Concrete slabs designed with finite element methods : modelling parameters, crack analyses and reinforcement design

Concrete slabs are today usually designed by traditional hand calculation methods. Powerful numerical calculation methods like the Finite Element Method (FEM) are not recommended in design handbooks for design of slabs, see e.g. Hillerborg et al (1990),(1996). In contrary, its distribution of reinforcement is considered to be unsuitable for practical use. Most FE-programs are also more adapted for analyses than for design. SKANSKA IT Solutions in Malmö, Sweden, has developed a FE-based design program called FEM-Design. The program handles e.g. FE-analyses and design of frames, trusses, beams, shear walls and plates. In this report the program is checked and improved regarding the crack analysis, the calculated effects of actions and the design of reinforcement. Another importent issue discussed is the extreme-value problem of moments in centre of interior columns/walls in flat slab floors. The models used are checked regarding influence of mesh density, element types, column widths and modelling of column stiffness. The FE-analyses show that the mesh density and the modelling of the column stiffness mainly affects the size of the support moments, whereas the field moments are almost independent of all modelling parameters. Fem-Design's automatically generated mesh gave good results with respect to the size of the support moments. However, the result of moment distribution or actually the reinforcement distribution could be improved by distributing the column stiffness over one plate element. The multi spring concept is also suggested for interior walls wider than 0.2 m. The iterative crack analysis is compared with Abaqus/Explicit smeared cracks and an experimental test, McNiece (1978). FEM-Design's crack analysis is found to be adequate for design, despite that the crack propagation differs quite much in comparison with Abaqus. FEM-Design's load-displacement curve shows better agreement with the experimental test. The difference depends on the implemented crack theory i.e. when a crack is considered to be a crack. The distribution of design moments for a simple flat slab floor are compared with designs by two hand calculation methods, the strip method and the yield line theory. The comparison shows that FEM-Design's design moments or actually the required reinforcements have to be chosen at certain points and redistributed by a design method. A FE-based design method is developed with respect to the capabilities of FEM-Design and FE-analyses performed with the traditionally distributed reinforcement. Comparisons between the three methods show that the FE-based reinforcement design method (FED) distributes less total amount of reinforcement than the two traditional methods with respect of both bending- and final design. The thesis concludes that FE-analyses can be used to get a practical reinforcement design in concrete slabs - if the reinforcement like for other methods are redistributed in appropriate areas/strips. This is in contradiction to statements by Hillerborg et al (1990), (1996). Finally, FEM-Design has proven to give reliable analyses and designs, for all tested cases. Actually, there are very few drawbacks with the use of a FE-based design, especially since FEM-Design's plate module is found to be a very user-friendly design tool. However, three improvements/implementations are suggested to make the program even better: • A distributed stiffness (the multi spring concept) to model interior columns/walls. • A more available and clear input check option. • A distribution method for reinforcement. The distribution method (FED), proposed in the thesis is suggested as one suitable method to implement, because it combines FE-theory with theories behind traditional design methods.Validerat; 20101217 (root