Lap length and anchorage length

A bar bending schedule tells you what bars to buy and what shape to bend them. It does not tell you where one bar hands its force to the next, or how far a bar must run past the point it stops being needed. That is detailing, and it is where a design that is arithmetically correct still falls down.

Everything here is BS 8110-1:1997 section 3.12.8, for high yield deformed bars — type 2 in the code's language, which is what T bars sold in Nigeria are. Older drawings write the same bar Y; the letter changed, the steel did not.

The number is about bond, not about steel

A bar carries force because the concrete grips it. The grip is a stress on the surface of the bar, so the force a bar can develop grows with its circumference times its length, while the force it has to develop grows with its area. Area beats circumference, which is why anchorage is always quoted as a multiple of bar size and never as a fixed number of millimetres.

That is the whole origin of the site rule. Grade 25 is the usual concrete in Nigerian residential work, and at grade 25 the arithmetic lands on exactly forty diameters. Change the concrete and it moves, because bond follows the square root of f_cu.

The bond coefficient β

Compression bond beats tension bond because a bar in compression is also bearing on the concrete at its end and is not pulling a crack open around itself. That is why a column starter needs less anchorage than a beam bottom bar of the same size.

Table 3.27, as multiples of bar size

For high yield deformed type 2 bars, f_y 460:

The compression lap is 1.25 times the compression anchorage, which is why the two right-hand columns do not match. The tension lap column is the basic value, before the multipliers below. The table rounds down slightly from the formula — at grade 30 the arithmetic gives 36.5φ and the code says 36φ.

Notice what grade 25 does. Its tension lap and its compression lap are both 40φ, so on a grade 25 job one number covers both and nobody has to think about which is which. That is the rule of thumb working by coincidence, and the coincidence ends at grade 30, where the compression lap is 36φ and the tension lap of a top bar is 50φ.

The two multipliers nearly everyone misses

A tension lap is the basic length above, multiplied by:

• 1.4 if the bar is at the top of a section as cast and the minimum cover is less than 2φ
• 1.4 if the minimum cover to a corner bar is less than 2φ, or the clear gap between adjacent laps is less than 75 mm or 6φ
• 2.0 where both of those apply — the classic case being a corner bar in the top of a beam over a support

And no tension lap is ever less than 15φ or 300 mm, whichever is greater, however small the bar.

The top-bar rule is about how concrete behaves while it is still wet. Everything below a bar settles and bleeds a little, leaving a weaker skin underneath it, so a bar near the top of a deep pour is gripped less well than the identical bar near the bottom. It is not a safety factor somebody added — it is a measured effect.

Tension laps in millimetres, grade 25

Millimetres, high yield deformed bars. Round up to something a steel fixer can measure — 640 becomes 650, 896 becomes 900 — never down. The 300 mm floor applies to the T8 row before anything else does.

Three worked cases

1. Bottom bars of a beam, lapped near mid-span

2T20 in the bottom of the 225 × 450 beam from the beam example, grade 25, cover 25 mm.

Bottom bars, and it still took the 1.4. The corner rule catches almost every beam bar in Nigerian sizes, because a 225 wide beam with 25 mm cover has no bar that is not a corner bar. Lifting the cover to 40 mm would remove the multiplier and shorten every lap by 320 mm — one of the rare places where more cover buys you steel back.

2. Top bars over a support

Which is why laps do not belong over supports. Move them into the span, where the top steel is barely stressed, and the required lap drops with the stress in the bar.

3. Column starter bars

4T16 starters out of a pad footing into a 225 square column, grade 25, the bars in compression.

That last line is the commonest detail failure on a small building. A pad footing is rarely deep enough to take a straight compression anchorage, so the starter has to turn through 90° and run along the bottom, sitting on the bottom mat. A starter that stops short inside the pad is anchored by hope.

Bends and hooks count as anchorage

When there is not enough straight length, a bend does some of the work. BS 8110 gives the effective anchorage of a bend as a multiple of the internal radius r, capped:

The radius cannot be chosen freely either — BS 8666 sets a minimum former size, and bending tighter than it cracks the bar or crushes the concrete inside the bend:

With r = 3φ, a 90° bend gives 12φ — the cap. So a bend is worth at most about a third of a straight tension anchorage at grade 25, and the rest still has to be straight length. A hook is worth 24φ, which is most of it, and is why hooks survive in mild steel links and short cantilever bars.

Curtailment: where a bar is allowed to stop

Bending moment falls away from mid-span, so not every bar needs to run the full length. The rule that always applies is the simplest one:

A theoretical cut-off point is where the moment diagram says the bar is no longer required. It is not where the bar stops. The bar stops an effective depth further on, because a diagonal shear crack can cross the section and put the moment demand at one place onto the steel at another.

BS 8110 also has simplified detailing rules — the figures in section 3.12.10 — that give curtailment points as fractions of the span for beams and slabs of ordinary proportions with roughly uniform loading. They are a shortcut past drawing the moment envelope, and they carry the same validity limits as Table 3.5's moment coefficients: outside them, draw the envelope.

At a simply supported end

Bottom steel does not stop at the face of the support. It carries on past the centre line, because the bar is still carrying tension where the beam sits down — commonly detailed as 12φ beyond the centre line of a simple support, or bent up where there is no room.

Where laps go, and where they do not

• Not at a point of maximum stress. Beam bottom laps belong near the supports and top laps near mid-span — the opposite of where the steel is working hardest.
• Staggered. Lapping every bar of a column at the same level doubles the steel in one section and leaves a plane with none of it continuous. Stagger the laps, or the section becomes congested exactly where it is weakest.
• Column laps sit just above the kicker, so the lap is in the storey it belongs to and not straddling the construction joint. Cranked bars change plane at not steeper than about 1 in 10, and the crank is below the lap, not inside it.
• Links continue through a lap at the normal spacing. A lap is where two bars are trying to burst apart sideways; the links are what stops them.
• Concreting round a lap needs room. If the clear gap between laps is under 75 mm the lap gets longer, which is the code telling you the section is congested.

What Structura does, and what it deliberately leaves to you

Structura schedules bars to BS 8666 and packs them into the 12 m commercial lengths you actually buy, largest first, so the order reflects that an off-cut is only useful if another bar fits inside it. Where a bar is longer than the stock length, it is flagged for lapping and never lapped.

That is on purpose, and this page is why. A lap length depends on where the lap is, which face it is on, what the cover is there, and how congested that section happens to be — none of which is in a member design. The calc sheets say so rather than inventing a number, and every schedule carries a note that laps and anchorages are to be confirmed at detailing.

Next

• Bar bending schedules
• Rebar weight chart
• Cover, grades and mixes
• Beam design
• Column design
• What size should a column be?
• Ground beam design
• Cube tests
• All worked examples