The design of the fiberglass portions of these tanks follows guidance from the American Society of Mechanical Engineers Standard. ASME-RTP-1-95 (REINFORCED THERMOSET PLASTIC CORROSION RESISTANT EQUIPMENT), and ASTM-4097-95a (STANDARD SPECIFICATION FOR CONTACT-MOLDED GLASS-FIBER-REINFORCED THERMOSET RESIN CORROSION RESISTANT TANKS), as noted through out this document. These standards were written for the design of large fiberglass tanks used for the storage of corrosive chemicals and are considered very conservative by the fiberglass industry.
For all static, sustained load cases a margin of safety of 10 will be adhered to above the laminates ultimate tensile strength as prescribed in ASTM-RTP-1, section 3A-210. In general, this gives an allowable stress for sustained load cases of 1500 psi. For transient, or live load situations a margin of 5 will be maintained above the laminates ultimate tensile strength as prescribed in ASTM-RTP-1, section 3A-440. In general this gives an allowable stress of 3000 psi for these cases. See Appendix A for select information from these standards.
The deflection criteria which is used for the design of these tanks while under the hydrostatic load are as follows:
A) For the design of the FRP panels and top flange a deflection limit of L/300 is adhered to.
B) For the design of the perimeter of the window opening along each span a deflection limit of L/300 is adhered to.
No deflection requirements are adhered to for the seismic load case.
The criteria for the steel stands are to have a maximum stress allowable of 22,000 psi in any member, for any load case.
MATERIAL PROPERTIES AND LAMINATES
The structural laminates considered in this analysis are of general-purpose ortho-polyester resin with E-type fiberglass reinforcement. The laminates utilized in the construction of these tanks will be of the Type II Laminates as given in ASTM-RTP-1, table 2A-1, 3 (See appendix A). The thickness for different regions of the tanks are as described in shop drawings for each individual tank, and are noted throughout this report.
The laminate properties listed here along with the minimum thickness as described herein are considered design minimums, which must be maintained during the manufacturing of the tanks. Thicknesses are considered minimum structural thicknesses. This thickness’ do not include any corrosion barrier or suffice finish enhancement layers. These thicknesses’ can be increased at the discretion of the manufacturer for reasons such as surface finish etc.
PROPERTY -------------------------------Type II Laminate = Alternate Layers 24 oz. WR, 1.5 oz. CSM
Density (lbs/cu.in.)----------------------.05 - .07
% fiber volume--------------------------30 or greater
Tensile modulus (psi)--------------------1.3 e6
Shear modulus (psi)---------------------0.40 e6
Ult. Tensile strength (psi)----------------15000
Ult. Compressive strength (psi)----------15000
Ult. Flexural strength (psi)---------------19000
Ult. Shear In-Plane (psi)-----------------7800
Interlaminar Shear (psi)------------------1500
The structural core to be used in the sandwich panel regions of the fiberglass tanks will be of end grain balsa wood typically used in marine construction.
The density and mechanical properties of the cores used in this analysis are provided below.
PROPERTY Balsa Properties
Density (lbs/cu.ft.) 7.5 min.
Shear strength (psi) 432
Shear modulus (psi) 23,100
Compressive modulus (psi) 590,000
SOURCES:
1. American Society of Mechanical Engineers, ASME-RTP-1-95 standard.
2. ASTM 4097-95a standard.
3. American Society of Civil Engineers, Structural Plastics Design manual, 1984.
4. Divinycell Corp., Grapevine, TX.
5. American Bureau of Shipping, Guide for Classing and Building High Speed Craft 1990.
6. Baltek Corp., North Vale, NJ