Dynamic Response of Steel Framed Structures Fire-Retrofitted with Viscoelastic-Damped Braces

A ten storey steel office building, designed for a low-risk zone under the former Italian seismic code and in line with Eurocodes 1 and 3, is considered as test structure. More specifically, the dynamic response of the test structure in a no fire situation is compared with what would happen in the event of three fire scenarios, on the assumption that the fire compartment with a uniform temperature is confined to the area of the first (i.e. F1), fifth (i.e. F5) and tenth (i.e. F10) level, with the parametric temperature-time fire curve evaluated in line with Eurocode 1. For each fire scenario, two retrofitting structural solutions are examined to upgrade the fire damaged test structure, by inserting diagonal steel braces with or without viscoelastic dampers along the perimeter of the level where the fire compartment is hypothesized only. Frame members are idealized by a bilinear model, which allows the simulation of the nonlinear behaviour under seismic loads, while an elastic linear law is considered for diagonal braces. Finally, viscoelastic dampers are idealized by means of a frequency-dependent model obtained as an in-parallelcombination of two Maxwell models and one Kelvin model. Dynamic analyses are carried out in the time domain using a step-by-step initial stress-like iterative procedure, assuming time histories of wind velocity, based on an equivalent spectrum technique, and artificial accelerograms, whose response spectra match those adopted by Italian seismic code. Results highlight the reliability of the VEDBs to control discomfort and deformability thresholds, under wind loads, and damage and buckling thresholds, under seismic loads, especially when F1 and F5 fire scenarios are considered.