# Experimental Study on Dynamic Effects of a Long-span Railway Continuous Beam Bridge
## 5 Analysis of Impact Factors
### Impact Factor Calcalated from Experimental Data  
The IMs calculated from deflections are given in Figure 10. The IMs of the Section A-A are greater than that of Section C-C. The reason is that the static and dynamic deflections of the deflection control section of the side span is smaller than those of the mid-section in the central span. Because the IM is the ratio of a dynamic increment to the static response and it can also be large when the increment is not large but the denominator (the static response) is small. By comparing the results of Figures 10b and 11a, it can be concluded that the IM calculated from deflections is smaller than the IM calculated from strains. This finding is consistent with the conclusions of some existing studies [24; 25]. However, some researchers presented different opinions that the IM calculated from deflections is greater than that calculated from strains [26; 27]. Therefore, it is not reasonable to assign the same IM for different load effects.  
Figure 9: Comparison of time history curves of the static and dynamic deflection when two freight trains run on the bridge.  
As shown in Figures 10 and 11, the IMs obtained from the Case 1 (Single line) are larger than those from the Case 2 (Double line). Many studies have reported the same findings [28; 29]. Furthermore, some studies [30] pointed out that the IMs of the bridge part subjected to smaller vehicle loads are usually greater than those of the bridge part subjected to larger vehicle loads. The same conclusion that the IMs of the loading side are smaller than those of the other side with one lane loaded, can be obtained by comparing the IMs of the two pairs of points symmetrically arranged about the longitudinal axis of the bridge in Figure 11a,b. As shown in Figure 11c,d, the IMs of Section C-C are greater than that of Section B-B, which suggests that the impact effect of moving trains on the section in central span is greater than the impact effect of moving trains on the section near support point. The dynamic test results at different speeds show that the IM is not sensitive to the train speed. Unexpectedly, the maximum IM is obtained at the speed of 20 km/h, which is even greater than the IM obtained at  
Figure 11: Impact factors calculated from strains of test points: (**a**) P1/P2; (**b**) P5/P6; (**c**) P3/P7; (**d**) P4/P8.  
Figure 10: Impact factors calculated from deflections: (**a**) Section A-A; (**b**) Section C-C.  
the speed of 80 km/h. One possibility is that the loading frequency approaches the natural frequency of the bridge and the quasi-resonance of the train-bridge system occurs when the train passes the bridge at a speed of 20 km/h.  
The time history curves of the left strain test point at the lower edge of the mid-section in the central span (P1) with one train running on the bridge at different speeds are given in Figure 12. A train running at 20 km/h causes the greatest dynamic strain of point P1 among all test speeds, which suggests that the quasi-resonance is likely to occur at a speed of 20 km/h compared with other speeds. When one lane is loaded, the mean and maximum IMs of Section B-B are 0.13 and 0.17, respectively, and the mean and maximum IMs of Section C-C are 0.15 and 0.19, respectively. When two lanes are loaded, the mean and maximum IMs of Section B-B are 0.10 and 0.15, respectively, and the mean and maximum IMs of Section C-C are 0.11 and 0.14, respectively. The ultimate IM obtained from the dynamic tests is 0.19, which appears at the lower edge of the mid-section in the central span with a train speed of 20 km/h.