Dynamic Analysis of Double Tee Floors Subjected to Human Activity : A comparison between a standard- and a modified cross section

Abstract: Building with larger spans and more slender elements is more common today since the demand for open floor plans has increased. The consequence is that low frequency floors and vibration problems arise. Such floors tend to undergo large vibrations since their low eigenfrequency coincides with the frequency which human activities have. The coincidence results in resonance which may cause discomfort for the occupants of the floor. Double tee floors can be categorized as low frequency floors and can suffer from large vibrations. In this thesis, a modified cross section of double tee is proposed with the aim to increase the eigenfrequency and solve the vibrations problems caused by human activities. This thesis also contains a lot of useful information about double tee dimensions and properties. A floor with the modified double tees and a floor with double tees that are currently used in practice were modelled in software BRIGADE/Plus with the aim to predict their steady-state response when subjected to human activities. A walking person was simulated on the floor and the predicted response was compared to the vibration criterion from ISO. In addition, it was investigated if it is possible to have a multi- function floor, consisting of an office and a gym, without getting adverse comments from people in the office during rhythmic activity in the gym. The floor model was divided into an office and a gym area. Jumping people were simulated over the gym area and the response was predicted in the office area and later compared to the criteria which for offices is 10 times lower than for gyms. A parameter study was also conducted in the thesis to investigate which parameter has the largest impact on the fundamental frequency of the double tee. The studied parameters were: I) Topping thickness II) Young’s modulus of topping and III) Intermediate beam height of the modified section. The results showed that the fundamental frequency for the floor with the modified section increased by 17%. Moreover, the modified floor had in general lower accelerations when subjected to dynamic loading. In addition, this modified floor had one resonance peak less which means fewer critical loading frequencies. Regarding the multi-function floor, the dynamic response showed smaller accelerations for the modified floor. However, the vibration criterion was still violated for some jumping frequencies, which makes it necessary to use expansion joints. With this in mind, one must in the early design stage, think of the multi- function floor plan to avoid unnecessary large vibrations. Otherwise, it would be costly to rearrange afterwards. The parameter study showed that topping thickness has the largest impact on the fundamental frequency. The effect of Young’s modulus was marginal. Higher intermediate beams had a large effect but using too high beams is not appropriate due to construction height regulations. It was also observed that the local deformation in between the stems, across the double tee, were marginal compared to the global deformation along the double tee. Thus, only global deformation needs to be considered during dynamic analysis of double tee floors.