Why do army tanks have tracks, bulldozers, excavators? To create more surface area. Army tanks can go through the mud with tracks on them right? Now put car tires on them and what happens when they try going through mud? They sink.
Can't really say your 120g should have collapsed in the middle like you stated above, because 3 sides where supported right? I know I can get away with just supporting the very ends, making only 2 points of contact with 120g tank, only if the tank can support the weight itself though, and the uprights being used to support the tank is perfectly plumbed and don't move with any kind of vibrations, but I wouldn't trust it with my little kids lives by any means. Any kind of movement will put stress somewhere on a poor design like that though and fail at some point.
I love learning so please explain...
Here's the reason I don't believe the above theory and I'll try to explain it. Its kind of like why army tank have tracks.
The tank stands we use are not built on 4 legs or suspended by cables, but could be if one wanted to. Most stands are built with only 1 point of contact to the floor, a rectangle or square shaped foot, no legs. But lets say for kicks we are using a table for a tank stand. If I were to take a basic 4 legged table and set a tank on top, yes the weight would put pressure at all 4 corners because that is how its designed. It only has 4 legs to support the weight, right? Thats if none of the horizontals break that is carrying the weight to the ends. Simply putting a scale under those 4 legs would further more state that if everything is level, the weight would be distributed evenly to each leg. Now lets say we now added 2 more legs and we put them in the middle of the table, dead center, and we now have a scale under all 6 legs. You telling me that only the legs on the 4 corners are going to carry the weight, and the scales under the middle legs will display significantly less numbers? Whether it be 6' or 600' long, every vertical upright will take on weight. The middle is usually most always the weakest point if left unsupported because it would be the largest span left unsupported, right? But to irradiate my comment up above, thats why I stated it really depends on how its designed. You can build it to distribute weight evenly, or put stress on the ends, or just make a poor design.
You do raise good points and I understand what you are saying, but I'm still not 100% that anything we build, weight will automatically be carried to the ends.