PSI - Issue 42

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Assad et al./ Structural Integrity Procedia 00 (2019) 000 – 000

Maha Assad et al. / Procedia Structural Integrity 42 (2022) 1668–1675

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2.1. Element Types There are two types of elements for each material constituting the strengthened RC slab. One is for the thermal analysis and the other is for the mechanical analysis. The element used to model the concrete and CFRP materials is SOLID70 (ANSYS, 2019), which is an 8-noded linear brick element. Steel reinforcement is modeled using the truss element LINK33 (ANSYS, 2019). These two element types can simulate the heat transfer through the slab by conduction, convection, and radiation. The second phase of analysis involves finding the displacements and stresses developed in the slab during a fire exposure. It is done by applying the nodal temperatures obtained in the first phase of the analysis on the nodes comprising the structural model along with the beams’ sustained loads. This type of analysis requires converting the SOLID70 element to SOLID65 (ANSYS, 2019) for concrete, which is an 8-noded linear brick element that is able to crack in tension and crush in compression. SOLID 70 elements that represent the CFRP laminate are converted into SOLID185 (ANSYS, 2019) which is also an 8-noded linear brick element. The element used to represent the steel reinforcement in the mechanical analysis is LINK180 (ANSYS, 2019). It should be noted that these element types are also used for the structural analysis of the slab at ambient temperatures. The debonding of the CFRP laminate from the concrete substrate was simulated using cohesion elements INTER205 (ANSYS, 2019) and the cohesive zone material model (ANSYS, (2019)). Debonding in the FE model occurs when the stress in the FRP reaches the ultimate shear stress specified in the model and is calculated using a previous developed bond-slip model data available in the literature (Nakaba et al., (2001)). 2.2. Material properties The properties of concrete, steel reinforcement, and CFRP laminates at ambient temperatures were taken from Azevedo et al., (2022) study. As mentioned previously, the model incorporates the variation of the thermal and mechanical properties of the constituent materials with temperature. The models that represent the degradation of the materials under elevated temperatures are taken from Eurocode (EC) 2 (Eurocode 2, 1992) and published literature (Hawileh et al., (2015); Naser, (2019)). Thermal temperature dependent properties include thermal conductivity, specific heat, and density. The mechanical properties required to simulate the structural behavior of the slab under fire are the modulus of elasticity, P oisson’s ratio, stress-strain behavior, and thermal expansion and their variation with temperature. For concrete and steel reinforcement, these properties were calculated as per the provisions of EC 2(Eurocode, (1992)). However, the code does not contain material models for the CFRP and other new materials. Several published papers tested different grades of CFRP under elevated temperatures and presented useful models that can be used in the FE analysis. The material models of the temperature-dependent modulus of elasticity were taken from (Hawileh et al., (2015)) and the thermal expansion model is assigned as suggested by (Naser, (2019)). 3. Results and Discussion Three types of models were developed, the first model was created to analyse the slab under ambient temperature. Using this model, a control slab, an EB slab, and a NSM slab were constructed and analysed. Another two models were created to simulate the heat transfer in an EB slab and a NSM slab specimens, respectively. Finally, two structural models were developed to perform the stress analysis of the strengthened slabs using the two different techniques. This section summarizes the analysis results of these models and their validation with the experimental data. 3.1. Behavior of the slab under ambient temperature A FE model was created to simulate the behavior of the slab at ambient temperature. The first step towards obtaining an accurate model that can predict the response of the slab under fire is to validate its response with experimental data under ambient temperature. Thus, three slabs were modeled and analyzed. The load-deflection curves and their comparison with the experimental load-deflection curves published in (Azevedo et al., (2022)) are shown in Fig. 3. It can be clearly seen that there is a good agreement between the experimental and numerically