About

Reinforced concrete relies on the bond between steel and concrete to transfer forces. When a reinforcing bar runs out of length, it must be overlapped (spliced) with another bar to maintain structural continuity. Similarly, at corners or ends, bars must be anchored deeply enough to prevent them from pulling out under tension.

This calculator determines the required Lap Length and Anchorage Length based on the concrete grade, bar diameter, and bond conditions (Good vs. Poor). Inadequate laps are a leading cause of structural failure in seismic events.

Formulas

The basic anchorage length is calculated by equating the bond force to the yield force of the bar.

l_b,rqd = φ × f_yd4 × f_bd

For design lap length, coefficients (α) are applied for cover, confinement, and splice percentage:

l₀ = l_b,rqd × α₁ × α₆

Reference Data

Concrete Grade	Bond Strength (fbd)	Typ. Lap (diameters)
C20/25	2.3 MPa	50d - 60d
C30/37	3.0 MPa	40d - 50d
C40/50	3.7 MPa	35d - 45d
Condition: Poor	x 0.7 Bond	Increase lap by 43%
Epoxy Coated	Reduced Friction	Increase lap by 20%
Compression Lap	-	Typically shorter than Tension
Min Lap	200 mm	Absolute minimum code rule.

Frequently Asked Questions

Bond conditions are considered 'Poor' for bars located in the top 300mm of a pour (or top half of slabs < 300mm) because air and water rise during settling, weakening the concrete grip around the top bars.

The tool defaults to Tension laps, which are longer and more critical. Compression laps (like in columns) can often be shorter (approx 0.6 - 0.8 times the tension lap) but require stirrup confinement.

The bar may slip inside the concrete before it reaches its full strength (yield point). This leads to sudden, brittle failure of the structure without warning cracks.

Anchorage is the length needed to secure a bar end into concrete (e.g., beam into column). Lap is the length needed to transfer stress from one bar to another (e.g., extending a column upwards).