouldn't want to raise the height of the table so not sure if that reduces the utility of an apron?

What is the main issue I should be concerned about and seeking to address with the substructure: board bowing / pressures forcing leg pairs apart / other?

All thoughts much appreciated.

Answer 1

Not sure how much use this is going to be to you since you've clearly given this a lot of thought already, but I didn't want the Question to go unanswered as there could be many future searchers seeking similar advice given that bamboo panel materials are likely to become more popular in the coming years.

I'd like some advice on the necessary sub-structure (if any) to support the table.

Although the span here is very long the panel could easily be stiff enough to not need support.

Note: based on the cross section provided your panel is likely to be substantially stiffer in one direction than another, and hopefully the central strips are aligned with the long axis. However it is possible that oriented the other way this material will be stiff enough to also require no additional support — it has quite substantial cross-laminations, plus due to the overall thickness.

Regardless of the orientation of the laminations the material IS essentially a complete unknown and perhaps the only way to get a read on its performance is to do a rudimentary test on it as-is. So rest it on the legs in the planned position, load the centre of the span¹ and see if you get any deflection. If you get essentially none, I think you're good to go without any form of reinforcement....... but you might want to add some anyway, for long-term peace of mind².

I'd be interested in opinions on these options or anything else I've not thought of.

Any of your three proposed stiffening options are viable methods to increase stiffness, and could eliminate any tendency to sag depending on the obvious variables such as metal gauge or depth of the wood 'apron' pieces.

Any benefit to C over L?

C will be stiffer as a rule (for a given gauge). Obviously the two profiles aren't equal in terms of sweat equity if the intention is to mount fully flush — it's perhaps double the work to embed C channel.

Would recessing profiles have any benefit over just screwing them to the underside face?

Assuming the metalwork is screwed firmly to the underside of the tabletop the main gain is simply the added stiffness of the profile itself, but I believe there is some benefit to fully recessing them.

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¹ Start with a heavy, but reasonable, weight and if you get very little deflection or none at all don't be afraid to increase it a lot and see how much sag you get.

² Many people will have some real-world experience of a phenomenon that bears on this — shelves that initially sagged only a little under their respective loads (you barely noticed), but as the years progressed the sag increased to the point it was clearly visible from across the room.