Per 4.4.1.2 of the AS 4100 "Second-order effects 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 Clause 4.4.2, provided the moment amplification factors (δ_b ) or (δ_s ) are not greater than 1.4; or (b) a second-order elastic analysis in accordance with Appendix E"

In RAM Frame the user can choose to consider either of these methods. If P-Delta is selected to be considered in the analysis (see RAM Frame Analysis Manual - Analysis Criteria and P-Delta Sections) then this is equivalent to 4.4.1.2(b) being considered. Without P-Delta the requirements of 4.4.1.2(a) are effectively considered. See the next section for a discussion on differences and advantages of the P-Delta and the Amplified First Order approach.

The following rules are applied to the determination of design moments considering the members sway status (braced or unbraced) and whether P-Delta was considered in the analysis.

Braced

4.4.2.2: M* = δ_b M*m

Unbraced

4.4.2.2: M* = max( δb, δ_s)M*_m

PDelta Considered

Appendix E: M* = δ_bM*_e

Where:

• Only design moments are amplified for the secondary affects. Axial and shear forces remain un-adjusted.
• P-Delta when selected will be considered for all members both braced and unbraced (cannot have P-Delta only apply to unbraced members in a structure). These secondary forces will be relatively small assuming the member is truly braced against sidesway by other frame members in the model. That is so that there is little lateral displacement of the braced members.
• When P-Delta is considered the secondary multiplier (δ_b) is applied directly to the moment calculated from the analysis. That is the gravity transverse loads are not considered as a simple load that is superimposed with the lateral load case forces, rather all forces are applied to the structure in separate load cases and superimposed in a load combination to produce final design forces.
• When calculating Nomb for consideration in the calculation of for δ_b the effective length (ke) is set to 1.0, that is the member is assumed braced.