Structural frame systems may be analyzed and designed based on forces from conventional first order theory when second order effects of axial forces on displacements in the compression members of the system are negligible. This is often the case. Extensive nonlinear analysis results are presented to document slenderness limits below which this is so for braced, end moment loaded compression members with various end restraints. Major factors that affect lower slenderness limit predictions of such members are investigated and a new lower slenderness limit is proposed and verified against the nonlinear results. Slenderness is defined in terms of a socalled normalized slenderness that, in addition to the geometrical slenderness, is a function of axial force and reinforcement. The limit itself is a function of the first order end moment ratio. The formulation is rational and the proposed limits are generally found to be more reliable than existing lower limit formulations.