Effects of Gravitational force
You are constantly being pulled to the center of the earth and this force which is acting on you is gravitational pull. This force normally is concentrated at the center of the objects and keeping any heavy objects at this point may harm the structure, which is why vastushastra says to keep center empty or have central courtyard. This courtyard will also act as an excellent source of ventilation apart from being a family area where everybody of family old and young can meet.
Gravitational force acts mainly on the structure and this formed part of vastushastra because in ancient times there were no architects and engineers to design and construct the house. The houses were constructed by common people on their own and many Vastu tenets gave details of construction e.g. vastushastra recommended size of rooms, shapes of rooms, dimension of rooms, windows / door sizes and locations, thickness of walls etc. Today as an architect when I read tenets of vastushastra pertaining to structure part, I wonder about extensive knowledge of sages that wrote about vastushastra in the technical field also. As an architect when I sit to calculate the size of a wooden column with the help of a calculator it works out the same as prescribed in Vastu, is it not amazing?