NUMERICAL STUDY ON PUNCHING SHEAR RESPONSES OF FLAT PLATE STRUCTURES STRENGTHENED WITH ENGINEERED CEMENTITIOUS COMPOSITES

Abstract

Flat plates are one of the conventional structural systems found in the construction sector due to their several advantages including simplicity, lowered expenses, and architectural mobility. However, the flat plate systems are vulnerable to punching shear failures which happen suddenly and catastrophically. Thus, it is crucial to develop strengthening techniques to augment the punching shear resistance of such structures. The strengthening techniques include shear reinforcement, drop panels, and column capital. One of the modern techniques for retrofitting flat plates is the use of engineered cementitious composites (ECC), applied as thin layers on both sides of slab surfaces. There are only limited studies that examined the response of flat plates with ECC retrofitting that improved the punching shear resistance. Most previous research has employed experimental methods to assess the impact of ECC on the punching shear capacity of the slab considering limited design parameters due to economic constraints. Furthermore, some inconsistent results were reported from two separate studies. In one study, the tension side retrofitting with ECC showed a noticeable contribution to punching shear resistance but in another, it was insignificant. Therefore, it is necessary to investigate the behavior of flat plates subjected to concentric vertical loading and examine if the ECC is valid as a retrofitting technique through numerical simulations considering various design parameters. This thesis evaluates the impact of the ECC strengthening technique on the punching shear response of flat plate structures. For that purpose, analytical models of the interior slab-column assemblage were developed in finite element software ABAQUS. First, the model was calibrated from test results in the literature. The numerical simulations were carried out on flat plate models subjected to gravity loading to investigate the global response of the flat plate and its failure mechanism. The contributions of concrete and ECC were determined to examine the effectiveness of ECC strengthening. Moreover, the cracking pattern was visualized by presenting the contour plots that displayed the flat plate model’s maximum principal equivalent plastic strain. In this thesis, the main study parameter was the placement of the retrofitting: on the tension, compression, and both slab surfaces. The other study parameters include compressive strength, thickness, and width of the ECC. The numerical study results showed that the punching shear behavior of flat plates was improved with the application of the ECC retrofitting technique. The addition of a thin layer of ECC on the compression side of the slab provided a direct shear strength contribution near the column face, therefore no improvement was observed as the width of the ECC increased. Moreover, the deformation capacity of the slab improved with low-strength ECC retrofitting of the slab on the compression side, whereas normal-strength ECC showed a more brittle response. The addition of a thin layer of ECC on the tension side of the slab has no direct shear force resistance contribution. However, it can lower the neutral axis and enlarge the concrete compression zone, increasing the concrete contribution to the punching shear strength. This increase is only effective if the ECC width is large enough to cover the punching cracking region. The double-sided retrofitting can significantly increase the strength and deformation capacity of the flat plate, resulting in a better performance under punching load. However, it must be noted that the strength does not result from the superposition of strengths of one-sided retrofitting since peak strengths occur at different displacements for ECC retrofitting on the tension and compression sides. Overall, the parametric results revealed that the higher the compressive strength and thickness of the ECC, the greater the punching shear response of the slab. Accordingly, the compressive strength of 20 MPa, thickness of 30 mm, and width of full slab length applied on both sides of the slab were determined to be optimal parameters for improving both strength and deformation capacity.