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

instrument implementation / implementation-contract

# Rebar Development Length Calculator implementation

A source-bound implementation contract for Rebar Development Length Calculator; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

**Record owner**: Guth Instruments

**Lifecycle**: verified-clean

**Evidence ID**: t7-093

**Effect**: Computation or source lookup only

## Input and validation requirements

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.

## Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

## Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

````
{
  "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."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/rebar-development-length-calculator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/rebar-development-length-calculator/mechanism/>)[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`