Over the past few years, punching shear has been in the forefront of both research teams and professional public due to a new approach to its verification according to Model Code 2010. From this topic, the task of flat slabs strengthening against punching shear has arisen. This problem, and in particular the problem of flat slabs strengthened by additional concrete column heads, is the focus of this paper. Structures are analysed using a 3D FEM models including material and geometric nonlinearity. The way of modelling is validated against experiments on non-strengthened flat slabs subjected to punching shear. At first, strengthening with a rigid connection at the interface between structures is considered and then, several different types of connection at the interface are evaluated. Finally, strengthening of structures with varying lengths of top flexural reinforcement in the slab is modelled while minimum anchorage length outside the additional column head is verified.

The paper focuses on punching shear strengthening of flat slabs. In the study, different, in practice commonly used systems of strengthening are introduced, systems used less frequently are also mentioned. Chosen methods of strengthening are modeled using FEM software considering material nonlinearity and crack development in structures. 3D models with brick finite elements and discrete modeling of reinforcement are used. The way of modeling of the structure itself is verified against known results of tests of concrete slabs under punching shear. Comparison of individual systems of strengthening is performed from the viewpoint of absolute value of punching shear resistance and it is supplemented by calculation of punching shear resistance according to design code EC 1992. Simultaneously, results are compared to the approach of Critical Shear Crack Theory that places great emphasis on deformation capacity of the slab itself, which can be easily determined from FEM models which allow for material nonlinearity.

Construction systems with recession in their ground plan along increasing height of a building are often used in current time. This lay-out is advantageous for the global stability of a building, which acts as a vertical cantilever with a widen socle. Attention must be paid with this type of global lay-out solution, because significant shear forces appear thanks to various types of grand plan. Usually buildings with massive elements, which can bear major of shear forces, don’t have any problem with the global vertical shear forces. Contrarily buildings with slight construction system such as flat slabs supported by columns could be unsafe designed if this effect is not taken account. There is no recommendation or exact approach to this problem in current standards and therefore it only depends on the designer’s approach and their experience. In the paper different types of approaches to the analysis of vertical shear forces and their distribution are presented. They are demonstrated on models with various levels of detailing. The scope of models starts at very simple linear model of whole structure and ends by models which take account of non-linear base supports and construction stages.

An additional concrete head or drop panels are not common solutions for strengthening flat slabs and have not yet been an adequately studied solution in the field of laboratory research. The investigation of drop panels in a laboratory is quite complicated, so that an examination directly at a construction site could be a better solution. Long-term measurement sensors were installed inside a concrete drop panel, which is on the bottom surface of a flat slab. Gauging was performed during various loading situations, including the loading of heavy machines, the ambient temperature load, and a uniformly distributed load. Results from the measurements are given in comparison with analytical models, which have been studied with respect to the influence of shrinkage and the bond-slip behaviour of a reinforcement.

The paper deals with strengthening structures against punching shear with focus on strengthening of existing concrete foundation slabs. These can be slabs of basements of residential, administrative and other buildings. Also, foundations slabs placed on piles can be considered. This is very specific case of strengthening where only one surface of the structure is accessible. Strengthening is often designed using massive new bearing elements, which are space-demanding. Attention is paid to the development of technology that minimizes the resulting constraints on the surrounding area and is sufficiently effective. Inspiration is strengthening of ceiling flat slabs with additional shear reinforcement. Compared to the ceiling slabs, a larger load must be bear to the foundation structures and therefore the shear reinforcements demand larger dimensions. Mechanical expanded anchors are used for anchoring on the inaccessible side. A significant contribution of mechanical anchoring is demonstrated in the tests results which make possible more efficient design of additional punching reinforcement.