PSI - Issue 78

1868 Ettore Sorge et al. / Procedia Structural Integrity 78 (2026) 1863–1870 alone, , to obtain an equivalent “uncoupled” moment . When fully coupled time-history analyses are impractical — owing to limited data or prohibitive computational cost — the square-root-of-the-sum-of-the-squares (SRSS) rule can be adopted to estimate , as expressed in Eq. (3): =√ 2 + 2 (3) Because SRSS treats the two responses as statistically independent, it can underestimate demand when W and E actions interact (e.g., near resonant or simultaneous excitation). Fully coupled simulations, which capture the interplay between aerodynamic damping and ground motion, provide a more realistic picture but are markedly more expensive (Asareh et al., 2016a; Santangelo et al., 2016). Thus, any simplified rule must be validated carefully to avoid either over-design or unconservative results (Asareh et al., 2016b; Avossa et al., 2017). The peak base moment from a coupled analysis is here denoted = ( , , ̈ ( )) to emphasise its dependence on both aerodynamic disturbances and ground acceleration. Alternative combination procedures introduce an interaction factor that scales the uncoupled response according to the governing load type (Meng et al., 2020). Although (IEC 61400-1, 2009) and the (Guideline for the Certification of Wind Turbines, 2010) recommend = 1 for parked turbines, field experience with utility-scale machines supports a reduced factor of =0.75 . The corresponding linear rule, adopted here and shown in Eq. (4), tends to reproduce coupled results more faithfully in seismic-dominated conditions (Yang et al., 2018): = 0.75 (1.4 + ) (4) In this study, 63 combined scenarios (8 wind records × 8 earthquake records, excluding the single case in which both actions are absent) were analysed. Figure. 4a confirms that SRSS systematically underestimates the coupled response, whereas the linear combination in Figure. 4b yields only a slight over-estimation and matches the fully coupled model far more closely.

(a) (b) Figure 4. Coupled and uncoupled (SRSS (a) and linear formulation (b)) comparison.

4.1. Performance of the HSFD Regarding the performance of the HSFD, Figure. 5 contrasts the baseline WTG with the same tower equipped with the HSFD a demanding combined load case — Wind Case 6 superimposed on Earthquake ID 7. The uncontrolled WTG develops a peak base moment of +352 MN·m and a trough of – 231 MN·m (Figure. 5a). Although the seismic record