Source: https://guthlabs.ai/docs/instruments/rebar-development-length-calculator/mechanism/

instrument mechanism / verified-clean

# Rebar Development Length Calculator mechanism

The inspectable method record for Rebar Development Length Calculator, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

Verdict is NEEDS_REVIEW because the specification has both a materially incorrect coefficient/formula and unresolved unit, ACI-edition, confinement-definition, and splice-class ambiguities. The test vectors use the corrected ACI-style SI equation and explicitly stated ACI 318-19 conventions; they are verification vectors for the required correction, not validation of the formula as currently written.

- The stated coefficient 1.1 is not the standard ACI 318 SI coefficient. The standard SI form is ld = (3/40)·(fy·ψt·ψe·ψs)/(λ·√f'c)·db/((cb+Ktr)/db), with 3/40 = 0.075 when fy and f'c are in MPa and lengths are in mm.

- The factor expression db/(c+Ktr)/db is ambiguous and dimensionally incorrect as written. It should be db/((cb+Ktr)/db), where cb is the smaller of concrete cover or one-half the bar clear spacing, and Ktr is the transverse-reinforcement index.

- The specification does not identify an ACI edition, so applicable modification-factor limits, minimum lengths, and lap-splice provisions are not uniquely defined.

- Lap splice length cannot be determined from ld alone without specifying tension or compression splice and splice class. The vectors below explicitly assume tension lap splices under ACI 318 Class A or Class B provisions.

- The calculator must enforce ACI constraints, including the permitted range and cap for (cb+Ktr)/db, permitted ψ-factor values and applicability conditions, minimum development length, and the relevant bar-size and reinforcement detailing requirements.

## Units and conventions

- The corrected calculations use ACI-style SI units: fy and f'c in MPa, bar diameters and lengths in mm, and λ dimensionless.

- The concrete density factor λ is 1.0 for normalweight concrete and 0.85 in the applicable lightweight-concrete test vector.

- Minimum straight tension development length is taken as 300 mm under the cited ACI 318 convention.

- Class A tension lap splice length is taken as max(1.0·ld, 300 mm); Class B is taken as max(1.3·ld, 300 mm).

- The input c_mm is treated as cb, the effective confinement dimension used by ACI, not automatically as nominal concrete cover.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "bar_size": "#4",
    "db_mm": 12.7,
    "fy_MPa": 420,
    "fc_MPa": 28,
    "lambda": 1,
    "psi_t": 1,
    "psi_e": 1,
    "psi_s": 1,
    "c_b_mm": 31.75,
    "Ktr_mm": 0,
    "splice_type": "tension",
    "splice_class": "B"
  },
  "expected_outputs": {
    "ld_raw_mm": 30.24,
    "ld_required_mm": 300,
    "lap_splice_length_mm": 390
  },
  "reference_data": {
    "bar_diameter_mm": {
      "#4": 12.7
    },
    "bar_diameter_source_convention": "ACI 318-19 reinforcing-bar designation table; ASTM A615 standard bar-size value",
    "fy_MPa": 420,
    "fy_source_convention": "ASTM A615 Grade 60 minimum specified yield strength, widely tabulated standard value",
    "fc_MPa": 28,
    "fc_source_convention": "ACI 318-19 normalweight-concrete design-strength convention",
    "minimum_tension_development_length_mm": 300,
    "minimum_length_source_convention": "ACI 318-19 Ch.25, SI equivalent of 12 in minimum",
    "class_B_lap_factor": 1.3,
    "class_B_lap_source_convention": "ACI 318-19 tension lap-splice convention"
  },
  "working": "Corrected ACI SI equation: ld_raw = 0.075·(fy·ψt·ψe·ψs)/(λ·√f'c)·db/((cb+Ktr)/db). First, (cb+Ktr)/db = (31.75+0)/12.7 = 2.50. Then √28 = 5.29150. Then 0.075·420/(1.0·5.29150) = 5.95294. Then db/ratio = 12.7/2.50 = 5.08 mm. Therefore ld_raw = 5.95294·5.08 = 30.24 mm. Apply the 300 mm minimum: ld = max(30.24,300) = 300.00 mm. Class B lap splice = max(1.3·300.00,300) = max(390.00,300) = 390.00 mm."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The instrument has an undefined-result failure when c+Ktr=0, accepts a physically unacceptable zero-cover boundary, and can produce materially different results if Ktr sign convention is not enforced. Its branch condition for psi_t also requires an explicit top-bar placement rule.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/rebar-development-length-calculator/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/rebar-development-length-calculator/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/rebar-development-length-calculator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche7/tranche7-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`