Try this simple experiment to find out why the arch shape is used as a weight-bearing feature in structures such as bridges:
Carefully cut or crack two eggs in halves. Clean out the insides (this is easier if you boil the eggs first).
Ensure that the edges of the cut are smooth and not jagged. You can wrap a little tape over the edge to help with this. Place the 4 halves down on a table to mark out the four corners of a rectangle.
Carefully place one book or magazine on top of the four egg halves so that they support the weight. Keep stacking on top until an egg half collapses.
Use a kitchen scales to find the total weight supported by the egg halves. You might be surprised at how much it is!