10.01: Analytical methods for the prediction of fire resistance of “reinforced” slim floor beams

Abstract

The present paper describes a new analytical method for the prediction of temperature in longitudinal reinforcement bars of slim floor beams subjected to fire. This method is based on the isotherms of EN 1992-1-2 Annex A developed for concrete beams and slabs and is consequently applicable only to slim floor beams subjected to standard ISO fire curve. In addition to vertical heat fluxes, this method also considers 2D temperature field induced by the hot part of the beam web and by the lateral heating in case of long span composite flooring systems. An adaptation is proposed for reinforcement bars when situated close to the beam web. This new method leads to very good temperature predictions for R60, R90 and R120 fire resistance requirements. For less severe requirements, the reinforcement usually remains below 400°C and the resistance of hot-rolled reinforcement bars is not reduced for this range of temperatures. The better predictions of the sagging bending moment of the section under ISO fire by use of this new method have also been demonstrated within the present paper.

Fire design of slim‐floor beams

“…This finding was already noticed by Cajot, L.G et al [17]. However, the model from Hanus et al [14] shows closer results for 90 and 120 minutes of fire exposure. It seems therefore clear that the usage of previous formulas results on safe predictions but slightly more precise ones may be useful for the reinforcing bars temperature assessment.…”

“…Bottom plate -Z&F [10] (tpl = 30) Rebar -Z&F [10] Rebar -Hanus et al [11] been assessed. As can be seen, both Zaharia and Franssen [13] and Hanus et al [14] models provide higher temperature than that obtained through the FE model. This finding was already noticed by Cajot, L.G et al [17].…”

“…More recently, Hanus et al [14] developed an additional simplified formula for the temperature assessment of the reinforcing bars. In this case, the formula was developed based on the temperature evolution within concrete slabs provided by EN 1992-1-2 [15], including some corrections to take into account the possible horizontal heat flux in steel-deck composite floors.…”

“…Through this type of model, other three-dimensional phenomena can be assessed, apart from the Once all the available models have been described, it is considered useful at this point to compare the validated FE model developed by authors [8] against the simplified temperature formulas developed by Zaharia and Franssen [13] and Hanus et al [14]. Specifically, the bottom plate, the reinforcing bars and the web profile temperature evolution has been compared, see Fig.…”

10.31: Simplified analytical determination of the temperature distribution and the load bearing resistance of slim‐floor beams

“…can be achieved by adding reinforcement span [10]. at room temperature for serviceability limit state (SLS) and for hot rolled section resistance in fire xposed concrete cover above the the steel eff = for the 9].…”