# Effect of Vinyl Flooring on the Modal Properties of a Steel Footbridge
## 2 Unexpected High Experimental Damping Ratios of the Jorge Manrique Footbridge
### Experimental Identification of the Modal Parameters of the Jorge Manrique Footbridge
The relative differences, \(\Delta f_{exp,j}^{num,j}\), were inside the usual variation range predicted during the design phase of a footbridge [28], and all the \(MAC_{exp,j}^{num,j}\) ratios are greater than 0.90. At this point, it is necessary to highlight two facts: (i) although the natural frequencies of the structure are inside the frequency ranges which characterize the human rhythmic action [13; 14] and the pedestrian traffic on the footbridge is high no vibratory problems have been reported in this footbridge after the installation of the vinyl flooring; and (ii) the experimental damping ratios, \(\zeta_{exp,j}\) (being \(j\) the number of the considered vibration mode), of the three identified vibration modes are clearly greater than the usual values recommended by the most advances design guidelines (Table 2). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Damping Ratio, \(\zeta_{j}\), [\%]**} \\ \cline{2-7}  & \multicolumn{2}{c}{**French Guidelines**[13]} & \multicolumn{2}{c}{**European Guidelines**[14]} \\ \cline{2-7}
**Construction Type** & \multicolumn{2}{c}{**Serviceability**} & \multicolumn{2}{c}{**Large**} & \multicolumn{2}{c}{**Serviceability**} & \multicolumn{2}{c}{**Large**} \\  & \multicolumn{2}{c}{**Conditions**} & \multicolumn{2}{c}{**Vibrations**} & \multicolumn{2}{c}{**Conditions**} & \multicolumn{2}{c}{**Vibrations**} \\ \cline{2-7}  & **Minimum** & **Mean** & **Mean** & **Minimum** & **Mean** & **Mean** \\ \hline Reinforced concrete & 0.80 & 1.30 & 5.00 & 0.80 & 1.30 & 5.00 \\ Prestressed Concrete & 0.50 & 1.00 & 2.00 & 0.50 & 1.00 & 2.00 \\ Steel welded joints & 0.20 & 0.40 & 2.00 & 0.20 & 0.40 & 2.00 \\ Steel bolted joints & 0.20 & 0.40 & 4.00 & 0.20 & 0.40 & 4.00 \\ Composite & 0.30 & 0.60 & - & 0.30 & 0.60 & - \\ Timber & 1.50 & 3.00 & - & 1.00 & 1.50 & - \\ Stress-ribbon & - & - & - & 0.70 & 1.00 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Damping ratio, \(\zeta_{j}\), recommended by different design guidelines [13; 14] being \(j\) the number of the considered vibration mode. As the unique modification performed on the footbridge during its overall life cycle was the installation of the vinyl flooring, it was assumed that the improvement of its comfort level was caused by this fact. Additionally, as it was checked that the modification of the natural frequencies of the footbridge due to the presence of the vinyl flooring was small (Table 1), it was assumed that the main effect of the vinyl flooring on the footbridge was the increase of its damping ratios. Thus, the experimental verification of this assumption motivated the development of this study. In the next section, an overview of the different mechanisms that generate damping in civil engineering structures is included. Additionally, the comparative method considered herein to estimate experimentally the effect of vinyl flooring on steel footbridges is described in detail.