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.
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.
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.