Computational study on prestressed concrete members exposed to natural fires

The present paper is aimed at investigating the structural behaviour in bending of prestressed concrete members exposed to natural fires, i.e. fires with a heating and a cooling phase. The fire scenarios considered are characterized by a heating phase that coincides with the ISO834 standard fire and a linear cooling branch. To accurately track the structural behaviour, the usual constitutive models for concrete, ordinary and prestressing steel at high temperature are adapted to account for the different behaviour of the materials upon unloading and cooling. Parametric analyses are carried out on typical prestressed sections (an I-girder and a double-tee), in order to highlight the detrimental effect of longer fire durations (up to 120 min) and lower cooling rates (3 °C/min) as well as the variability of the structural behaviour with the variation of the load level. The results show that in members characterized by massive sections (I-girder in the present study) and exposed to natural fires, limiting the attention to the heating phase is not sufficient, as the maximum temperature in the prestressing steel may be reached long (even hours) after the onset of cooling (in accordance with tests reported in the literature), leading to delayed failure. Moreover, within the range of variation of the cooling rate (3–10 °C/min, ranging from slow to fast cooling) and load level (M/M u = 0.15–0.30, ranging from low to high load ratio), the structural behaviour exhibits significant variations in the cooling phase of the fire, from an almost complete recovery of the initial configuration to runaway failure.