## 3 Digital Twin Calibration and Validation  
The calibration and validation of the digital twin for the Cho'ponota L1 Bridge were conducted by integrating high-fidelity field data into a finite-element model, originally developed using design project documentation. The dynamic characteristics obtained from OMA were compared with the numerical modal properties predicted by the initial finite-element model. Modal analysis results provided natural frequencies and mode shapes for each span, which were also used to guide sensor placement and evaluate the bridge's dynamic behavior more precisely. As presented in Table 7, the initial model exhibited a maximum frequency discrepancy of 13.22% compared to the OMA results, exceeding the 5% threshold commonly accepted in the literature [85]. These deviations indicated a clear need for model refinement. Such differences often stem from simplifications in modeling assumptions, variations in construction materials, support boundary conditions, or unmodeled as-built deviations. Accordingly, the FE model was iteratively \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Mode**} & \multicolumn{6}{c}{**Frequency Values for S1, S2, and S3 Spans (Hz)**} \\ \cline{2-10}  & \multicolumn{2}{c}{**S1 Span**} & \multicolumn{2}{c}{**S2 Span**} & \multicolumn{2}{c}{**S3 Span**} & \multicolumn{2}{c}{**Diff. (\%)**} \\ \cline{2-10}  & **Before** & **After** & **Diff. (\%)** & **Before** & **After** & **Before** & **After** & **Diff. (\%)** \\ \cline{2-10}  & **Loading** & **Loading** & & **Loading** & & **After** & **Before** & **After** & **After** & **Diff. (\%)** \\ \hline
1 & 1.12 & 1.11 & 0.89 & 1.12 & 1.11 & 0.89 & 1.12 & 1.11 & 0.89 \\
2 & 1.87 & 1.89 & 1.07 & 1.89 & 1.88 & 0.53 & 1.89 & 1.86 & 1.59 \\
3 & 2.12 & 2.12 & 0.00 & 2.13 & 2.13 & 0.00 & 2.12 & 2.07 & 2.36 \\
4 & 2.53 & 2.57 & 1.58 & 2.51 & 2.54 & 1.20 & 2.56 & 2.51 & 1.95 \\
5 & 2.75 & 2.78 & 1.09 & 2.75 & 2.75 & 0.00 & 2.74 & 2.75 & 0.36 \\
6 & 3.34 & 3.35 & 0.30 & 3.35 & 3.34 & 0.30 & 3.35 & 3.34 & 0.30 \\
7 & 4.03 & 4.07 & 0.99 & 4.04 & 4.03 & 0.25 & 4.04 & 4.02 & 0.50 \\
8 & 4.28 & 4.25 & 0.70 & 4.28 & 4.25 & 0.70 & 4.25 & 4.25 & 0.00 \\
9 & 4.58 & 4.55 & 0.66 & 4.57 & 4.53 & 0.88 & 4.55 & 4.55 & 0.00 \\
10 & 5.26 & 5.23 & 0.57 & 5.25 & 5.23 & 0.38 & 5.28 & 5.23 & 0.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Frequency values obtained before and after the loading test for S1, S2, and S3 spans. Figure 17: The first 10 operationally obtained mode shapes of the bridge’s S1, S2, and S3 spans. updated--particularly by adjusting material stiffness and support representations--to achieve better alignment with the measured data. One critical hypothesis considered was the potential difference in the actual concrete quality used during construction compared to the project specifications. Through this calibration process, the digital twin was validated as a reliable representation of the bridge's true dynamic behavior, enabling diagnostic and predictive analyses under service conditions. The primary updating parameter was selected as the concrete material strength. Manual calibration was performed by incrementally adjusting this property until the natural frequencies derived from the numerical model closely matched those obtained from field measurements. This adjustment reflects the hypothesis that the concrete used during construction may differ from the design values. Through iterative updates, the FE model was refined to accurately represent the actual stiffness distribution of the bridge. Table 8 presents the comparison of the updated model's frequency values with OMA data. The results indicate that, for all modes except the eighth, frequency deviations were reduced below 5%, thus meeting widely accepted validation criteria. Even the eighth mode--where deviation slightly exceeds 5%--remains within tolerable limits for higher-order modes.