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We send the calculation,
not just the thickness
Anyone can quote a wall thickness. Fewer can show you the finite element model behind it, the stress classification, the allowable stress it was checked against, and the anchor bolt design that holds it down. That calculation package is part of what we deliver.
ASME Sec VIII Div 2
AISC 15th Ed
MIDAS/GEN 3D FEM
von Mises
Sealed by licensed structural engineer
Method
How a tank gets designed here
A GRP tank is not sized by rule of thumb. It is modelled, loaded, analysed and checked against a stated allowable stress. Below is the framework we work to.
CODE 01
ASME Sec VIII Div 2
Governs stress classification and allowable stress for the GRP plate. Stresses are separated into primary general membrane and primary local membrane, each with its own limit.
CODE 02
AISC 15th Ed, LSD16
Governs the anchor bolt design. Tension check, bolt capacity and development length are carried out against the reaction data from the model.
SOFTWARE
MIDAS/GEN 2023
3D finite element model of shell, base, roof and rib plates. Results are evaluated on the von Mises yielding criterion, with graphical stress output included in the report.
VERIFICATION
Licensed Engineer
The calculation sheet is reviewed and sealed by a licensed structural engineer before issue. It is a submittal document, not an internal note.
3D FEM Model · Shell / Base / Roof / Rib · 8 Anchor Restraints
Every plate is meshed, not assumed
The model includes the shell, the base, the roof and the rib plates as separate elements, each with its own thickness. Anchor points are modelled as restraints, so the reactions that come out of the analysis are the same numbers used to size the bolts.
- Shell hoop and axial stress from static head and self weight
- Base bending between anchor points, usually the governing plate
- Roof live load and any equipment mounted on it
- Ribs stiffeners modelled where they actually sit, not smeared into the shell
Meshing the plates separately is what makes the base plate result meaningful. A single shell model would miss it.
Allowable Stress
The numbers we check against
Publishing the allowable stress basis is unusual, and deliberate. If you are reviewing our submittal, these are the values your engineer will want to see first.
| Parameter | Value |
|---|---|
| GRP tensile strength (Fu) | 104 MPa |
| Modulus of elasticity | 8,276 MPa |
| Density | 14.7 kN/m³ |
| Primary general membrane, allowable | 34.7 MPa (Sm = Fu / 3) |
| Primary local membrane, allowable | 52.0 MPa (1.5 × k × Sm, k = 1.0) |
| Load combination factor k | 1.0 design / 1.2 with seismic or wind |
| Anchor bolt material | SUS304 — Fu 515 MPa, Fy 205 MPa |
Values shown are from a completed project calculation. Material properties and allowables are re-established for each project against the resin system and laminate actually specified.
Worked Example
A real calculation, start to finish
PROJECT REFERENCE
Nghi Son 2 BOT Thermal Power Plant
Client: Doosan Vina | Scope: Coagulant (alum) storage tank and dosing tank, GRP | Calculation issued: April 2023
Both vessels were modelled, analysed and checked to the framework above. The results below are taken directly from the issued calculation sheets.
| Item | Storage Tank | Dosing Tank |
|---|---|---|
| Dimensions | 2.4 m dia × 4.7 m H | 1.4 m dia × 1.6 m H |
| Capacity | 21.25 m³ | 2.0 m³ |
| Fluid | 8% alum | 8% alum |
| Empty weight | 2,860 kgf | 820 kgf |
| Operating weight | 24,520 kgf | 3,520 kgf |
| Plate thickness | Shell 10T / Base 10T Roof 10T / Rib 9T | Shell 6T / Base 9T Roof 9T / Rib 9T |
| Anchorage | 8 × 5/8" SUS304 | 6 × M16 |
Storage Tank · All Model
Stress bands follow the static head. The lower shell carries the highest membrane stress, exactly as the fluid column predicts.
Peak 29.3 MPa
Storage Tank · Shell
Shell plate in plan. Concentrations appear at the anchor positions, which is why the bolt layout is part of the same calculation.
29.3 / 34.7 = 0.844 O.K
Storage Tank · Base
Base plate. Bending between anchor points governs here, which is the case a shell-only model would miss entirely.
29.3 / 34.7 = 0.844 O.K
| FEM Result | Calculated | Allowable | Ratio | Check |
|---|---|---|---|---|
| Storage tank, shell | 29.3 MPa | 34.7 MPa | 0.844 | O.K |
| Storage tank, base | 29.3 MPa | 34.7 MPa | 0.844 | O.K |
| Dosing tank, shell | 18.1 MPa | 34.7 MPa | 0.522 | O.K |
| Dosing tank, base | 27.6 MPa | 34.7 MPa | 0.795 | O.K |
Why the ratios matter. A utilisation of 0.844 means the laminate was sized to the load, with margin but without waste. A very low ratio would mean the vessel is heavier and more expensive than it needs to be. A ratio above 1.0 means it does not pass. We publish these numbers because they are the fastest way for a reviewing engineer to see whether the design was actually calculated.
Deliverables
What the calculation package contains
Issued as a bound submittal document, in English, ready to go into your design review.
SECTION 1
Design Criteria
Design summary, load summary, applicable codes, material properties, allowable stress derivation, analysis software.
SECTION 2 – 3
Frame Plan and Analysis Model
General arrangement, plate thickness schedule, and the 3D finite element mesh used for the analysis.
SECTION 4 – 5
Design Load and Combinations
Dead load including connection and contingency allowances, live load, fluid load, pressure and temperature load, and the load combinations checked.
SECTION 6
Plate Stress Check
Tabulated calculated stress against allowable for each plate, plus graphical stress distribution output from the model.
SECTION 7
Anchor Bolt Design
Reaction data from the model, tension stress check, bolt capacity check and development length check.
ISSUE
Reviewed and Sealed
Signed and sealed by a licensed structural engineer, with revision number and issue date, suitable for submission to the client.
What We Need
To run the calculation
The list is short. If you have these, we can start. If some are still open, tell us what you have and we will work with assumptions marked as such.
Fluid
- Chemical and concentration
- Specific gravity
- Operating temperature
- Continuous or intermittent service
Vessel
- Capacity or dimensions
- Vertical or horizontal, flat or dished bottom
- Nozzle schedule and locations
- Manway, ladder, platform loads
Site
- Indoor or outdoor
- Seismic zone and wind speed
- Foundation type and anchor detail
- Client specification, if any
Send the tank data sheet
We will come back with the laminate specification, the resin selection and the scope of the calculation package. If your specification requires a sealed structural calculation, say so at enquiry stage and we will confirm it in the quotation.
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