why should we ignore the directions of minimum principal and only be concerned about maximum principal?nMost materials fail in Tension at a much lower level of stress than Compression. The highest level of tension is shown by Max Principal stress. The highest level of compression is shown by the Min Principal stress. The directions of these two stresses are orthogonal.nA true truss structure is assembled using pins at the ends of each strut. The pins prevent any bending loads from entering the strut. Only axial forces can enter the strut. Some bridges have struts welded to plates at the joints so that bending loads could enter, but then it is no longer a true truss structure.nIn a true truss structure, you can compute by hand the tension and compression in each strut from the applied loads and the reaction forces at the supports. In the figure you show, the applied load P is vertical, so Ha = 0. Once you weld the joints, it is much more difficult to compute the stress in each strut by hand because bending will be allowed, so a FE model would be used to compute the solution instead.nWhile exceeding the strength of the material is a failure mode for struts in tension, there is an additional failure mode for long slender struts in compression: buckling. ANSYS can do a linear Eigenvalue Buckling analysis to compute the critical buckling load.n