10.13: Bending resistance evaluation of shallow floor beams exposed to fire: Fid-Bau Portal

10.13: Bending resistance evaluation of shallow floor beams exposed to fire

Leskela, Matti V.

For the moment, the Eurocode 4 standards give detailed analysis rules merely for the standard or ordinary composite beams where the concrete section rests on the top flange of a steel beam. In contrast to shallow floor beams (abbreviated as SF‐beams), where only the bottom flange is directly exposed to fire, the ordinary composite beams do not have any inborn fire protection. Although the SF‐beams are not excluded from the scope, no special design rules are given for them, and they only appear in one schematic figure in EN 1994‐1‐2. The basic method for evaluating the bending resistance of fire exposed composite beams is based on the employment of rigid‐plastic stress blocks in steel and concrete. However, it is also possible to utilize elasto‐plastic analysis, where the flexural strain distribution and the corresponding stresses are taken into account, and the maximum compressive strain of the concrete could be set to control the straining. The analysis results between the rigid‐plastic and elasto‐plastic sagging bending resistances of fire exposed SF‐ beams are compared and the additional useful information obtained from the latter analysis is illustrated. For any strain distribution that fulfils the equilibrium of internal stress resultants of the composite section, there is a well‐defined bending moment, curvature and bending stiffness. The data following from the elasto‐plastic stress analysis can further be utilized for evaluating nominal deflections that cannot be associated with the rigid‐plastic resistance analysis.