The millenium bridge was not simply a case of a bridge designed in 2-d instead of 3-d (it would have had to be designed in 3-d to account for wind loading). It was a math error, but it was much more complicated - essentially, the slight sideways motion people imparted to the bridge while walking caused the bridge to sway slightly, which in turn caused the people to walk in step with it, creating a positive feedback loop.