Joint efficiency (Table 4.2 / 4.3) calculator
Shell joint efficiency E by construction standard and build-year edition band, welded or riveted.
For estimating and checking. The inspector of record is responsible for every number that goes into a report. Verify against the current edition of API 653 — this page was checked against API 653, 5th Edition, Addendum 2 (May 2020). Not a substitute for the standard or for engineering judgment.
Joint efficiency (E) knocks a construction quality factor into the t-min formula — a fully radiographed butt weld gets full credit, an old lap-welded or riveted joint gets much less. API 653 assigns E by the standard the tank was built to and, for most standards, the year it was built. Pick a standard and a build year below to see the row that applies, alongside the full table.
- Standard: API 650 — Seventh and later (1980–present)
| Standard | Edition | Joint | Applicability | E |
|---|---|---|---|---|
| API 650 * | Seventh and later (1980–present) | Butt | Basic Standard | 1.00 |
| API 650 * | Seventh and later (1980–present) | Butt | Annex A, spot RT | 0.85 |
| API 650 * | Seventh and later (1980–present) | Butt | Annex A, no RT | 0.70 |
| API 650 | Third to Sixth (1966–1978) | Butt | Basic Standard | 0.85 |
| API 650 | Third to Sixth (1966–1978) | Butt | Annexes D or G | 1.00 |
| API 650 | First and Second (1961–1964) | Butt | Basic Standard | 0.85 |
| API 650 | First and Second (1961–1964) | Butt | Annex D | 1.00 |
| API 12C | 14th and 15th (1957–1958) | Butt | — | 0.85 |
| API 12C | Third to 13th (1940–1956) | Lap — full double | 3/8 in. (10 mm) max t | 0.75 |
| API 12C | Third to 13th (1940–1956) | Butt | — | 0.85 |
| API 12C | First and Second (1936–1939) | Lap — full double | 7/16 in. (11 mm) max t | 0.70 |
| API 12C | First and Second (1936–1939) | Lap — intermittent | 1/4 in. (6 mm) max t | 0.50 |
| API 12C | First and Second (1936–1939) | Butt | — | 0.85 |
| Unknown standard | Welded, code/edition unknown | Lap — full double | 7/16 in. (11 mm) max t | 0.70 |
| Unknown standard | Welded, code/edition unknown | Lap — intermittent | 1/4 in. (6 mm) max t | 0.50 |
| Unknown standard | Welded, code/edition unknown | Butt | — | 0.70 |
| Unknown standard | Welded, code/edition unknown | Lap — single welded | — | 0.35 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Lap | 1 rivet row | 0.45 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Lap | 2 rivet rows | 0.60 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Lap | 3 rivet rows | 0.70 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Lap | 4 rivet rows | 0.75 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Butt | 2 rows/side | 0.75 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Butt | 3 rows/side | 0.85 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Butt | 4 rows/side | 0.90 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Butt | 5 rows/side | 0.91 |
| Riveted | Table 4.3 — within 6 in. (150 mm) of rivets | Butt | 6 rows/side | 0.92 |
The formula
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
E | Joint efficiency used in the one-foot method / t-min formula | Table 4.2 |
Worked example
Using the pre-filled example above (US customary units):
- Standard: API 650 — Seventh and later (1980–present)
- Butt — Basic Standard: 1.00
- Butt — Annex A, spot RT: 0.85
- Butt — Annex A, no RT: 0.70
Where the inputs come from
- Standard and build year come from the tank’s construction record, nameplate, or original design drawings — not a guess based on the tank’s apparent age.
- When the construction record genuinely does not identify a standard or edition, the unknown-standard row is the conservative default API 653 assigns.
- A riveted tank uses Table 4.3 instead of Table 4.2 — pick "Riveted" to see its joint efficiencies and its own flat allowable stress.
Built by TankForge — API 653 inspection software
TankForge keeps the shell layout’s joint efficiency wired to the Construction tab’s build year and standard, so a change to either one updates the other automatically instead of drifting out of sync.
Related
Frequently asked
Why does the same standard give a different E for a different build year?
API 653 groups editions of API 650 and API 12C into bands, because welding and inspection requirements changed as those standards were revised — an older edition band generally gets a lower joint efficiency for the same basic construction.
What is Annex A / spot RT vs no RT?
Annex A construction under the current API 650 editions can be built with or without spot radiographic testing (RT) of the welds. Spot RT earns a higher joint efficiency than the no-RT option in the same edition band.
My tank has no build year on record — what should I pick?
Leave the year blank to use the most recent (and, for a welded standard, most favorable) edition band, or switch the standard to "Unknown / not documented" if the construction standard itself is not known either — that is the most conservative starting point.
Does joint efficiency change over the life of the tank?
No — E is fixed by how the tank was originally built and does not change with age, corrosion, or repairs. A repair performed under a different code does not retroactively change the original shell’s E.
Is this the same E used for the shell t-min calculation?
Yes — this is the same joint efficiency the one-foot-method t-min formula uses. Enter it directly on the shell t-min calculator once you know it, or default to the Table 4.2 unknown-standard value if the construction record does not identify one.
Where to find labeled drawings
- API 650 Annex H (internal floating roofs) and Annex C (external floating roofs): figures of the roof and its fittings.
- API 653 Annex C checklists: the inspection names for most of these parts (C.1.4 roofs, C.1.5 roof accessories, C.2.8–C.2.9 floating roofs and seals).
- EPA AP-42, Chapter 7.1 (Organic Liquid Storage Tanks): cutaway diagrams of guide poles, deck fittings, rim seals and vents, each part named. EPA AP-42 Chapter 7
- Vendor catalogs for close-up photos: Enardo, Protectoseal, Groth, Varec, Shand & Jurs (vents, hatches, gauges); HMT, Matrix (floating roofs, seals).
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tankforge.us/guide/calc/joint-efficiency — TankForge