Experimental and Numerical Analyses of the Dynamic Behaviour of Hollow-Core Concrete Floors
Time: Fri 2020-05-15 13.00
Subject area: Civil and Architectural Engineering, Structural Engineering and Bridges
Doctoral student: Fangzhou Liu , Bro- och stålbyggnad
Opponent: Professor Bassam Izzuddin, Imperial College London
Supervisor: Professor Jean-Marc Battini, Byggkonstruktion, Byggvetenskap, Bro- och stålbyggnad, Mekanik; Adjungerad Professor Costin Pacoste, Bro- och stålbyggnad
Precast hollow-core (HC) concrete slabs are widely used in construction, especially in Nordic countries. The combination of prestressing and low self-weight due to the voids makes it possible to build long-span floors. However, this also implies that the HC concrete floors become more sensitive to vibrations from human activities. These vibrations are usually not a safety problem for the structure but they can induce important comfort problem for people living or working in the building. The overall aim of this thesis is to propose a methodology to predict the dynamic behaviour of HC concrete floors in the design process. For that, both a FE model and excitation loads are needed in order to calculate the accelerations and to quantify the intensity of the vibrations. The magnitudes and positions of the excitation loads are defined in guidelines. This thesis mainly focuses on how to develop an accurate FE model of HC concrete floors. At first, an experimental slab which consisted of 6 HC elements was studied. Nine accelerometers were used to record the vertical accelerations. The natural frequencies and eigenmodes were obtained in two different tests by using a vibration exciter and a heavy-duty impact hammer. Moreover, single person walking tests were also performed. The purpose of this study was to get an accurate FE model of the slab itself. In Paper I, an accurate solid element model was performed in order to understand in depth the behaviour of the experimental slab. Then, in Paper II, three different shell element models were developed for practical use. The results indicate that the best alternative is to take an orthotropic shell model. In Paper III, four different numerical load models were taken from the literature to reproduce numerically the single pedestrian walking tests. The results show that the four pedestrian loads give rather different numerical results and that an accurate numerical modelling of a single pedestrian loading is not an easy task. After that, six in-situ dynamic experiments performed in four buildings were studied. In each case, the natural frequencies and eigenmodes were obtained in two different tests by using an impact hammer and a free-falling of a person from a height of 30 cm. Some single person walking tests were also performed. The purpose of these studies was to get a more accurate FE model not only for HC concrete slabs itself but also considering the effects of surrounding structural members. In Paper IV, the results show that the orthotropic shell model proposed in Paper II gives good results in all the studies. They show also that the surrounding structural members should be incorporated in the FE model in order to get accurate results. Finally, Paper V focuses on the vibration response of HC concrete floors predicted by two design guides, the SCI P354 and the Concrete Center. For that, the floors studied in Papers I to IV and the corresponding FE models are used. Several recommendations that structural engineers can use to perform the dynamic assessment of the HC concrete floor are proposed.