The sign convention for bar elements is based on the convention used for sectional forces. Sectional forces are of the same sign if they produce the same result at the end of a bar as the result produced by positive nodal forces applied at the beginning node of the bar. These forces are those whose direction agrees with the direction of the axes of the local coordinate system. Therefore, compression forces are positive, while the tension forces are negative. Positive bending moments MY produce tension in the beam fibers that are located on the negative side of the local coordinate axis z. Positive bending moments MZ produce tension in the beam fibers that are located on the positive side of the local coordinate axis y.
Positive directions of forces and moments:
