Nonbuilding and Special Structures
Nonbuilding structures essential to public safety and everyday community activities should be properly designed to withstand seismic forces. Nonbuilding structures, such as water tanks, typically do not have the same type of life safety concerns as buildings because they are often not occupied and rather serve other important functions. For water tanks classified as essential facilities, the expectation is that their seismic performance characteristics will allow for continued operation during and after a design-level seismic event to carry out disaster relief operations and for the normalization of the community they serve. In recent years, there has been more attention paid to the seismic performance of water tanks and what design requirements are needed to ensure continued operation.
The dynamic behavior of steel water tanks is complex. Seismic behavior include effects from hydrostatic forces, hydrodynamic forces (including convective and impulsive forces), steel shell behavior, anchorage behavior and foundation behavior. Observed damage to steel water tanks in earthquakes include elephant’s foot buckling (buckling of thin-walled shell liquid storage tank), yielding/fracture of anchors, tank piping failure, foundation rocking, and tank wall and roof damage due to sloshing.
Three real-world case studies will be presented describing the details of the seismic analysis and upgrade of three existing ground supported steel water tanks. The existing steel water tanks are located in the greater Seattle, Washington area, and are of different ages and have different aspect ratios. One of the tanks has had its seismic upgrade construction completed and the other two tanks are in various stages of evaluation, design and construction.
The case studies will explore the complexities of water tank seismic analysis, the advantages and disadvantages of conducting finite-element and modal analyses, tank soil structure interaction, the benefits of site-specific seismic hazard analysis, and seismic upgrade solutions given tight municipal financial budgets. The case studies will also explore the procedures and requirements of the building codes governing the analysis and design of steel water tanks and existing structures including ASCE 7 Minimum Design Loads for Buildings and Other Structures, ASCE 41 Seismic Evaluation and Retrofit of Existing Buildings, and AWWA D100 Welded Carbon Steel Water Tanks for Water Storage.