12 Temperature Effects on the Modal Properties of a Suspension Bridge 91 0.13 0.135 0.14 0.145 0.15 f (Hz) Mean+RMS Mean awdata 0.215 0.22 0.225 0.23 f (Hz) 10/10 12/10 14/10 16/10 18/10 20/10 1.22 1.23 1.24 1.25 1.26 Time (DD/MM) f (Hz) Fig. 12.3 Evolution of HS1 (top), VA1 (middle) and TS1 (bottom) between the 20/09/2015 and 30/09/2015. Data binning has been applied to better estimate the fluctuating mean value and RMS of the eigen-frequencies 01/07 01/08 01/09 01/10 01/11 01/12 0.22 0.23 (VA1) Time (DD/MM) f (Hz) 0 10 T (◦C) 20 Fig. 12.4 Evolution of the temperature and the frequency of the mode VA1 from July 2015 to December 2015 As expected, the damping ratios for HS1, HS2, HA2, VS1, VA1 and TA1 fluctuate little with the temperature. For a given temperature, multiple wind velocities are recorded. Because the total modal damping ratios increase with the wind velocity, due to added aerodynamic damping, a large variety of total modal damping ratios are recorded for a given temperature. Consequently, a large dispersion is expected when the modal damping ratios are expressed as a function of the temperature alone. On Fig. 12.5, VS1 and VA1 have an averaged modal damping ratios of about 0.75% and 1% respectively and are characterized by a considerable spreading. This is not surprising given that the modal damping ratios associated with VS1 and VA1 are highly affected by the wind induced aerodynamic damping effect [4]. A reduction in modal damping with increasing temperatures and the underlying reduction in wind speed is generally expected. This is in particular the case for the vertical modes for which the aerodynamic damping can be relatively high. The highest damping values are still observed for temperatures between 5ıCand 10ıC at which the strongest winds occur. The decrease in the damping ratios of TS1 for increasing temperatures is somewhat more distinct. As already pointed out, lower temperatures are typically associated with higher wind speeds, for which a weakly increasing TS1 damping ratio has been observed [4]. It is however a bit surprising that the corresponding contribution of the aerodynamic damping, which is much stronger for the vertical modes, is not so obvious in the average damping estimates presented. To improve the reliability of these results, an analysis using at least 1 year of temperature, velocity and acceleration data is required.
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