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Project is outdoor granite topped table. Table legs are two isosceles trapezoids crossed together at 90 degrees, with the legs having an internal angle of 80 degrees, made from 100 x 100 x 4mm 316L Stainless Steel equal box section. Granite top will weight either approx. 158.95Kgs (350.5 Lbs) @ 20mm thick or 238Kgs (525 Lbs) @ 30mm thick. Clearance from ground to underside of table top = 700mm. I am stuck on the support frame (which will be back 150mm from perimeter of the Granite) to be welded to the legs . As I only have 40mm height to play with (a downstand of granite will cover the support), and given that the legs are 100 x 100mm box section, what 316L would be the most rigid/strong enough to take the weight of the granite and be welded to the legs? All answers gratefully received, but please bear in mind I have no technical knowledge whatsoever, so just a shopping list please! I have to give the fabricators a drawing that makes sense and just knowing what I want in my garden won't help them! Many thanks in advance.

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  • $\begingroup$ Bear in mind, it's not just about taking the weight of the granite - you also need to resist bending loads eg if someone sits on it. Don't want the granite to crack! $\endgroup$ Feb 28, 2021 at 15:47
  • $\begingroup$ Absolutely! @Jonathan R Swift But where I come from, tables were made for glasses, not children's ***** so I will definitely be policing it! We're only doing this once! $\endgroup$
    – MrsJLO
    Feb 28, 2021 at 16:09
  • $\begingroup$ Sure - should have no problem if it's just glasses. My kitchen island is a 'bridge' shape, and needed to expect fairly heavy impact from kneading dough, smashing out chocolate moulds, etc. No way I would have been happy with a flat bar in that situation - but yours may see a quieter life! $\endgroup$ Feb 28, 2021 at 17:42
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    $\begingroup$ Hi MrsJLO. This community is here to provide technical advice and education, not professional design services. Out of consideration for your risk, our liability, and the guidelines of this site, I am voting to close the question. However, I think based on what you've written that you are underestimating your technical ability! If you would consider revising (editing) your question to focus on the technical problem, rather than a shopping list, that could be a great fit for this site. $\endgroup$
    – Air
    Mar 1, 2021 at 18:43
  • $\begingroup$ 316 is an expensive alloy! I would lean toward a box or channel, will give much more strength per volume (aka cost) of material $\endgroup$
    – Pete W
    Mar 1, 2021 at 20:45

3 Answers 3

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The cross-section of the legs will be doing most of the supporting for the granite weigh in this case. Even the maximum of weight 238kg, is not going to be too taxing on the supporting frame. (For example if you had vertical legs a flat bar of almost any thickness more that 3 mm would be OK).

Since you have this small inward angle, there will be some small danger for buckling horizontally.

For simplicity, I would go for a flat bar with width of 100mm and thickness between 10 to 15[mm] (that may also be overengineered) but you will be on the safe side.

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  • $\begingroup$ Perhaps I didn't explain it properly - I only want to see the legs, not the support frame, so need 316L that's short in height, but strong enough to take the weight of the granite, that's how I got to thinking whether a flat bar would work. So, a couple of questions: 1. If I have to use box section, then am I correct in thinking that equal box is stronger than unequal box? and 2. If I prefer using flat bar, what thickness should I use? Many thanks. $\endgroup$
    – MrsJLO
    Feb 28, 2021 at 14:51
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    $\begingroup$ in my answered , I focused on the shopping list, and I did not read through your question (because probably soon will be closed). $\endgroup$
    – NMech
    Feb 28, 2021 at 15:28
  • $\begingroup$ Understood. Thank you for your help and time. Much appreciated. $\endgroup$
    – MrsJLO
    Feb 28, 2021 at 15:32
  • $\begingroup$ Didn't see this comment! "flat bar with width of 100mm and thickness between 10 to 15[mm] (that may also be overengineered) but you will be on the safe side". That's a great help - thank you! I would much prefer to go with a Flat bar so that I can conceal it. I'm happy with it being 'over engineered' - just as long as it's safe:-) Once again, thank you for your time. Have a good day. $\endgroup$
    – MrsJLO
    Feb 28, 2021 at 15:36
  • $\begingroup$ @MrsJLO If I didn't misunderstand your wish - use a easy to hide flat bar for the support frame, that is to be welded to the 100x100x4 legs; the suggested 10mm - 15mm thick bar will definitely work, but the width should be wide enough to accommodate the welds on both sides of the leg. It can be done with a flare bevel weld that requires less width. You should check with your welder before order the flat bar. $\endgroup$
    – r13
    Feb 28, 2021 at 21:54
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The only reasonable possible concern is flatness of the metal top for the granite to sit on . Welds must be ground smooth at the top of the structure and should be as flat as possible as, no doubt ,the granite is flat. The traditional way to do this is to have the steel firmly held to a "strong back" ( welding table ) during welding. I have a 915 mm X 1730 kitchen island granite that sits on conventional wood frame ,apparently the wood frame was slightly accommodating as the top always sat solid. Your stainless is much stronger and not very accommodating. Granite does well in a garden , I have granite on a carbon steel frame as a planting bench; However, It is a few irregular pieces of granite that were free. Because of limited size ,flatness is no problem.

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  • $\begingroup$ Good point @blacksmith37 - I will emphasis the flatness requirement to the fabricators. Getting a bit nervous now - a "back of the envelope" idea is turning into a lot of precision engineering! $\endgroup$
    – MrsJLO
    Feb 28, 2021 at 19:38
  • $\begingroup$ Re: flatness: comment 1: it doesn't need to be ground flat everywhere, just at the high points where there is contact. comment 2: steel frames often can flex slightly to take up slight non-flatnesses. comment 3: a layer can be added between, e.g. rubber sheet. Helps spread out the load too! $\endgroup$
    – Pete W
    Mar 1, 2021 at 20:48
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Without checking into details or performing calculations, I will go for the flat bar as the first choice, the next choice will be a rectangular box shape with the wider face laying flat.

The reason is that the rigid granite tabletop can span itself to a certain length without relying on the top support frame to carry the load to the support legs, thus the rigidity of the frame member is immaterial, but the thickness of the bar, or the wall thickness of the box shape, shall be sized to resist the potential punching shear from the leg, and to satisfy the requirement of making a sound weld joint.

On your design, you shall make sure the lateral stability is achieved by attaching diagonal bracings to the legs, or rigid brackets that restrict lateral movement, especially in the transverse direction (onto the page).

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