Course · lesson 3 of 6

Laps, anchorage and cover in practice

RC Design 1 gave the formulas. This lesson is about using them on real drawings: where to lap, which multiplier applies, starter bars, anchorage at supports, cover by exposure and fire, spacers and chairs, and keeping room for the concrete.

1. Learning outcomes

After this lesson you can:

choose where to lap, and stagger laps;
pick the right lap multiplier from where a bar sits;
detail starter bars and compression laps;
anchor bars at supports with straight lengths, bends or hooks;
set cover by exposure and fire, and see what it does to bar positions;
place spacers and chairs, and check that the bars leave room for the concrete.

2. Before you start

RC Design 1, lesson 8 (anchorage and lap formulas, cover, spacing) and lesson 2.
The free RC beam and RC column calculators.
A pocket calculator and about 45 minutes.

3. Where to lap

Where to lap. Put a lap where the bar it splices is lightly stressed: bottom bars of a continuous beam near the supports, top bars near mid-span, column bars just above the floor where the kicker is. A lap at the peak moment needs the full tension lap and crowds the bars where the concrete works hardest.

Staggering. Laps that all fall at one section weaken it and pack the bars together. Stagger them so the laps are spread out; the RC beam calculator’s detailing checks also test that the bar sizes lapped at any section add up to no more than 0.4 times the width, and that a lap fits in one layer.

4. Lap lengths on real drawings

The lap multipliers you meet on real drawings (SANS 10100-1 4.11.6.6.3). A tension lap is the anchorage length times 1.0, 1.4 or 2.0, by where the bar sits. STATIX uses 2.0 when the position is not given, so tell it the cover and the gap and the laps come down.

Top bars bond less well (SANS 10100-1 Table 24 note). In a member over 300 mm deep, a top bar takes 0.7 of the bond stress, so its anchorage and laps are about 1.4 times longer. This is separate from the 1.4 lap factor: a top bar with thin cover can have both.

Where both lapped bars are larger than 20 mm and the cover is less than 1.5 times the smaller bar, links must run along the whole lap (4.11.6.6.5); the RC beam calculator adds the check where it applies.

5. Starters and compression laps

Starter bars and compression laps. Column starters come out of the base (shape 37, foot on the mat) and lap with the column bars above the kicker. Where the column stays in compression the lap is 1.25 times the compression anchorage (4.11.6.6.4); where the bars can go into tension, the tension lap applies.

6. Anchorage at supports

Anchorage at a simply supported end (SANS 10100-1 4.11.7.1.2(a)): the bottom bars need an effective anchorage of 12 bar sizes past the centre line of the support, and a bend or hook only counts if it begins at or past the centre line. On a narrow bearing that is often the limit.

Bends and hooks against straight bars (SANS 10100-1 4.11.6.8.2). A bend or hook lets a bar anchor in less room: a 90° bend counts as the smaller of 4r and 12φ (or its real length, if that is more), a 180° hook as the smaller of 8r and 24φ. Straight bar beyond the bend counts only when the bearing stress inside the bend is checked (4.11.6.9).

7. Cover

Cover by exposure and fire. Durability cover comes from SANS 2001-CC1 Table 3 by exposure and concrete class (to the outermost steel, usually the links). Fire cover for beams and columns is the average cover to the main bars, SANS 10100-1 Table 43 a), by the rating; slabs use Table 45 a). Use whichever governs, and never less than the bar size.

How cover drives bar positions. The nominal cover is to the link; the main bars sit a link size further in, so the fire cover to the main bars is the cover plus the link. Every millimetre of cover or link comes off the lever arm, which is why cover is fixed before the bending design and the schedule.

Where the fire table asks for more than 40 mm of cover, SANS 10100-1 (7.2.6) asks for supplementary reinforcement to hold the concrete in a fire; STATIX flags it on the fire row. It goes by the cover the table requires, not the cover you give: extra cover for durability does not call for it. Check the clause for your member.

8. Spacers, chairs and congestion

Spacers and chairs. Spacers under and beside the bars hold the cover; their height is the nominal cover. Chairs hold the top mat of a slab or footing at its level; their height is the depth less both covers and the bars of both mats. Chairs are usually scheduled as their own bar mark or supplied by the fixer.

Congestion (SANS 10100-1 4.11.8.1). The clear gap between bars must pass the stone and the poker: at least the aggregate size plus 5 mm and at least the bar size. Check it where it is tightest: at laps, where bars cross at a column, and with two layers.