Design of a 120 in.-Diameter Steel Bifurcation with a Small Acute Angle for a High-Pressure Penstock

The design of the shell and reinforcing members of a 120 in. x 120 in. bifurcation (wye branch) for the Olivenhain-Lake Hodges Pumped Storage Project in San Diego County, CA, was a major challenge because of the small acute angle of 34.6° and the high design pressure of 472 psi. The project design criteria for the shell of the wye and reinforcing elements limit the general membrane stresses to the lesser of 2/3 of yield strength or 1/3 of tensile strength of the steel and the surface stresses at points of geometric discontinuities to 3 times the allowable membrane stress. Although AWWA Manual M11 design procedure does not include calculation of discontinuity stresses, preliminary design of the wye and reinforcing elements in conformance with AWWA procedures does not lead to designs that are feasible to fabricate and transport due to the required thickness and depth of the crotch plates. Finite element analyses (FEA) were performed to verify a modified-preliminary AWWA baseline crotch plate reinforcement design and to predict the membrane and discontinuity stresses. Since the project allowable stresses of the baseline design were exceeded, additional FEA analyses were conducted using different crotch plate thicknesses and depths, different shell thicknesses, and additional localized reinforcement schemes. Crotch plates with T cross-sections could not be used because of the small acute angle. The design assumptions and the FEA results are the focus of this paper. The advantages of the nonlinear analysis due to the high discontinuity stresses will be discussed and the supporting data presented. The use of a steel reinforcing pin at the intersection of the crotch plates to reduce the maximum discontinuity effects is also discussed.