# Estimation of Live Load Distribution Factor for a PSC I Grider Bridge in an Ambient Vibration Test
## 4 Estimation of LLDF in the Ambient Vibration Test  
In general, a vehicle loading test is conducted to measure the LLDF of a bridge. It is difficult, however, to conduct the test on bridges where there is heavy traffic because traffic control is required. Therefore, if LLDF estimation is possible under ambient vibration conditions, it will be efficient and economical because LLDF can be estimated at bridges where traffic control is impossible. The ambient vibration test was conducted to examine the possibility of estimating the LLDF of a bridge where vehicles with unknown weight are traveling. In the test, LVDTs were attached in the same manner as in the vehicle loading test. A camcorder was installed on the roadside to identify vehicle types and driving lanes in the ambient vibration test. The measured responses were also classified according to the vehicle type and lane by measuring the camcorder and LVDTs at the same time. The ambient vibration test was conducted for three hours. Table 4 shows the vehicle types that passed the bridge and their driving lanes in the test. Figure 10 shows the images of the representative vehicles captured using the camcorder installed on the roadside in the ambient vibration test. The vehicles were classified as cars, buses, trucks, dump trucks, and special vehicles. Figure 11 shows the displacement response measured in a certain section. When small vehicles, such as cars, passed the bridge in the ambient vibration test, they were excluded from the estimation of LLDF \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Vehicle Type** & **Remarks** & **1 st Lane** & **2 nd Lane** & **Total** \\ \hline car & small, medium, and large & 70 & 17 & 87 \\  & vans, etc. & 21 & 23 & 44 \\ bus & large buses, etc. & 21 & 29 & 50 \\ truck & 1-, 5-, and 10-ton trucks, etc. & 18 & 11 & 29 \\ dump truck & large dump trucks, etc. & 10 & 14 & 24 \\ special vehicle & tanker trucks and trailers, etc. & 10 & 14 & 24 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Vehicle types that passed the bridge and their driving lanes in the ambient vibration test. Figure 9: Error rates in each LC. because a deflection of less than 0.2 mm occurred. The LLDF of the bridge was estimated using heavy vehicles that caused significant vertical displacement. To prevent errors in the LLDF estimation, two or more vehicles travelling on the bridge almost at the same time were excluded. LLDF was estimated when a heavy vehicle passed the bridge in one lane, and four vehicle types (bus, truck, dump truck, and special vehicle) were considered. Table 5 shows the number of the vehicles used to estimate LLDF. Table 6 shows the average maximum displacement of the static component extracted from the displacement response measured for each vehicle type and lane in the ambient vibration test. When vehicles travelled in lane 1, the maximum displacement occurred at G2 and ranged from \(-0.52\) to \(-1.73\) mm. In lane 2, the maximum displacement occurred at G3 and ranged from \(-0.50\) to \(-1.42\) mm. Dump trucks exhibited the largest displacement, while buses yielded the smallest displacement. Figure 11: Displacement response measured at each girder in the ambient vibration test: (**a**) displacement response in a specific section; (**b**) displacement response measured between 50 and 66 s.  
Figure 10: Representative vehicles in driving lanes in the ambient vibration test: (**a**) Representative vehicles that travelled in lane 1; (**b**) Representative vehicles that travelled in lane 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Lane** & **Dump Truck** & **Bus** & **Truck** & **Special Vehicle** & **Total** \\ \hline
1 & 6 & 11 & 8 & 7 & 32 \\
2 & 4 & 9 & 12 & 6 & 31 \\ total & 10 & 20 & 20 & 13 & 63 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Vehicle types and lanes used to estimate LLDF in the ambient vibration test. Figure 12 shows the average LLDF estimated for each vehicle type and lane, and the result was compared with the LLDF measured in the static loading test.