When joining/attaching elements that are in different planes, especially at 90 degrees, the strength is either in the joint or the means of attachment..... HUH?
Joints can be structural in themselves, like mortise and tenon, half laps, or dovetails. If you don't make the joint itself structural, the strength comes only from the screws or glue holding it together ... not good.
A butt joint, especially involving end grain to either end grain or long grain relies on the glue bond which is the weakest type. Long grain to long grain is the strongest bond glue will have.
As far as your table design goes, the 90 degree joints in the frame corners will not withstand any lateral forces, like a push from the side or end, IF miters or butt joints are used. Typically, that design is made by laminating thin layers over a form with radiused corners providing great structural strength. Then the issue of attaching the flat top bar to the bottom of the top becomes primary. Screws up through the bar would be typical. A recess for the bar to sit in, a shallow mortise, would add strength.
To make that type of table that will endure the rigors of time requires a thoughtful analysis of the forces and the methods of joinery. Glue is simply not sufficient.
Similar table have been nailed and glued or screwed together, but they will not last. Typically, these days a welded steel frame is used, not wood and that provides the strength required.