A method for hybrid fire testing: Development, implementation and numerical application

Hybrid Fire Testing (HFT) is a technique that allows assessing experimentally the fire performance of a structural element under real boundary conditions that capture the effect of the surrounding structure. To enable HFT, there is a need for a method that is unconditionally stable, ensures equilibrium and compatibility at the interface and captures the global behaviour of the analysed structure. A few attempts at conducting HFT have been described in the literature, but it can be shown, based on the analytical study of a simple one degree-of-freedom elastic system, that the considered method was fundamentally unstable in certain configurations which depend on the relative stiffness between the two substructures, but which cannot be easily predicted in advance. In this paper, a new method is introduced to overcome the stability problem and it is shown through analytical developments and applicative examples that the stability of the new method does not depend on the stiffness ratio between the two substructures. The new method is applied in a virtual hybrid test on a 2D reinforced concrete beam part of a moment resisting frame, showing that stability, equilibrium and compatibility are ensured on the considered multiple degree-of-freedom system. Besides, the virtual HFT succeeds in reproducing the global behaviour of the analysed structure. The method development and implementation in a virtual (numerical) setting is described, the next step being its implementation in a real (laboratory) hybrid test. (NS) the response of which is analyzed aside during the test in a finite element model or by a predetermined matrix. The NS refers to the remainder of the structure and the response of this structure will have an influence on the boundary conditions of the tested element. Few attempts at HFT have been done in the past. The first documented attempt to perform HFT was made by Korzen et al. (2002) at BAM (Germany) on a column specimen extracted from a building. Only the axial degree-of-freedom is controlled during the test and the NS is defined by a constant matrix. Robert et al. (2010) at CERIB (France) presents a HFT performed on a concrete slab where the behavior of the NS is modeled by a predetermined matrix. Three degrees-of-freedom are controlled, namely the axial elongation and the rotations on the two supports. Mostafaei (2013a, 2013b) at NRC (Canada) performed a HFT on a concrete column while the remainder structure, i.e. a moment resisting frame with some parts frame exposed to fire, was modeled in the nonlinear finite element software SAFIR (Franssen, 2005). More recently, researchers worked on the development of the HFT methodology and validation has been done in the numerical environment (Tondini et al., 2016) or by experiments performed on small-scale specimens, i.e. Whyte et al. (2016) and Schulthess et al. (2016). In most cases, the methodology applied in the former HFT has been tailored for the analyzed case studies and for the capability of the fire facility where the test has been performed. The research objective is here to develop a methodology which is applicable independently on the type of the case study and the capability of the furnace. In this paper, the methodology considered in the former HFT performed on real structural elements, i.e. Korzen, Robert and Mostafaei, will be analyzed in details. Moreover, for a better understanding, the methodology considered in these three hybrid fire tests will be referred as the “first generation method”. It will be shown that the considered methodology is applicable only for specific cases and a new solution will be proposed in order to perform HFT in a general context, independently on the analyzed case study. The capability of the proposed method will be analyzed on a case study consisting of a concrete beam extracted from a moment resisting frame. 2 INTEREST OF HYBRID FIRE TESTING In order to highlight the potential of HFT, the behavior of a concrete beam extracted from a moment resisting frame will be analyzed numerically in different configurations (details about the analyzed moment resisting frame and the fire load can be found in Section 5). The configurations considered are: (1) a test of the whole structure, only possible in a virtual environment (2) a test on the beam simply supported with no bending moment introduced at the supports (3) a test on the beam simply supported with constant bending moments introduced at the supports (through cantilever extensions, for example), (4) test on the beam with rotations fixed on the supports and free thermal expansion, (5) test on the beam under fixed rotations and fixed thermal expansions and (6) a hybrid fire test. For all these cases, the evolution of the mid-span displacements as a function of time is presented in Figure 1. Figure 1. Mid-span displacement of the beam in different testing configurations -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0 60 120 180 240 M id -s p a n d is p la c e m e n t (m ) Time (min)