Designing & Bracing Rectangular FRP Tanks

A. Hi, Raja. Sorry, I don't know what STP stands for in this case. Also, you haven't mentioned if this is a vertical round tank or horizontal round tank, or rectangular open-top tank. Bracing of rectangular plastic tanks is not a dirt-easy subject, but fortunately your tank doesn't sound too big. Please tell us the shape and dimensions of the tank, and the temperature and specific gravity of the STP and then someone can hopefully give you a starting point.

Regards,

A. Hello again Raja. Although several strength of materials books address the issue of design of plates with "triangular" or "trapezoidal" loading like you get on an open top tank (because the pressure increases proportional to solution depth), the only book I've ever come across that addresses the question of design of rectangular tanks in an integrated manner is a terrific old book "Design of Weldments" by the Lincoln Arc Welding Foundation. I used it throughout my career, including as the basis for computer programs I sold to two manufacturers of plastic tanks for calculating required bracing. It's well worth hunting for!

For a tank this size, which is very "ordinary" for jobshop metal finishing, I doubt that you would want toe braces, but rather a rim and a couple of belly bands. You can roughly estimate tank loads, making approximations per that Blodgett book, or you can get it significantly closer with the method of two moments applied at each belly band, or you can do a finite element analysis although that didn't exist in my school days and I can't instruct about it. Some vendors have prepared "design books" or computer programs for modest sized tanks like this so each tank needn't be recalculated individually.

Remember that we're speaking of a fiberglass tank here, Freeman. Although its tensile strength is perhaps half the strength of steel, that's not the whole story. With steel tanks you can go with a small factor of safety because the plate is made (uniformly) by a mill, and we have more than a century of incredibly broad experience proving steel to be as strong and reliable as we think. But for fiberglass, laid up by craftspeople unknown to us, of unknown skill, who may wet the cloth not enough or too much, or may let air bubbles in, or may think it okay to have joints in the roving, you need a much larger factor of safety. I'd probably expect three belly bands; maybe only two if I knew and trusted the company and its craftspeople.

Raja, if you are an engineer, try the book I referenced. If not, remember that tanks must be designed by engineers. Good luck.

Regards,

A. Hi Yuli. For plating shop applications, corrosion resistance is important, and vinyl ester is probably the most universal resin for this purpose. Good luck.

Luck & Regards,