Validation and improvement of N1 method for pushover analysis

• Study of seismic behaviour of steel and R/C frames based on pushover analysis.
• A modified version of the N1 method for non-linear pushover analyses is presented.
• Analytical validation of N1 method was made by comparison with the N2 method.
• Validation was conducted on both regular and irregular steel and R/C frames.
• N1 corrected method is more accurate for seismic evaluation of frame buildings.

Many of the non-linear static methods for seismic assessment of buildings according to modern structural codes are based on the well-known N2 procedure. A more intuitive pushover procedure, N1, has recently been proposed. Its main advantage is that the explicit evaluation of an equivalent SDOF system is not required. The N1 method has been proved to provide the same accuracy as N2, but only when a lateral load distribution proportional to the first mode shape is involved. After a brief description of the main differences between the two methods, an improved version, the N1 corrected method, is presented here. It is more consistent with N2, also when constant acceleration lateral load patterns are applied. The N1 corrected method is validated according to an extensive parametric investigation of a set of case studies on steel and R/C frames, with regular and irregular mass distribution in height.