Hi, I teach a unit on bridge building for American high schools. We use software to show which members are in compression and tension and adjust design prior to physically building (ie doubling up members in compression). We use the equation: 2*Joints = Members + Reactions as a way to enable the software to work (advanced classes do it by hand). Typically I say something like "The software won't be able to compute the forces unless the equality is true". I was looking for something more related to real world experience of an actual engineer...what is the significance of this equation in practice?