The interface shear anchorage of the U-Bahn weight-saving floor system

The U-Bahn floor system is an innovative weight-saving floor system that consists of a prefabricated floor and a structural screed. To connect these two layers, only lattice girders are applied. This means that all stresses in the complete floor system must be taken up by these lattice girders. The U-Bahn floor system is built without propping, a feature that speeds up the building process enormously. Therefore, all building phase depend on each other and together determine the capacity in the use phase. However, to correctly determine the capacity in the use phase, all stresses need to be determined per building phase. The capacity of the correctly anchored lattice girders must after all withstand these stresses. The aim of this research is to find out whether the lattice girders can resist the shear stresses of the U-Bahn floor system. The following research question has been drawn up: In which way does the design of the interface shear of the U-Bahn weight-saving floor system sufficiently anchorage the structural screed? To answer this question, extensive research was first conducted into the various building phases of the U-Bahn floor system. The stresses and forces for each phase individually were calculated. Subsequently, it is examined which type of lattice girder is usable in the U-Bahn floor system and how this type is incorporated in the floor. The stresses that these lattice girders must resist and transfer, have been calculated by means of two methods, an analytical calculation and finite element analysis calculation. As there is no product standard for the U-Bahn floor system yet, the calculations are based on the Eurocode and the floor plank standard. The results of the analytical calculation regarding the anchoring of the lattice girder showed the following : (i) the connection reinforcement, according to the floor plank standard, is sufficient, (ii) the shear force, according to the Eurocode, is not sufficient and, (iii) the detail requirements, as described in the Eurocode, have not been met. The results of the analytical calculation regarding the interface shear capacity of the total floor system shows that the resistance of the lattice girder in the U-Bahn floor system is lower than the acting stresses in the use phase, which results in a unity check of 1.40, which exceeds the maximum unity check of 1.0 as given in the Eurocode. To verify the analytical calculation and to determine to what extent the anchoring of the lattice is insufficient, the floor has been modelled in the finite element program, Diana. Due to the complexity of the program and the calculation, it has been decided to use a linear analysis for the calculation of the different phases of the U-Bahn floor system. Due to this, no result of the crack width and redistribution of the forces in the floor can be given. Nevertheless, the results do show that the concrete cannot take up the calculated stresses, which suggests that the floor will crack. Future research can further investigate the limits of the U-Bahn floor system by means of a non-linear calculation in a finite element analysis. In this non-linear calculation, the factors, deflections, creep and shrinkage must also be included, since this research has shown that these are co-determining factors for the interface shear capacity of the floor. These results will then have to be verified with destructive tests. Finally, if the results are confirmed, they can be converted into a rule-of-thumb, in order to facilitate tenders in the future.Civil Engineering | Structural Engineerin