Efeito da modelagem do carregamento, do impacto do calcanhar humano e do amortecimento estrutural na resposta dinâmica de passarelas mistas

Abstract

The demands imposed by daring architectural projects produced pedestrian footbridges have been designed and constructed with light and large span structures. This fact have generated very slender structural footbridges and consequently changed the serviceability and ultimate limit states associated to their design. Another consequence of this design tendency is the considerable increase of vibration problems. In the particular case of footbridges this phenomenon happens when the structure fundamental frequency is the equal or even near of the pedestrians step frequency. Human activities as walking, running or jumping produces dynamic forces. In some cases these dynamic forces can produce excessive vibrations and disturb the pedestrians that are crossing the footbridge. As the primary purpose of the footbridges is the pedestrian, they need to be safe and to present a behavior that doesn't offer discomfort to the users. This investigation was developed to evaluate the influence of the loading modelling, related to the pedestrians, human heel impact effect and structural damping on the composite footbridge dynamic response. The analysis was carried out based on a more realistic loading model developed to incorporate the transient impact of the human heel due to walking. In this loading model, the movement of legs that cause an ascent and descent of the effective mass of the human body at each step was considered and the position of the dynamical loading is changed according to the individual position and the generated time function, corresponding to the excitation induced by people walking, has a space and time description. The investigated structural model was based on several composite footbridges in use. The footbridge dynamical response was obtained, based on the peak accelerations, and compared with results supplied by design standard objectifying human comfort evaluations. Based on the results reached in this work it was possible to demonstrate the importance of the investigated parameters (dynamical load modelling, human heel effect and structural damping) and how these elements substantially influence on the evaluation of the footbridges dynamical response. The results clearly indicated that structural designers should be aware of some distortions caused by disregarding the dynamical effects on this type of structure. The maximum acceleration values violate the human comfort serviceability conditions when compared to limit acceleration targets present in several references and design codes. Hence it was detected that this type of structure can reach high vibration levels, compromising the footbridge user s comfort and especially its safety.