Tay Bridge disaster
The original design was for lattice girders supported by brick piers resting on bedrock. Test drilling showed the rock was not far below the river. At either end of the bridge the single rail track ran on top of the bridge girder, most of which lay below the pier tops.  However, in the middle section of the bridge (the “high girders”) the railway ran inside the bridge girder, which could then run above the pier tops. This made the bridge high enough to allow sailing ships to travel up the river to Perth.