# Analytical and Measured Effects of Short and Heavy Rail Cars on Railway Bridges in the USA
## 3 Results of Case Studies
### Riveted Truss Spans
#### 3.1.2 Results from Test of Truss Bridge  
The strain gauge data were collected on a two-span truss bridge, with spans of 61 m (200 ft) and 33.5 m (110 ft), for two months to compare the structural response under typical 16.2-m (53-ft) coal cars as well as shorter railcars of cement trains and rock trains and to show the differences due to railcar lengths. A total of 24 strain gauges were installed on main members of trusses to record axial stresses, and four additional strain gauges were installed on stringers to record bending stresses [3]. Based on the results from initial FEM analysis, several members of both trusses were selected to be instrumented to verify the differences due to various railcar lengths. On the 61-m (200-ft) span, 14 strain gauges to evaluate axial forces were installed on the bottom chord (L2L3) and top chord (U3U4) close to the center span, end post (L0U1), and diagonals (U1L2, L2U3, and U3L4). Similarly, on the 33.5-m (110-ft) span, strain gauges were installed on the bottom (L1L2) and top (U1U2) chords close to the center span, end post (L0U1), and diagonal (U1L2). The bottom chords were instrumented on both inside and outside of the cross-section. Data were collected under a total of 465 trains, which were comprised of 299 empty unit trains, 120 mixed freight trains, and 46 loaded unit trains [3]. The research focused specifically on loaded trains; therefore, data from the 46 loaded unit trains were then separated by the length of their cars. Thirteen loaded unit coal trains of 16.2-m (53-ft) common length railcars, nineteen loaded unit rock trains of 13.1-m (43-ft) railcars, and eight loaded unit cement trains of 12.8-m (42-ft) short railcars were analyzed to validate the analytical results. Figure 10 compares the stress histories calculated from measured strains for a train with 12.8-m (42-ft) railcars and a train with 16.2-m (53-ft) railcars. As predicted by analysis, the 12.8-m (42-ft) short railcars caused higher stresses in the top chord and end post of the 61-m (200-ft) span. Figure 11 illustrates the typical peak stresses caused by unit trains of 16.2-m (53-ft) common length railcars and unit trains of  
Figure 10: Stress histories calculated from measured strains in 61-m (200-ft) span: top chord (**top**) and end post (**bottom**). 12.8-m and 13.1-m (42- and 43-ft) short railcars on the top, bottom, and diagonal members of the 61-m (200-ft) span. The top and bottom chords experienced stress increases of 31 and 28%, respectively. The end post saw a 20% stress increase, whereas the diagonal U1L2 saw a 21% stress increase. A 4-6% stress increase was recorded in the intermediate diagonals. The test yielded similar results for the 33.5-m (110-ft) span. Figure 12 shows the comparison of stresses between 12.8-m and 13.1-m (42- and 43-ft) short railcars and 16.2-m (53-ft) common length railcars along with the members of the 33.5-m (110-ft) span. The top and bottom chords each experienced a 30% increase in stresses due to the short railcars. The end post saw an 18% increase, and the diagonal U1L2 saw a 13% stress increase due to short railcars. Stress ranges were calculated to determine the fatigue cycles that each member experienced. Figure 13 shows the stress ranges for the sets of the 12.8-m and 13.1-m (42- and 43-ft) short railcars and 16.2-m (53-ft) common length railcars on the 33.5-m and 61-m (110- and 200-ft) spans. The stress ranges of the top chord were not included, because they were close to zero. All of the stress ranges calculated from measured strains for the main span members were well below the variable amplitude fatigue limit of 41 MPa (6 ksi) according to AREMA provisions [8]. For the main span members, fatigue was not a concern for either of the trusses due to the 16.2-m (53-ft) common length railcars or to the 12.8-m (42-ft) short railcars. Figure 11: Typical peak stress calculated from measured strains, comparison for 61-m (200-ft) span. Figure 12: Typical peak stress calculated from measured strains, comparison for 33.5-m (110-ft) span.