Rebar Lap and Anchorage Length Calculator

What are anchorage and lap lengths?

The anchorage length is the straight length a reinforcing bar must be embedded in concrete to transfer its force. The lap length is the length over which two bars overlap side by side to pass force from one to the other. Both depend on the tensile strength of the concrete, the strength of the steel and the bar diameter, which is why they are often quoted as multiples of the diameter, such as 40 ø.

The TS 500 expression

For a ribbed bar in tension with a straight anchorage:

l_b = (0.12 × f_yd / f_ctd) × ø, at least 20 ø

Here f_yd = f_yk / 1.15 is the design strength of the steel and f_ctd = 0.35 × √f_ck / 1.5 is the design tensile strength of the concrete. For bars in an unfavourable bond position (Position II) the length is multiplied by 1.4, and above 32 mm by 100 / (132 − ø). A tension lap grows with the share of bars lapped at one section: l₀ = (1 + 0.5 r) × l_b.

The Eurocode 2 expression

First the bond strength: f_bd = 2.25 × η₁ × η₂ × f_ctd. The basic required length is l_b,rqd = (ø / 4) × (σ_sd / f_bd). The design anchorage length is that value times the α coefficients, with a lower limit. A lap also uses α₆ (1.0 to 1.5), which depends on the share of bars lapped in the same zone.

Example

C25/30 concrete, 500 MPa steel, 16 mm bar, good bond: f_ctd = 1.197 MPa, f_bd = 2.693 MPa, f_yd = 434.8 MPa. The basic length is 16 / 4 × 434.8 / 2.693 = 646 mm, about 40 bar diameters. With all bars lapped in one zone, α₆ = 1.5 and the lap length is 969 mm.

Limits

This tool applies only the basic expressions above. Seismic detailing rules for laps in columns and beams, hooked and mechanical anchorages, bundled bars, welded fabric and National Annexes are outside its scope. The result is preliminary. The lengths on the drawings and the code in force always take precedence.