A reinforced concrete flat plate structure is a type of structural system that consists solely of slabs and columns. Flat plate structures are popularly adopted worldwide due to their economic and architectural advantages. However, an inherent issue associated with this type of structures is that the slab-column joints (SCJs) are prone to brittle punching shear failure. In addition, the absence of beams in flat plate structures results in a lack of load redistribution path. Consequently, once a single SCJ fails, the initial failure has a high risk of propagating to the adjacent SCJs, which may eventually trigger a progressive collapse of the entire structure. Progressive collapse often causes severe loss of life and property. Since the progressive collapse incident of the Ronan Point Apartment Tower in 1968, scientific and engineering communities have been working devotedly to improve the progressive collapse resistance of building structures. A direct way to improve such resistance of flat slab structures is to enhance the punching shear and post-punching capacities of the SCJs, the latter is regarded as a second line of defence to progressive collapse. Strengthening methods, which are commonly applied in engineering practice, include strategies like enlarging the slab thickness at the SCJ, using various types of shear reinforcement and incorporating a post-tensioning system. To date, substantial researchefforts have been made to investigate the mechanical behaviours of the SCJs with these strengthening methods up until punching shear failure, which is within the small deformation stage. However, research on the mechanical behaviours of strengthened SCJs in the post-punching stage (i.e., the large defamation stage) remains scarce. To fill this knowledge gap, a comprehensive study consisting of experimental, numerical and analytical investigations on the strengthened flat plate SCJs are conducted in this PhD research. The strengthening methods include adding drop panels, shear studs and prestressing tendons in SCJs. [...]