In any case, because of the development of the calculation technique of the time or because of their own resources, neither Amancio Williams nor Robert Maillart seem to have considered this three-dimensional character of their structures in their analyses.

 There is a notable difference in concept from the mechanical point of view between the two works analysed: Maillart introduces a ball-and-socket joint in the keystone of the arch, transforming the structure into an isostatic one. Thus, the tri-articulated arches, like the Salginatobel bridge, due to their isostatic character, have a better behaviour than the hyperstatic arches, such as the one built in the case of the House over the Stream.