Spillway Gate Failure
The hydraulic load on this type of spillway gate (Tainter gate) is transmitted from the cylindrical skin plate, which is in contact with the reservoir, through a number of struts to a convergence at the trunnion hub. The hub collects the load from the struts and transfers it across an interface to the trunnion pin, which is stationary and is connected to the dam. When the gate is operated, the hub rotates around the pin. The struts are primarily compression members, but friction at the pin-hub interface induces a bending stress during gate operation. Typically, and in this case, the struts are oriented such that the trunnion friction stress is applied to the weak axis of the struts (see Section modulus). In order to better handle these loads, the struts are connected with diagonal braces that take the stress as axial loads.
On the morning of July 17, 1995, the Folsom Dam power plant was shut down and Spillway Gate 3 was opened to maintain flows in the American River. As the gate was operated, the failure initiated at a diagonal brace between the lowest and second lowest struts. Increasing corrosion at the pin-hub interface raised the coefficient of friction and, therefore, the bending stress in the strut and the axial force in the brace. The capacity of the brace connection was exceeded and it failed. This caused the load to redistribute and the failure progressed, eventually buckling the struts.
After a year-long investigation, the United States Bureau of Reclamation attributed the failure to a design flaw: the Corps of Engineers, which designed the dam, did not consider trunnion friction (at the pin-hub interface) in the gate analyses. While this is true, this was one of five identical service gates that operated under the same circumstances for nearly 40 years without problems being observed. This suggests that the failure resulted from a condition that changed over time; specifically, there was a gradual increase in the coefficient of friction at the pin-hub interface. While one would expect maintenance frequency to increase as a gate ages, Reclamation decreased the frequency of regular maintenance and lubrication over time due to budget constraints. In addition, the lubricant used by Reclamation did not conform to the Corps' original design specifications; it was a new, environmentally-friendly lubricant that was not sufficiently waterproof, allowing water to enter the pin-hub interface and cause the corrosion that resulted in increased friction.
The failure resulted in the uncontrolled release of nearly 40 percent of Folsom Lake at a peak rate of approximately 40,000 cubic feet per second (1,100 m3/s). The freshwater reaching San Francisco Bay was atypical for the summer season and confused salmon and striped bass, whose instincts told them that fall rains had arrived; they began their annual fall migrations months ahead of schedule. However, this failure caused no fatalities and it had a significant positive impact on the dam industry. A renewed focus was placed on maintenance and monitoring of radial gates, many of which were retrofitted to strengthen struts and bracing and ensure sufficient lubrication.
Read more about this topic: Folsom Dam
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