4.4.1 Assumptions
Individual members shall be assumed to remain elastic under the action of the factored design loads for all limit states.
The effect of haunching or any variation of the cross-section along the axis of a member shall be considered, and where significant, shall be taken into account in the determination of the member stiffness.
4.4.2 First-Order Elastic Analysis
In a first-order elastic analysis, the equilibrium of the frame in the undeformed geometry is considered, the changes in the geometry of the frame due to the loading are not accounted for, and changes in the effective stiffness of the members due to axial force are neglected. The effects of these on the first-order bending moments shall be allowed for by using one of the methods of moment amplification of 4.4.3.2 or 4.4.3.3 as appropriate. Where the moment amplification factor , calculated in accordance with 4.4.3.2 or 4.4.3.3 as appropriate, is greater than 1.4, a second-order elastic analysis in accordance with Annex B shall be carried out.
4.4.3 Second-Order Elastic Analysis
4.4.3.1
The analysis shall allow for the effects of the
design loads acting on the structure and its members in their displaced and deformed configuration. These second-order effects shall be taken into account by using either:
- a) A first-order elastic analysis with moment amplification in accordance with 4.4.2 provided the moment amplification factors and are not greater than 1.4; or
- b) A second-order elastic analysis in accordance with Annex B.
4.4.3.2 Moment amplification for members in non-sway frames
For a member with zero axial compression or a member subject to axial tension, the design bending moment is that obtained from the first order analysis for factored loads, without any amplification.
For a braced member with a design axial compressive force as determined by the first order analysis, the design bending moment shall be calculated considering moment amplification as in 9.3.2.2.
4.4.3.3 Moment amplification for members in sway frames
The design bending moment shall be calculated as the product of moment amplification factor [see 9.3.2.2 (C_my, C_mz)] and the moment obtained from the first order analysis of the sway frame, unless analysis considering second order effects is carried out (see 4.4.3).
4.4.3.4
The calculated bending moments from the first order elastic analysis may be modified by redistribution upto 15 percent of the peak calculated moment of the member under factored load, provided that:
a) the internal forces and moments in the
members of the frame are in equilibrium with applied loads.
- b) all the members in which the moments are reduced shall belong to plastic or compact section classification (see 3.7).