When sketching a truss, there are two "rules" that must be followed in order to have an acceptable design:
- The truss must pass the 2j=m+3 rule as discussed in the Truss Stability section of Research. In English, this means that 2 times the number of joints (2j) must be equal to 3 plus the number of web members (m+3). The purpose of this rule is to ensure that the method of joints can be applied to solve the internal forces within the truss. A truss which adheres to the 2j=m+3 rule is statically determinate.
- All truss web members must form triangles. This sounds obvious but can sometimes be deceiving. The attached screenshot illustrates a sketch that appears to be comprised of all triangles but actually is not. The red lines indicate that the "triangle" actually has four sides.
If the truss sketcher is not allowing you to apply a sketch, it is likely that you have broken one of these rules.