Computational analysis of reinforced concrete structures subjected to fire using a multilayered finite element formulation

Abstract

Fire is a critical risk in reinforced concrete (RC) structures and appropriate structural resistance against it has to be ensured. In this contribution, an approach using corotational layered beam finite elements is employed in which the cross-section temperature is derived from a low-cost closed form model, as opposed to the more commonly used fully computational thermal analysis. The effect of geometrical and material nonlinearities (constitutive behavior fitted to experimental data for concrete and steel), material degradation as a function of temperature rise, and the contributions of thermal, transient, and creep strains are incorporated in the structural analysis. The computational results are favorably compared to experimental data from the literature for an RC beam and for a larger RC frame. Taking benefit of the layered beam formulation offering local insight into the cross-sectional and material behavior, the relationship between the structural degradation and data extracted from the cross-sectional behavior is successfully established. Noteworthy originalities of the contribution are the use of ultimate strain and its evolution as a function of temperature for both materials and the explanation of the observed structural response in fire conditions from cross-sectional data.

Probabilistic investigation of reinforced concrete structures under fire loading accounting for material and geometric uncertainties

“…Commonly, fire analysis involves at each time step: (i) a thermal computation of the cross sectional temperature distribution using FE and (ii) a subsequent thermo-mechanical analysis of the structural response. In the proposed multilayered beam approach a computational thermal analysis is never performed, a closed form thermal model proposed by Kodur et al (2013) is employed instead to approximate the thermal field in the structural members’ cross sections (the interested reader is referred to Kodur et al (2013) and Sosso et al (2021) for more details).…”

“…The thermo-mechanical response is derived through a corotational multilayered Bernoulli beam approach for planar frames (2D), developed in Iribarren et al (2011), Oliveira et al (2014), Sosso et al (2021). In this work the Bernouilli beam theory is employed, as in Kodur and Dwaikat (2008), Prakash and Srivastava (2018a, 2018b), Sosso et al (2021), a simplification that could be removed by using Timoshenko kinematics (Lin and Zhang (2013)) in the future.…”

“…The nominal material parameters used as reference set in the applications treated in this work were identified as best fit to experimental data in Sosso et al (2021) and are given in Table 1.…”

“…The softening parameter for concrete and the hardening parameter for steel were kept constant in all analyses, at μ c = 0 (perfect plasticity) and μ s = 1.52, respectively. Note that the chosen nominal value of the reinforcement ultimate strain ( ɛ u , s ) is high, compared to values adopted in the literature, however, the influence of ɛ u , s is rather limited considering that fracture was never observed as a failure mode in Sosso et al (2021) even though employing lower ɛ u , s values.…”

“…The stress in each material is a function of the mechanical strain component obtained from the total strain decomposition (Sosso et al (2021)):where εx,mechi, εx,thi are, the mechanical (i.e elastic and plastic) and thermal strain, respectively and εx,LITSi is the Load Induced Thermal Strain (LITS). σx,toti and T i are layer axial stress and the cross section temperature at layer i , respectively.…”