Lap and bundle detailing

What this page computes

The detailing rules for lapped splices in EN 1992-1-1 §8.7.2, §8.7.4, §8.7.5, together with large diameter bars §8.8 and bundled bars §8.9.

Not lengths — those come from the anchorage page. Here: where the bars sit, how far apart, and what holds the lap together.

This is the part of Section 8 that gets missed most often, for a simple reason: it does not produce a number you compare against a capacity. It produces conditions that are either met or not. And when they are not, the consequence is not "less safety" in the abstract — it is spalling of the concrete over the lap zone, a failure without warning.

Too large a distance is not rejected, it is priced

§8.7.2(3) limits the clear distance between the two lapped bars. But the code does not say "not permitted". It says something more interesting:

if this condition is not satisfied, the lap length should be increased by a length equal to the clear space between the bars

So it is a price, not a barrier. A bar placed 80 mm further away costs 80 mm of extra lap. The page computes the increase and shows the corrected l₀.

A genuine ambiguity in the text

The code writes the limit as "4φ or 50 mm". Figure 8.7 shows both with a "≤" sign. If both must hold, then the smaller governs — and that is the interpretation used here, being the conservative one. The note appears on every run, with both values shown, so you can decide otherwise if you have a reason.

The 100% is conditional

§8.7.2(4) permits 100% of the tension bars to be lapped in one section — but only if the bars are in one layer and the arrangement complies with (3). In several layers, the percentage drops to 50%.

The logical order is often missed: not "100% if one layer", but "100% if (3) is satisfied and one layer". The page checks (3) first, then decides the percentage.

In compression the whole discussion disappears: all compression bars and all secondary reinforcement may be lapped in one section.

Transverse reinforcement: two alternative exemptions

§8.7.4.1(2) exempts you from dedicated transverse reinforcement if φ < 20 mm or if the lapped percentage is below 25%. Alternative, not cumulative — either one suffices.

Note how the exemption is worded. The code does not say "no transverse reinforcement is needed". It says that what is already there for other reasons is deemed sufficient. In a beam with no links in that zone, the exemption gives you nothing.

Where you are not exempt, §8.7.4.1(3) requires ΣA_st ≥ 1.0·A_s, with A_s the area of a single lapped bar. And §8.7.4.1(4) says where it goes: half in each end third of the lap, not spread uniformly. The middle of the lap needs nothing — splitting starts at the ends.

When straight bars are no longer enough

The second paragraph of (3) is easy to read past: if more than 50% of the bars are lapped in one section and the distance a between adjacent laps is ≤ 10φ, the transverse reinforcement must be links or stirrups anchored into the section — not straight transverse bars.

The difference is real: a straight bar can move with the block of concrete that breaks away; a closed link cannot.

§8.7.5 — for fabric, the choice of plane changes three things

Lapping panels in the same plane or in different planes is not a matter of workmanship. It changes:

• α₃ — in the same plane the favourable effect of the transverse bars is ignored, so α₃ = 1.0;
• the permitted percentage — in the same plane Table 8.3 applies (that is, α₆); in different planes, 100% below 1200 mm²/m and 60% above;
• the crack width check — in different planes, if the lap is not in a zone with σ ≤ 0.8·f_yd, the stress used in Tables 7.2N/7.3N is increased by 25%.

That last point is the easiest to miss, because it applies in an entirely different check from the one where the decision was made.

And a note that simplifies things: §8.7.5.1(7) — no transverse reinforcement is required in the lap zone of welded fabric.

§8.8 — above φ_large, a different rule set

The rules of §8.8 replace those of §8.4 and §8.7; they are not added to them. The threshold φ_large is a nationally determined parameter, with a recommended value of 32 mm — check the Annex before relying on it.

What changes: additional transverse reinforcement appears in the anchorage zone, A_sh = 0.25·A_s·n₁ parallel to the tension face and A_sv = 0.25·A_s·n₂ perpendicular to it, at a spacing of at most 5φ.

And lapping becomes the exception: §8.8(4) discourages it, with two ways out — sections at least 1.0 m across, or stress below 80% of the design strength.

§8.9 — the bundle is a fictitious bar

φ_n = φ·√n_b ≤ 55 mm. Same area, same centre of gravity — but the diameter grows with the square root of the number of bars, and this φ_n then governs spacing, cover and anchorage length.

Three things that get missed:

• the maximum n_b depends on the situation: 4 for vertical compression bars and for bars inside a lap, 3 otherwise;
• bundles of more than three bars are not lapped at all (§8.9.1(3), last sentence) — not even where four bars are permitted in the bundle. These are two different rules, easily confused;
• staggering buys you the diameter: §8.9.2(2) — if the cut-offs are staggered by more than 1.3·l_b, l_bd may be evaluated with the individual bar diameter instead of φ_n. On a bundle of three bars, that is a difference of over 40% in anchorage length.

A correction made to the anchorage page

§8.7.3 contains a note that is easy to skip: for the calculation of α₃ in a lap, ΣA_st,min = 1.0·A_s·(σ_sd/f_yd) is taken, not 0.25·A_s as for anchorage.

The consequence: the same transverse reinforcement reduces l₀ less than it reduces l_bd — sometimes not at all. The anchorage page now computes the two α₃ values separately and shows both.

What it does not cover

• §8.7.1 — welding and mechanical coupling devices.
• §8.6 — anchorage by welded transverse bars.
• §9.2.3 and the rest of the detailing rules of Section 9.
• §8.10 — prestressing tendons and everything attached to them.

The texts of §8.7.2–§8.9 are transcribed from EN 1992-1-1:2004, pages 129–134.

Related calculations

• Anchorage and laps — the l_bd and l₀ lengths this page assumes are already computed.
• Cover — c_nom, which feeds into c_d and therefore into α₂.
• Reinforcement ratios — minimum reinforcement, the other family of detailing rules.