Tainter Gate Safety Evaluation

On July 17, 1995, Spillway Gate No. 3 at Folsom Dam failed during a morning operation with nearly a full reservoir, resulting in an uncontrolled release of water from the reservoir. In response to this failure, the Aluminum Company of America (ALCOA) retained ECI, a water resources engineering consulting firm, to conduct a study of the tainter gates at the Tapoco and Yadkin systems in Tennessee and North Carolina, respectively. The Tapoco and Yadkin systems include six dam and hydroelectric plants, built between 1917 and 1957 with 65 tainter gates similar to the gates at Folsom Dam.

ECI's scope of work for this study included: 1) a site inspection to examine the gates and hoists, to verify the "as-built" condition compared to the drawings and to collect available data such as specifications, original calculations, operation logs and operation and maintenance (O&M) manuals, 2) a three-dimensional finite element structural analysis of the gates to determine the level of stresses and deflections in the gate struts, strut bracing members and connections, for all loadings including trunnion friction, 3) an evaluation of current lubrication practices and recommendations for future lubrication procedures, 4) recommendations for painting and other regular maintenance, and 5) recommendations for both physical testing to determine the actual trunnion friction coefficient during gate operation and rehabilitation of gate members, joint connections, guide rollers, gate seals and hoist chains.

The three-dimensional finite element structural analysis was performed for the gates at each of the dams to determine the forces and stresses in the gate struts, strut bracing members and connections for the expected usual, unusual and extreme loading conditions. The models were based on the drawings provided, data gathered during the preliminary site inspection and assumed physical properties for the steel and trunnion friction. The results were compared to allowable stresses and design code strength requirements for members subjected to axial load and biaxial bending.

As a continuation of the initial study, ECI is currently performing physical testing for ALCOA on select tainter gates. Strain gages, attached to specific gate struts and strut bracing members will be used to measure actual forces in the members. This information, used in conjunction with the finite element structural analysis, will then lead to a determination of the actual trunnion friction coefficients and an evaluation of the overall structural safety of the gates.

This paper discusses the design aspects and results of the initial finite element structural analysis, the strain gage testing procedure and measurement results, and an evaluation of the structural safety of the gates based on the results of the strain gage measurements.

Based on the findings of the Bureau of Reclamation as to the cause of the 1995 Folsom Dam tainter gate failure, the Federal Energy Regulatory Commission (FERC) is now requiring review of design criteria, inspection, maintenance and lubrication procedures for all tainter gates at "High" and "Significant" hazard potential dams. A similar review is recommended by FERC for all "low" hazard dams as well. 11 pp., 12 references.