Notional lateral loads in a second-order P-Delta analysis: perturbation or real equivalent effect?

The role of notional loads in the second-order P-Delta analysis of structures is unclear in current guidance. Using the commentary to CSA S16:19, notional loads might be interpreted as simply a perturbation of the initial lean to develop the full P-Delta effect during a second-order analysis. This would require the first-order effect of the notional loads on the member forces to be removed from the analysis results to avoid double counting them. In this paper, a simple model for analysing structures under gravity and lateral loads is used to compare the results of a second-order P-Delta analysis where the initial lean is directly modelled to an analysis where notional lateral loads are used to capture the equivalent effects. The analysis demonstrates that notional loads cannot fully perturb the initial lean unless the structure is unstable and that removing the first-order effects of notional loads from the analysis is unconservative.