Forces on Structures
Forces of Wonder - scroll to bottom and click on the file to download it.
PBS - Loads Lab - this site lets you apply forces like wind and earthquakes to structures
PBS - Shapes Lab - this site reviews arches, triangles and other shapes
PBS - Materials Lab - this site shows how different materials affect the stability of the structure
These sites are mostly interactive, that is, you have to adjust them and see what happens. There is often information about the force.