## 1 Introduction
### Strain Field Measurement Scheme of a Simply Supported Composite Beam Bridge  
Strain measurements were conducted on the upper and lower flanges of the I-beam in the composite section of the simply supported beam bridge on Shanghai Metro Line 3. In field strain detection, due to the bridge's simply supported structure, critical sections include the mid-span, support, and quarter-span sections, which experience the most severe stress conditions. The mid-span section bears the maximum positive bending moment, while the support section endures the maximum shear force. The quarter-span section experiences significant bending moment and shear force. Given practical constraints in deploying strain gauges, the focus was primarily on the upper and lower flanges of the I-beam in the mid-span and quarter-span sections. Figure 3 illustrates the layout and numbering of strain measurement points for these sections. The bottom I-beams of the composite girder are labeled 1 st to 4 th from left to right. Due to the presence of a green belt, strain measurement points were arranged on the three I-beams on the right side, omitting the 1 st I-beam. Strain measurement points in the quarter-span section are denoted as Q-\(i\)-\(j\), and those in the mid-span section as M-\(i\)-\(j\). The first digit signifies the bottom I-beam number of the composite section, while the second digit indicates the serial number of the strain gauge on the I-beam. For instance, Q2-3 denotes the third strain measurement point on the 2 nd I-beam in the quarter-span section, and M2-3 denotes the third one on the 2 nd I-beam in the mid-span section.  
Figure 3: Layout of strain measurement points for the quarter and mid-span sections of the simply supported composite beam bridge (unit: mm).  
In the field strain measurement (Figure 1), two data acquisition instruments were utilized to ensure data accuracy and reliability. Specifically, strain data from the quarter-span and mid-span sections of the simply supported composite beam bridge were collected using DH 5902 and DH 5920 devices manufactured by Donghua Testing Technology. The DH 5902 data acquisition device features 32 channels and a maximum sampling frequency of 100 kHz, while the DH 5920 device includes 16 channels with a maximum sampling frequency of 128 kHz. Both devices have 16-bit A/D resolutions. Prior to strain measurement, the layout, temperature compensation, and channel connections of the strain measurement points were configured, followed by instrument calibration. During on-site strain detection, the sampling frequency of the data acquisition instruments was set to 1000 Hz, significantly exceeding twice the natural frequency of the simply supported composite beam bridge structure, thus meeting the requirements for accurate field strain detection.  
The strain of the composite section of the simply supported composite beam bridge on Shanghai Metro Line 3 was monitored in situ over three days, from 27 to 29 April, including both working days and holidays. This monitoring captured the structural response of the bridge under different traffic conditions. Due to power supply constraints in outdoor settings, the data acquisition instrument was activated only when trains passed through the bridge. After filtering out abnormal data, train crossings were recorded 21, 23, and 23 times on 27, 28, and 29 March, respectively. Trains traveled in both forward and reverse directions: on 27 April, there were 12 forward and 9 reverse crossings; on 28 April, there were 12 forward and 11 reverse crossings; and on 29 April, there were 12 forward and 11 reverse crossings.  
Numerical Modeling, Validation, and Strain Influence Lines Calculation of an In-Service Simply Supported Composite Beam Bridge