Identification of RC connection behavior for seismic retroffiting – ILISBAR
Identification of RC connection behavior for seismic retrofitting
Scientific and technological objectives
ILISBAR project focuses on the re-engineering of civil engineering structures and the seismic strengthening with External Bonded FRP application. Nowadays, the life-duration extension of existing structures and the management of the underlying risk require the implementation of innovative and robust strategies. FRP composites based techniques are known as being efficient in case of a seismic loading. Approaches currently used consider existing structures as systems with more or less ductile components. In most cases, the structures are over strengthened to avoid any failure. The structural assessment process is then based upon the assumption of a linear elastic behavior and consequently, in this case, FRP efficiency is underestimated. On the other hand, FRP is useful to increase not only the structural strength but also the ductility capacity. Structural assessment requires the use of nonlinear approaches which allow accounting for the increase of ductility that may be enough to withstand the earthquake loading. The ductility brought by FRP appears as being very interesting from a structural point of view. However, there are still many challenges to take into account in order to understand and to predict the ductility increase by FRP. The real interest in studying the nonlinear behavior of reinforced concrete (RC) structures strengthened by FRP is obvious.
The project aims to identify the mechanical response of junction of RC elements submitted to complex loading. The main goal is to develop a numerical method able to represent the behavior of a junction between a vertical element and a horizontal member reinforced or not to put it into an overall calculation model to characterize the impact of reinforcements on the overall behavior of the structure. To address this issue, five key steps can be identified:
• experimental identification of the behavior of unreinforced RC connections;
• assessing a numerical model capable of predicting the behavior of unreinforced connections;
• experimental identification of the behavior of reinforced RC connections;
• assessing a numerical model capable of predicting the behavior of reinforced connections;
• integration of local models in a comprehensive analysis of the behavior of unreinforced and reinforced structure.
This research requires two important phases, an experimental part and a numerical modeling one. To define a robust predictive model, a test campaign on experimental samples having different configurations is necessary for the definition and timing of physically identifiable numerical parameters, whether on unreinforced structures (initial) or strengthened.
The experimental identification of the behavior of reinforced concrete (including failure mechanisms) requires performing the tests on mock-up experimental samples. In order to be representative, they have to be built as large as possible, for instance to be able to use real materials of RC structures. The use of rebars available on the market (therefore compliance with the bond lengths) and concrete ready for use (involving a significant aggregate size) therefore led to make reasonably similar size test sample at full scale.
The project is initially divided into 3 parts corresponding to the joist local and global behavior, and the installation of a descriptive model of the phenomena observed in both first steps of work:
1- an in-situ experimental campaign based on preliminary numerical simulations conducted with Cast3M and benefiting from 3D tomography was prepared to study the behavior of small samples subjected to a flexion loading. Several configurations obtained by varying the anchor length and inclination are considered. - The observation of the interface degradation phenomena and microscopic failure modes will provide us with the information needed for the calibration of digital tools (in particular the constitutive law and the kinematic-concrete compatibility relationships) allowing to describe the behavior of the junction at the mesoscopic scale. The next step will be the identification of a strategy to perform the upscaling procedure and proceed to the simulation of junction behavior at the structure level.
2- At the «substructure« scale, specimens representing a connection between a slab and a wall were made and tested experimentally until failure. The dimensions of the specimens are of scale 1, in particular the thicknesses of the elements are in accordance with the usual rules. This step of work makes it possible to set up the different parameters of the composite reinforcement and to analyze the influence of these parameters on the performances of the reinforcement. This step made it possible to highlight a reinforcement configuration making it possible to increase the performance of the joist by approximately 60% which will be used in the reinforcement of a scale 1 structure taking into account the stiffnesses of the RC adjacent elements.
3- All the results obtained experimentally allows the calibration of the numerical model being developed
The analysis at different scales is continued in order to vary the different geometrical parameters or materials and to determine a hierarchy of the influencing parameters. This experimental work makes it possible to define the unavoidable parameters necessary for a modeling translating a behavior as realistic as possible of the numerical answer.
1. MECHANICAL CHARACTERIZATION OF A RC WALL-SLAB JOIST REINFORCED BY FRP UNDER ALTERNATING CYCLIC LOADING - Antoine Chalot, Laurent MICHEL, Emmanuel FERRIER – LYON1 - 9th International Conference on Fibre-Reinforced Polymer (FRP) Composites in Civil Engineering (CICE 2018) – Paris – 17-19 july 2018
2. Etude du comportement mécanique de liaison BA Voile-dalle renforcé par PRFC sous chargement cyclique alterné – Antoine Chalot, Laurent MICHEL, Emmanuel FERRIER – LYON1 – 36e rencontres Universitaires de Génie Civil, St Etienne, 20-22 juin 2018
The project aims to define a new concept of modeling for RC structures reinforced by composite materials capable of taking into account all the elements independently strengthened and their interaction to assess the global response of the structure loaded under seismic conditions. The numerical approach is to use both conventional elements (type multi-fiber beam) in the less stressed areas and complex 3D numerical models in the most stressed areas. This new tool will be calibrated using experimental testing to define easily identifiable physical model parameters. This new method of modeling is intended to avoid the definition of boundary conditions typically required to calculate and highly dependent on the choice of the calculator.
Monsieur Laurent MICHEL (Laboratoire des Matériaux Composites pour la Construction)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
CEA CEA SACLAY
LMT Cachan Laboratoire de Mécanique et Technologie
IFSTTAR Institut Français des Sciences et Technologies des Transports, de l’Aménagement et des Réseaux
LMC2 Laboratoire des Matériaux Composites pour la Construction
Help of the ANR 768,442 euros
Beginning and duration of the scientific project: December 2016 - 42 Months