On the accuracy of the N2 inelastic spectrum for timber structures

This paper investigates the accuracy of the N2 spectrum for timber structures by calculating the 'rigorous' inelastic spectra for natural seismic records selected to match, with their average, a chosen design elastic spectrum. A purposely developed software has been used to obtain the inelastic spectra for a Single Degree of Freedom (SDOF) system characterized by a slip-type hysteretic relationship with pinching typical of timber structures. Two different sets of records have been considered: the former is consistent, on average, with a given design spectrum from Eurocode 8, the latter is taken from a strong motion database by selecting the records having soil class A and PGA within a chosen range. Non-linear dynamic analyses have been carried out by varying the level of ductility and the natural vibration period of the SDOF systems. The effects of hardening or softening of the system behaviour have also been analysed. Two analysis procedures have been used with the aim to have a further confirmation of the achieved results. The comparisons between the rigorous and the approximated N2 spectrum demonstrate that, in general, the N2 method always give fairly good results in estimating the inelastic spectra even for timber structures.