The Green River Bridge is a reinforced concrete structure with an expansion joint on its West end. The joint allows the concrete to naturally expand and contract without cracking as the deck undergoes thermal changes and other loading conditions.
Passenger cars have very minor dynamic loading effects on the bridge. But semi-trucks with trailers weighing up to 80,000 lbm provoke measurable vibration responses. These trucks are common due to the nearby Amazon distribution center and numerous other warehouses.
The figure below shows a bridge response acceleration time history as measured on an Android Smartphone, vertical axis. The excitation was primarily due to a single semi-truck, with some small passenger cars also contributing.
A damped-sine curve-fit over the free decay portion is superimposed. The frequencies and damping ratios are given below the figure. Note the beat frequency effect due to the frequency difference between the first two modal frequencies.
| Mode | Amplitude (G) | Freq (Hz) | Phase (rad) | Damping |
| 1 | 0.0107 | 2.25 | 0.0809 | 0.85 % |
| 2 | 0.0045 | 2.41 | -1.6588 | 0.63 % |
| 3 | 0.0037 | 5.43 | 2.3790 | 1.5 % |
| 4 | 0.0047 | 9.84 | -0.1769 | 2.8 % |
| 5 | 0.0059 | 11.99 | 3.1357 | 0.54 % |
Bridge acceleration time history: Green_River_bridge.txt
– Tom Irvine
Thanks Tom, for explaining the real world to us. Keep it up. This is great!
Love this. Thanks