Unsupported span for table top?

[Chris152]

This is the table I'm working on at the moment:

And this is the substructure:

The top is maple (20mm thick), the legs etc sweet chestnut.

The table spans the two cross members which are 54 cm apart, as do the two side pieces along the length. My question is, can I expect the top and/ or the length pieces sag with time? ie, do you think I need to build in further support before I start applying finish and assembling?

Thanks for any advice,

Chris

 

[skipdiver]

Looks to me like racking could be more of an issue.

 

[Chris152]

Thanks Steve. I'd wondered about putting in a further set of supports further down the legs but like the design as it is so not sure. It's a side table (I think) so for putting in one place and leaving alone. Something to put a pair of gloves on. :roll: Each corner has 3 doweled joints so they should be strongish, but on the other hand I could do further supports would work well enough. Hmmm...

 

[skipdiver]

I'm no expert on furniture, so someone with more experience may come along and give you some reassurance. I'm looking at it as someone who has built a lot of big timber structures and it just doesn't look that stable in it's current form. If it is just an occasional table, it may be perfectly fine. I do like it though, it's different.

 

[MikeG.]

If the second photo is how it is actually going to be, then it needs some help to retain any sort of structural strength. If, however, those members are going to come together with some proper joints, then that will be less of an issue, although even then the horizontals are a bit skinny to resist much racking (the tendency to parallelogram if some leans against or bumps into the table).

It would certainly want some support across the grain, currently lacking. You appear to have only longitudinal support at the moment, which it doesn't need anywhere near as much as support across the grain. As mocked up, if someone were to sit in the middle of the table it would put the glue joints of the table top under great strain.

 

[Chris152]

Thanks again Steve.

Mike - I don't fully understand, with apologies. The table top is supported across its width as it will be attached to the cross members.

Do you mean it'd need another support as here?

Also, you mentioned proper joints - are dowel joints seen as inferior? I did read around this and thought they were strong joints, but happy to be corrected - and I do understand the real issue here is mechanics, not the strength of the joint.
Thanks, C

 

[MikeG.]

The boards of the top run from left to right in the top photos (the first ones you posted), don't they? The rail below them isn't hard up underneath the top, if your mock-up is anything to go by. In other words, the top is supported along its length, and not across its width. This is a serious weakness, unless I am entirely misunderstanding what you are proposing.

If that lot is to be doweled then it will fail the minute it has someone bump into it or lean on it clumsily. You will need to introduce either some additional rails at lower level, or radically thicken up the top rails. I stress, this isn't to resist load applied to the top of the table but to cope with a horizontal impact. The design is fundamentally weak.

 

[John15]

Chris,
As MikeG says, you really need to deepen the top rails preferably to around 50mm, and form a stronger joint to the legs to make the piece rigid. Unfortunately this will need the legs to be a larger cross section, 40 x 40mm would do.
Hope that helps.

 

[Chris152]

Thanks for the replies both.

I had no illusion that it was a strong structure and wanted to strip the design back as far as possible in favour of aesthetics. Don't some people have side tables just for putting a vase of flowers on?! In which case, when is a table strong enough? I'm asking that not as a challenge to the advice I've received which is no doubt right, but it is an interesting question. Some people have homes with rooms big enough to situate a table for more-or-less purely aesthetic reasons (and where nobody is likely to exert stronger forces on them) I'd have thought? I weigh 14 stone and can lean heavily on the base as it is and try to shove it side to side, and there isn't any sign of weakness. Maybe that won't last?

Again, this isn't to challenge the advice I've received, but I do think if strength can be compromised more possibilities for design open up?