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# POV of Tower Modeled as Single Shell Course vs. Tower Modeled with Multiple Shell Courses

## POV of Tower Modeled as Single Shell Course vs. Tower Modeled with Multiple Shell Courses

(OP)
I am checking a Compress calculation report and the designer modeled a 50 ft seam/seam tower with just a single shell course instead of multiple shell course, e.g. 5 shell cans that are 10 ft long.

Will the period of vibration be the same?

### RE: POV of Tower Modeled as Single Shell Course vs. Tower Modeled with Multiple Shell Courses

Modeling a "run" of cylindrical shells as a single cylinder component may be acceptable for a conceptual model. But it imposes possible problems and limitations for calculations intended for fabrication.

For example, radiography of the additional girth seams is unknown, undocumented, and untreated in the COMPRESS reports. Using only a single cylinder prevents varying the nominal thickness of the shell to account for variations in hydrostatic head, required thickness due to wind or seismic, etc. Other problems may result as well.

However, treating the section of cylindrical shell as a single component does not materially affect the calculated period of vibration of the vessel. COMPRESS calculates the natural frequency using a Rayleigh-Ritz method, with each shell component broken into a number of "slices" to consider the lumped mass. The number of slices results with the POV of the two models (single cylinder versus multiple) being pretty much the same.

Tom Barsh
Codeware

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