B-2.1 Analysis
In a second-order elastic analysis, the members shall be assumed to remain elastic, and changes in frame geometry under the design load and changes in the effective stiffness of the members due to axial forces shall be accounted for. In a frame where the elastic buckling load factor (λcr) of the frame as determined in accordance with 4.6 is greater than 5, the changes in the effective stiffness of the members due to axial forces may be neglected.
B-2.2 Design Bending Moment
The design bending moment under factored load shall be taken as the maximum bending moment in the length of the member. It shall be determined either:
- a) directly from the second-order analysis; or
- b) approximately, if the member is divided into a sufficient number of elements, as the greatest of the element end bending moments; or
- c) by amplifying the calculated design bending moment, taken as the maximum bending moment along the length of a member as obtained by superposition of the simple beam bending moments determined by the analysis.
For a member with zero axial force or a member subject to axial tension, the factored design bending moment shall be calculated as the moment obtained from second order analysis without any amplification.
For a member with a design axial compressive force as determined from the analysis, the factored design bending moment shall be calculated as follows:
where