On Fire Safety of Thin-Walled P/C Beams subjected to Cracking and Corrosion

The assessment of fire safety in a rather slender pretensioned simply-supported beam with a V section is presented in this paper. Such architecturally-valuable thin-walled beams have been used in Italy for the last forty years in the roofs of large industrial or office buildings, where a series of V girders (= secondary beams, with interposed concrete or glass panels) are supported by other girders resting on columns (= primary beams). After several decades since their construction, many of these members exhibit severe symptoms of distress, often in the form of longitudinal cracks, that can significantly reduce the bearing capacity in shear. (These span-wise cracks are mostly due to transverse bending and start propagating close to the supports, along the extrados of the secondary beams, where rain may accumulate because of insufficient or damaged water proofing). The beam in question, with thin and inclined webs, is checked both at the ultimate limit state (ULS) and in fire, with reference to bending, shear (where the traditional truss model is used), and shear-slip close to the supports, where the aforementioned longitudinal cracking may cause a sort of delamination resisted by the partially-corroded stirrups. Reference is mainly made to the provisions of Eurocode 2, but in the check concerning shear transfer along the longitudinal cracks ACI 318-08 comes into play as well. Even if it is rather peculiar for its unusual cross-section, the beam in question offers the opportunity to focus the attention on some general aspects concerning the behavior of prestressed concrete members in fire conditions, and on some weaknesses of past design provisions.