FRP is not a uniform, homogeneous, isotropic material like steel. The corrosion resistance is provided by the epoxy or polyester resin, while the structural strength is provided by strands or filaments of fiberglass. Wall thickness consists of a resin layer with little strength, and layers of fiberglass bonded with same or different resin to form the structural shell. The proportion of resin versus fiberglass in the wall thickness can be made to fixed ratios, or to fixed dimensions depending on the manufacturing process. Usually the corrosion barrier is a fixed dimension typically selected as 20 mils, 50 mils, 100 mils, etc. The wall's structural fiberglass thickness can be built up to strength requirements. OVERALL ALLOWABLE STRESS (ALSO WEIGHT, MODULUS) BECOMES A PROPERTY OF THE COMPOSITE OF RESIN + FIBERGLASS REINFORCEMENT.
Different manufacturing processes also cause the properties to vary. Pipe could be filament wound with different angles of fibers, or "hand lay up" of matt instead of filaments. Small fittings could be molded with mortar filler or short fiber resin compounds. Large fittings would only be made by "hand lay up" method or "chopped strand spray-up". There are some other manufacturing methods like centrifugally cast pipe, and filament wound fittings. These all have some variation due operator technique as well as size limitations of the tooling. The I.D. or O.D. control of dimensions also has consequences for design. There are also the joints to consider. Smaller diameters might be socket adhesive bonded, while the larger sizes of joints would be butt wrapped resin + fiberglass.
Vendor design and installation directions should be followed. Be cautious of mixing pipe or fittings of different suppliers. Conservative allowable stress must be used. The stress analysis is used more for design of the supports, rather than prediction of safety margin of stresses. MORE OFTEN THAN NOT, IT IS THE VACUUM COLLAPSE RESISTANCE THAT HAS DETERMINED WALL THICKNESS OF FRP PIPE SYSTEMS. Regards, Richard Yee