Numerical and experimental dynamic analyses of Hägernäs pedestrian bridge : Including seasonal effects

Abstract: Wood as a construction material has in recent years increased, in particular concerningpedestrian bridges. By utilizing wood, the ecological footprint can be reduced,and the material can be designed to comply with the increasing aesthetic demandbridge designers are facing. However, as the material weighs little with respect toits bearing capacity, combined with design becoming more slender, human inducedvibrations are becoming a problem.Having this in mind, the objective of the thesis is to conduct a case study on anexisting timber pedestrian bridge and assess its dynamic parameters by means ofexperiential testing and numerical modelling. The case study concerns the Hägernäsbridge, an arch bridge located in Hägernäs, Täby. The thesis also considers seasonaleffects by conducting experiments on two separate occasions. In addition, the thesisevaluates influencing parameters on the dynamic behaviour by conducting a sensitivityanalysis. To aid the above mentioned objective, a literature review coveringsimilar type of analysis is conducted. The literature review also studies the seasonaleffect, mainly from the asphalt layer, as its stiffness contribution is temperaturedependant.The results from the dynamic parameters showed that not all modes fall above therecommended values concerning damping ratio (with values above 1-1.5%). However,all modes fulfill design criteria concerning the magnitude of the natural frequencies.Furthermore, results showed that the natural frequencies are highly temperaturedependant. The measured values during warm weather (+17C) resulted in lowervalues than those from the cold weather experiment (-10C). Moreover, the greatestdifference, by 21% was on the 1st transverse mode and the lower difference was onthe 1st vertical mode, that decreased by merely 5%. Moreover, the damping ratiowas calculated and it was not possible to find any correlation between warm andcold temperature. Instead, decreased temperature caused some modes to increase indamping ratio and others to decrease. The most affected mode was once again the1st transverse mode which increased by 146% going from warm to cold temperature