If you look at these tension members (say member 50) you will find that the amount of compression developed in these members is practically insignificant. The small amount of compression is developed for some load cases like 1 because the member is subjected to very small tension for these cases at the top and the value is so small that even the selfweight of the member is enough to reverse the nature of the force to compression at the bottom end. For example for the member 50, if you look at the axial force for load case 1, the upper node 24 is subjected to a minimal tension force of 0.008 Kips. This very small axial tension is converted to a small axial compression of 0.005 Kip at the bottom node 8, due to selfweight of the member. Same behavior is observed for load case 101. It is not uncommon for TENSION members with such minimum tension to experience a small compression. The best thing to do in such cases is to bypass the slenderness check for these members or set a high value for the allowable slenderness such that the slenderness does not govern.
There following wiki explains the same phenomenon
