"Not bad, but in need of improvement"
I cant see the incredible parallels between this and tower of goo people keep talking about. Small fundamental differences make a huge difference on the end product. They are both tower builders, but have a lot of different functionality.
My issue is the "flex" or "stretch" in the girders. I realize making them all stiffer will require more number crunching and thusly CPU power, but as is its just lacking - no material used in construction anywhere has either a coefficient of expansion or a k constant anywhere near as high as whats seen here.
If all the materials had the same "stretch" per unit of length, creating a tower with more advanced nodes would add strength, just like bracing does. Making more diagonal braces would distribute the given force and distribute it across mode points, reducing the overall flex of the tower. As it is, the same force is increased - building a simple triangle acts just as strong as one thats been bolstered in 5 different directions.
Also, the comment of "girders on the pylons are 5 times stronger" - that also should never happen. Steel beams attached to the ground are no stronger than those in the air. Why have them be special? And more importantly, as something like that is critical to performance - why not mention that somewhere in the intro or rules?
Again, it is a good effort as something to goof around with for a few minutes, but the number crunching behind the system is pretty crude - household materials like popsicle sticks or drinking straws would offer better structural performance to the "girders" used here.