Numerical and experimental dynamic analyses of the Vega Pedestrian bridge including seasonal effects
Abstract: As timber structures become increasingly relevant and sought after – since they enable improvements in building time while reducing a structure’s life cycle impacts – streamlining their design can have meaningful economic and environmental implications. For timber footbridges, its design is frequently governed by serviceability criteria linked to excessive vibrations. To address this in design, it is necessary to correctly characterize the structure’s dynamic properties and understand what the leading parameters in its behaviour are. This thesis studied an existing timber arch footbridge, aiming to evaluate its dynamic behaviour both with experimental measurements and with theoretical models. The influence of temperature change over different seasons was considered, particularly around its effect on the asphalt layer – whose stiffness is highly correlated to temperature. The experimental results showed high correlation between temperature and natural frequencies: a variation of +21°C reduced the natural frequency for the 1st transverse mode of the deck by as much as 30.6% while the 1st vertical mode was reduced by 17.7% (variation of 0.029Hz/°C). The damping ratio was also measured, though a definitive correlation between its value and temperature was not identified. This change in behaviour cannot be explained by the influence of the asphalt layer alone however, as there is a high degree of uncertainty around many other components of the bridge and their interactions, such as the connections. Thus, to fully characterize the influence of each component with changing temperature, further experimental tests would have to be performed, or simpler structures with fewer connections should be considered. In designing a new structure, considering the asphalt layer as an added mass is a straightforward way to treat this material at the most critical condition (i.e. no contribution to stiffness). This strategy lead to sufficiently similar results between the computational model and the experimental results at warm temperatures. The asphalt stiffness could perhaps be considered for the 1st transverse mode of the deck, since it is in this mode that the asphalt layer plays its largest contribution.
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