Abstract
School buildings are susceptible to high incidents of fire because of carelessness, faulty electrical installation and arson, raising the attention on their seismic retrofitting after fire exposure. Hot and residual mechanical properties of a reinforced concrete (r.c.) structure exposed to fire depend on duration of the heating and cooling phases. As a consequence, seismic retrofitting of a fire-damaged framed structure may not be effective when the peak temperature during a fire is considered. For a successful retrofit, ultimate capacity resulting from residual properties after cooling needs to be taken into account. To this end, the state secondary school Collina-Castello of Bisignano (Italy), a three-storey r.c. framed structure designed in a medium-risk seismic region to comply with a former Italian seismic code, is considered as test structure. Thermal analysis of r.c. frame members is preliminarily carried out for two fire scenarios, on the assumption that the fire compartment is confined to the ground (i.e. F0) and first (i.e. F1) levels. Moreover, four fire-damage cases are examined, considering only the heating phase, at 45 minutes of fire resistance, and the overall fire cycle, for fast, medium and slow phases of cooling. Afterwards, the school is supposed to be retrofitted with hysteretic damped braces (HYDBs), in order to achieve the performance levels imposed by current Italian code in a high-risk seismic zone. Nonlinear static and dynamic analyses of the unbraced and damped braced structures are carried out, with reference to the degradation of r.c. frame members for different fire durations in the design procedure of the HYDBs.
Highlights
Fires have heating and cooling phases, yet the effects of the latter on temperature distribution and residual mechanical properties are generally not considered when evaluating structural behavior
Two approaches are applied to evaluate the residual capacity of the cross-sections subjected to different fire scenarios and decay rates: (i) the 500◦C isotherm method proposed by EC 2.1-2, combined with reference to 30 min (R30) and R45 fire resistances in the heating phase; (ii) the proposed variable isotherm method, using time-temperature profiles of the internal layers during the cooling phase
Columns are assumed to behave elastically under axial forces, accounting for the interaction with bending moment in the yield condition, while axial strains are not considered in the beams; shear deformations are neglected in all the r.c. frame members
Summary
Fires have heating and cooling phases, yet the effects of the latter on temperature distribution and residual mechanical properties are generally not considered when evaluating structural behavior. As a matter of fact, the use of dissipative braces provides significant additional stiffness in the structure, that usually affects maximum base shear and sometimes floor accelerations adversely This problem loses its relevance for post-fire resistance because a significant decrease in stiffness and strength is confirmed in the structural members exposed to fire, in comparison with the nofire condition (Mazza and Alesina, 2019), stiffer and stronger damped braces can be inserted in the fire-damaged level; these will be able to restore the corresponding initial values at that level. Residual strength and stiffness properties of r.c. frame members after fire are evaluated by experimental results, taking into account the fact that when concrete (Chang et al, 2006) and steel rebars (Slowanski et al, 1971) have cooled down an additional decrease in their performance may occur. Additional details on the steel reinforcement of columns and beams, constant along the height of the building, are reported in other works (Mazza and Vulcano, 2014a; Sorace and Terenzi, 2014)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
