GNGTS 2013 - Atti del 32° Convegno Nazionale

The coefficients C F were chosen with the aim to investigate systems with different energy dissipation, where the case with represents a non-linear elastic system, namely a system that practically does not dissipate energy. The case with represents a poor dissipative structural behaviour [slender masonry panels subjected to rocking collapse mechanism (Amadio et al. , 2011)], while the case with represents a dissipative capacity intermediate from the previous case and the full dissipative one. This last case is typical of timber frames with hybrid beam-column and wall-foundation joints where unbonded prestressed tendons are used to re-centre the structure at the end of an earthquake event in this way minimizing residual damage, and energy dissipaters are placed in parallel to provide dissipation (Priestley et al. , 1999). Finally, the slip-type model (Fig. 1b) is typical of timber structures, where significant pinching effect due to the plasticization of metal fasteners and crushing of the timber at the interface with the fastener occurs (Foliente et al. , 1995). Adopted software. Each inelastic spectrum was calculated with a software written in Fortran language and purposely developed for a SDOF system. Three versions of this program have been developed, the first for the fully dissipative behaviour, the second one for the flag- shape hysteretic behaviour, and the third one for the slip-type systems. As input data, the software requires a seismic record and the desired ductility value of the SDOF systems. Acceleration, velocity, displacement and energy spectra are returned as output. The program uses the bisection method to evaluate iteratively the yielding force F y of the SDOF in order to match the desired ductility value µ provided by the user as an input. The software carries out a time-history non-linear dynamic analysis with the seismic record given as input. The program uses the well-known Newmark integration method (Chopra, 2011). The considered SDOF system has unitary mass; in this way the stiffness can be easily calculated for each considered natural vibration period [Eq. (5)]. (5) The assumed damping ratio is 0.05 (5%) for all cases. Results for SDOF systems. Results were mainly obtained in terms of acceleration and displacement spectra. The ductility levels investigated are the typical values of 2 (low), 4 (medium) and 6 (high). The spectra obtained from the fully dissipative system are denoted with “EP”, the ones calculated with the flag-shape hysteretic models with “FS” and the ones for the slip-type model with “ST”. The EC8 elastic spectra are compared with the inelastic spectra Fig. 1 – Adopted hysteretic models: “fully-dissipative” and flag-shape (left, a), and slip-type (right, b). 133 GNGTS 2013 S essione 2.1