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Shear and Torsion in R/C Structural Members in Fire



Seldom structural designers are required to design from scratch or to check existing R/C structures or members in fire, under pure shear or torsion, because the structural response is in most cases controlled by either bending (with/without an axial force) or by shear with some bending. Furthermore, there is hardly a single test under prevailing shear or torsion in fire. How to design or to check a shear- or torsion-sensitive R/C member in fire is, therefore, an open issue, which needs a reasonable answer based on the available design models and on the knowledge of the shear-resistant mechanisms active in reinforced concrete. In this rather general paper, the design models wellknown in the calculations at the ultimate limit state in ordinary environmental conditions are recalled and their use in fire is discussed, with reference to both solid and thin-walled open sections. Both the effective-section method and the zone method are treated, as well as the fire-sensitivity of the various shear-transfer mechanisms, active in both shear and torsion, related to concrete-struts bending, aggregate interlock and dowel action along the cracks, not to mention the stirrups. On the whole, what stands out clearly in fire is the increasing role played by the stirrups in shear and that played by the rather cold concrete core in torsion.

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