Stainless steel structure: FEA verifies competition swimming pool design.

The design of the new Olympic-sized short-course pool at the Meli Park Community Center in Dania Beach, FL, is far from typical. Rather than being made of reinforced concrete, as many older swimming pools are, this pool is made from modular, stainless steel panels covered with an industrial-strength, 60-mil PVC liner. The pool was designed by Bradford Products, LLC in Wilmington, NC, and analyzed with FEA software from ALGOR, Inc. to verify that the stainless steel walls could withstand loads produced during the installation process.

Since 1982, Bradford has specialized in designing, developing and manufacturing stainless steel spas and pools for homes and commercial facilities. "Although most of our products, especially spas, can be verified with simple hand calculations, we use FEA on larger projects, unusual designs or if local engineering codes are involved," explained John Marciano, engineering design department manager. With an overall budget of about $1.2 million, the Dania Beach project fell into the category of a large project.

Construction for the in-ground pool started with excavating a hole larger than the pool's area. Stainless steel walls were then erected with K-shaped braces providing support on the outer surface. After the liner and filtration equipment were installed, Marciano recommended that the excavated area outside the steel walls be backfilled with soil as water was concurrently added to the pool to offset the hydrostatic pressure.

With the simultaneous weight of soil and water being applied, Marciano was confident that the walls would experience no significant stress of displacement, but he decided to take his engineering certainty a step further. "Since it was a million dollar project and anything could happen during construction, I wanted to verify that the walls could withstand either the hydrostatic pressure of being filled with water without backfill or the pressure of being back-filled while empty," said Marciano.

Although Marciano often uses AutoCAD for architectural layouts and CADKEY for solid modeling, in this case, he chose ALGOR's finite element model drawing tool, Superdraw, for building the pool's model. "For engineers like us that only need to use analysis software occasionally, ALGOR is a complete and very affordable solution with outstanding technical support," said Marciano.

The dimensions met the desired size standards of 25 x 25m and 2m deep. The beam and plate model was created with Superdraw's structured mesh generation tools. "Because the design is made entirely of sheet metal, there area lot of thin sections and beam-like supports which are best represented in the FEA model by a combination of beam and plate elements," said Marciano.

He applied material properties for stainless steel from the software's built-in material library. Constraints were then added along the bottom of the walls and the braces. In one design scenario, a hydrostatic pressure load was applied to the inner surface of the walls to represent the pool completely filled with water. Another design scenario considered a calculated pressure load to represent backfill on the outside of the walls. Since Marciano was not sure what type of soil was present at the construction site, he assumed a worst-case load of 100 lbs/[ft.sup.2], which was based on the weight of granite, and 30-degree sheering such as one would find with sand.

A linear static stress analysis was performed and then Marciano evaluated the results. "I primarily looked at the displacement results and was pleased to see only about 1 in. of displacement in the center of each panel," he said. "On a pool of this size, displacement of that magnitude is negligible. In addition, seeing displacement in the middle of the panels is far less significant than if the results would have revealed deformation at the top edge or at the seams of panels. The FEA design verification gave me an added level of confidence that my design could withstand any variation in the installation procedure."

The pool was completed last autumn and will be available for competitive use once the community center is completed later this year. With Bradford Products' pool business growing, Marciano already has plans to use software again. His next project is a swimspa that generates a current that allows a person to swim in place. The side walls of the prototype spa have experienced flexing in the laboratory. Marciano plans to compare FEA results to the behavior of the prototype and then use the software to optimize the design.--SG

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