again a useful distraction for physics majors
how many immovable rods does it take to hold up a fortress?
One for every 5000 pounds of weight (including occupants at mass capacity)
You guys don't know your physics very well. If you build a cube platform from immovable rods and then place immovable rods at 45 degree angles on the sides of the cubes, you can build a structure that would hold immeasurably more weight that the combined maximum endurance weight of the rods.
I think you're assuming that immovable rods would brace against whatever they brace against in the the same way that physical supports would brace against the earth. In that case you would apply all the downward force to the central platform, which would exert it against the ground, effectively removing it. In this case there's still the issue that all the mass is being supported by the magical energy of the rods, regardless of how the downward force is distributed against various elements of the construction.
Perhaps a good analogy to an actual physical construction would be if we thought of the building as hanging by a cable that can support exactly 5000*
n pounds before breaking. (
n being the number of rods used in the construction) It doesn't matter how we build the attachment point for the cable, it will still snap if the building weighs more than it can support.
The setup you described it very useful, however, since it reduces the chances the building would become unbalanced and helps make it more structurally sound. It would distribute any unbalanced weight (from, say, a visiting dragon) onto the central platform so all the supporting rods would bear a fraction of the added weight.