While its reinforced concrete and steel construction physically allows the st. petersburg pier to stand, from a distance the up-side down, triangular shaped pyramid suggests an un-stable form. this is a good example of?

This is a good example of the use of "Pendentives".
Pendentives is a construction device which placed some sort of elliptical dome over a rectangular room.
This type of structure makes the building able to hold large sized ornaments on the ceilings and providing a lot of room for people at the same time, which is really ideal if its used as museum or Churches