The goal of this exercise was to help us understand how to cast empty spaces using plaster. Therfore what we created was a negative of what is observable in reality. In this particular case, the empty area in the cast represents a section of a bridge located in Zürich. In order to make the cast, we had to create a mould, that required us to draw a monge projection of the bridge on a piece of cardboard, in which the plaster was subsequently poured in.
The pictures below show my mould and my cast, whose bridge section was too wide, which resulted in an excessively big empty space and a structure that couldn't sustain its own weight.