Duttilità e resistenza al degrado oligo-ciclico di connessioni nelle strutture in legno = Ductility capacity and low-cycle fatigue strength of timber connections

The capacity of timber connections to undergo plastic deformations limiting the strength degradation in fully reversed low-cycle tests represent a fundamental requirement to ensure large amounts of energy dissipation in timber structures in case of seismic event. Although the current Standard documents for the seismic design of timber structures (NTC18 and Eurocode 8) express the seismic demand for dissipative connections in terms of both ductility and impairment of strength, the European Standard for cyclic testing of joints made with mechanical fasteners (EN12512) does not provide a unique and detailed data processing method to calculate the low-cycle fatigue strength of joints. This paper presents a new proposal for the calculation of the ductility capacity of timber connections, taking into account in the determination of ultimate condition a fixed limit value of the impairment of strength between the 1st and the 3rd cycle. The proposal will be developed by integrating the current procedure of EN12512. The Equivalent Energy Elasto Plastic method reported in ASTM E2126 will be proposed as an alternative method for the evaluation of the yield deformation. The results obtained from cyclic tests within an experimental programme carried out on screwed and dowelled timber-to-timber connection as well as hold-downs will be analysed according to the new proposal. The relationship between ductility capacity and strength degradation for the tested connections will be presented and critically discussed.