There must be a state of equilibrium in all structures, meaning that all forces should be equal with each other. There are many ways to ensure equilibrium in a structure. Trusses (bracing) are a very good example that is used in all building/bridges. Example: every truss in a bridge is withstanding many forces, two common ones being tension and compression or even both, these forces must be equal to each other or else they fall apart. It’s also important to know which material is best to resist these forces. Concrete itself is good when under compression, very bad in tension, steel in the other hand is good with both, it has it’s limits.