# Load Testing and Analysis of a Large Span Through Simply-Supported Steel Box Arch Bridge
## 5 Dynamic Load Test
### Dynamic Load Test Results and Analysis  
#### 5.3.1 Results and Analysis of the Bridge's Natural Vibration Characteristics Test  
The natural vibration characteristics of a bridge are typically assessed using three methods: impact excitation, ambient vibration excitation, and forced vibration excitation. In the impact excitation method, a specialized hammer or similar device is employed to apply an impulsive force to the bridge, inducing vibrations. Although this method can replicate the impact loads encountered in real-world engineering scenarios, the complexity of such loads may introduce significant errors in test results. The forced vibration excitation method is commonly utilized to simulate specific frequency and amplitude vibration environments for evaluating system performance under vibrational conditions. This approach enables precise control over both frequency and amplitude parameters and allows for repeated testing; however, it necessitates high-end equipment and incurs substantial costs. On the other hand, the ambient vibration excitation method utilizes naturally occurring environmental stimuli like wind flow, water currents, or ground motion to stimulate structural responses. This cost-effective and time-efficient technique is also referred to as pulse testing--which has been chosen as the preferred methodology for this experiment. The pulse test involves the placement of highly sensitive sensors on the bridge to record its vibration over an extended period under environmental excitation, such as wind, water flow, or ground motion. Subsequently, the recorded vibration time history is processed and analyzed in both the time domain and frequency domain to determine the natural vibration characteristics of the bridge. The pulse test assumes that environmental excitation follows a stationary stochastic process. In the low- and mid-frequency range, ambient vibrations exhibit a relatively uniform excitation spectrum. When the frequency of environmental excitation matches or closely aligns with the bridge's natural frequency, energy absorption occurs, leading to an increase in amplitude. Conversely, when there is a significant difference between environmental excitation frequency and the bridge's natural frequency, a large phase difference arises, resulting in energy cancellation and, subsequently, smaller amplitudes, by analyzing amplitude and phase relationships of vibration response at different measurement points, mode shapes corresponding to various modal frequencies can be determined for the bridge. Averaging multiple time history curves enables the determination of natural frequencies for each mode exhibited by the bridge. It should be noted that ambient vibrations may introduce additional noise and interference, which can affect test result accuracy; hence, extra data processing, including the exclusion of abnormal data, is required. Performing time-domain and frequency-domain analyses on the pulsation test signals revealed the measured first-order frequency of lateral vibration for the main bridge to be 1.425 Hz, compared to a finite element calculated frequency of 1.14 Hz. The measured first-order frequency of vertical vibration was 2.95 Hz, whereas the calculated frequency was 1.71 Hz. Additionally, the measured first-order torsional frequency stood at 3.60 Hz, higher than the calculated value of 1.948 Hz. For the approach bridge, the measured natural frequency of the first-order vertical bending mode was 6.375 Hz. These results indicate that the vertical overall stiffness of the bridge meets the design requirements. Refer to Table 7 for detailed test results of the main bridge's pulsation natural frequencies. The mode shape diagrams of the main arch and main beam simulated by Midas Civil are shown in Figure 14.