It's always about the physics ...
No matter what part of woodworking you are discussing, whether building a project,or using machines or hand tools, physics plays an ever present and important role. In the case of a table top, and "warping" issues, use this as an example. Take a piece of cardboard about 6" x 12" and lay it on a flat surface. This is your ideal table top, supported all around and unable to flex. Now pick it up and twist it by hold one short edge and rotating the opposite edge. It will flex completely as much as 1" in that 12" length. Now if you glue a 1" wide perimeter border at 90 degrees on all four edges, like the open half of a box and try to flex it. you find it will hardly move at all. The wider/higher the border frame is, the stiffer the cardboard will become.
This is what happens when you add aprons around the perimeter of a table top. The aprons are typically set in from the edges so they don't show or bump your legs. Whether the aprons are steel or wood they will stiffen the top, steel being stronger inch for inch than the wood.
If a design is contemporary then steel can be used, but in a traditional design steel would not be suited and has not been used for centuries. The joinery in an all wood top provides the strength as does the arrangement and stiffness of the members. The greater the depth of the section, say 12" vs 6" the less resistant to breaking or deflection AND the species of wood, Oak vs alder or Pine has some effect, but the design itself plays a very important role. Triangles resist movement, rectangles do not and will collapse without bracing... it's that simple.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 01-16-2018 at 04:54 PM.