Influence of asphalt pavement on damping ratio and resonance frequencies of timber bridges

Since timber bridges are light-weight structures with a high life-load to death-load ratio, they are susceptible to vibration and thus satisfying vibration serviceability is a governing design criterion. A good knowledge of the damping properties of timber bridges is therefore a key aspect for guaranteeing a sufficient serviceability. Since observations suggest that timber bridges with an asphalt pavement increase the damping, this work investigates the influence of asphalt pavement on the damping and the resonance frequencies of two laboratory bridges: (1) a cable-stayed bridge with a main span of 15.6 m, a secondary span of 3.6 m and a width of 1.6 m and (2) a smaller simply supported bridge with a total length of 9.6 m and a width of 1.6 m. The deck plates were made from two different kinds of panels: cross-laminated timber (CLT) and a stress-laminated plate (SLP). Due to the asphalt pavement the first resonance frequency decreases 23%–26% for the cable-stayed bridge and 16%–18% for the simply supported bridge. This result suggests that the additional mass overtops the effect of an additional stiffness. The damping of the first vibration mode is about ζ = 0.5 % … 0.66 % for both bridges without pavement, and increases to ζ = 2.2 % … 2.6 % and ζ = 6.2 % … 7.4 % for the cable-stayed and the simply supported bridge, respectively, after deployment of an asphalt pavement. The comparison of the experimental results for the first resonance frequency and the damping with Finite-Element computations indicate that a noticeable amount of shear forces are transferred during the dynamic measurement and thus the pavement contributes to the stiffness of the bridges. The beneficial effect of the asphalt pavement at ambient temperatures on the damping has to be validated by measurements at different temperatures, since the asphalt is very temperature dependent.