Recreating the Past: Inspection and Analysis of Spillway Radial Gates

Over the past two years, HDR has worked with dam owners to investigate the condition of 44 spillway gates at seven dams. One of our first discoveries was that, while original design and fabrication drawings often exist, the original design criteria and design calculations do not. Without the original design calculations, determining the current capacity of the gate members becomes a two step process of inspection and structural analysis. In the course of the inspection and analysis we are recreating the original design basis for each gate. We are also determining the structure's ability to accommodate loads that were not included in the original design.

Trunnion friction was typically not considered in the design of the gate end frames. The significance of increased trunnion friction over time was dramatically demonstrated in gate failure at Folsom Dam in 1995. Some gates may also be subjected to significant earthquake forces that were not included in the original design. In addition to these new loads, the analysis and design philosophies used for some older gates inadequately predict the distribution of forces to secondary members. The original design was typically hand calculations considering only two-dimensional effects. However, the gate layout and connection detailing may induce significant three-dimensional distribution of forces.

HDR has found that the structural analysis can best performed by developing either a two-dimensional or three-dimensional computer model.

HDR engineers, who are trained in climbing techniques, perform hands-on inspections of the radial gates. HDR has developed safe, dam-specific inspection rigging that enables engineers to examine all sides of radial gate members. The type and condition of the end connection of each member is also examined to determine the appropriate modeling condition to use during the analysis.

The paper will discuss which types of computer modeling and structural analysis methods are appropriate for various gate designs and layouts. Short case histories for some of these dams will be presented. The paper will also discuss the HDR's rigging, climbing techniques and types of data collected during a full gate inspection. 11 pp., 13 figures.