Case Study for WWSP Welded Steel Pipe Seismic Design Approach

The $1.6 billion Willamette Water Supply Program (WWSP) is located in the seismically active Pacific Northwest, subject to a magnitude 9.0 Cascadia Subduction Zone earthquake. The new water transmission supply system includes over 30 mi (48 km) of 66 in. and 48 in. (1.6 m and 1.2 m) welded steel pipe (WSP). Due to limited information available from the water industry, the WWSP developed its own seismic guidelines for WSP based on best practices, including the use of a strain-based seismic design approach. This approach considered major limit states that were selected for WSP related to tensile performance, compressive performance, and joint strength; minor limit states related to the serviceability of cement mortar lining with double lap welded joints; and different approaches for addressing seismic performance for both transient ground shaking and permanent ground deformation (PGD). The use of joint efficiency values for double lap welded joints and butt-welded joints is described. For transient ground shaking, severe combined axial loading from thrust near bends, Poisson's effect due to internal high pressures, and ground strain due to ground shaking are described. For PGD, the development of finite element models, including the associated ground springs, is described in addition to lessons learned from the application of the finite element analysis (FEA) models. This work serves as a case study for the seismic design of WSP to achieve a seismic level of service and operational goals. Conclusions and recommendations related to the work are provided for use by owners and design practitioners for critical water conveyance pipeline infrastructure.