STOCHASTIC ANALYSIS OF STEEL STRUCTURES EQUIPPED WITH FREEDAM DAMPERS WITH DIFFERENT COATING MATERIAL

The “FREEDAM” beam-to-column devices are used in the steel structures in seismic areas to ensure that during a strong seismic event the structural components of the building (beams and columns) remain free from damage. FREEDAM connections are a specific type of partial-strength connections made by a friction damper located at the bottom flange level using a bolted T-stub. The seismic input energy is dissipated exploiting the cyclic force versus displacement behaviour of the friction damper whose stroke is designed to accommodate the expected displacement demands. The main purpose of this work is to provide a probabilistic approach to achieve a global collapse for a fixed goal reliability in the Moment-Resisting Frames equipped with “FREEDAM” dampers designed by the Theory of Plastic Mechanism Control in the α-θ plane. Using the design conditions to avoid undesired collapse mechanisms, the columns at each storey are designed to obtain a global collapse mechanism. However, various uncertainties can influence the outcome of the design, for this reason it is important to obtain an overstrength coefficient that takes them into account. Using a probabilistic approach based on the combination of the FORM and the Ditlevsen bounds, each undesired collapse mechanism is considered as a limit state event. These events are placed in series and the probability of failure refers to the probability that the design condition is not satisfied and therefore a collapse mechanism other than the global one occurs. In the probabilistic procedure, in addition to the variability of the steel yield strength, it is also necessary to consider the variability of the device components i.e., the static friction coefficient of the contact surfaces of the friction pads and the variability of the pre-tightening force of the bolts. These last two components are part of the moment of resistance of the FREEDAM, therefore, the random variables to be considered in the procedure will be the plastic moments of the columns and the resistance of the devices. The static friction coefficient for 3 types of covering materials: M1, M4 and M6 are considered. The aim of the work is to carry out a probabilistic analysis of the Moment Resisting Frames (MRFs) equipped with FREEDAM devices for different values of the static friction coefficient in order to evaluate how the statistical evaluation of tribological properties i.e., the static friction coefficient, affects the results of the probabilistic analysis.