Detailing and the bending schedule

How bars are detailed to EN 1992-1-1, on the Eurocode 2 route of the STATIX calculators: cover with Δc_dev, bond and anchorage, anchorage at supports, bends and formers, laps, spacing and curtailment, and the bending schedule with SANS 282 shapes.

1. Learning outcomes

After this lesson you can:

work out the nominal cover with Δc_dev;
find the bond strength f_bd for good and poor bond;
work out anchorage lengths, including at a simple support;
work out lap lengths with α₆ and stagger laps;
choose bend formers to Table 8.1N, and say why the schedule uses SANS 282 shapes.

2. Before you start

Lesson 1 (the beam) and lesson 3 (the shear truss).
The RC beam calculator, set to Eurocode 2 (EN 1992-1-1).
A pocket calculator and about 45 minutes.
The same lesson on the SA code: RC Design 1, lesson 8 (SANS 10100-1); for the schedule itself, Detailing 1.

What the beam calculator’s Eurocode 2 checks use: cover by 4.4.1 with Table 4.4N (S4) and Δc_dev 10 mm; f_bd by 8.4.2 with η₁ = 0.7 for top bars of members deeper than 250 mm; anchorage by 8.4.3 and 8.4.4 with α₁ to α₅ all 1 (the safe side) and l_b,min; laps by 8.7.3 with α₆ 1.5 unless the share lapped is known; formers by Table 8.1N; spacing by 8.2 with k₁ = 1 and k₂ = 5 mm; end anchorage by 9.2.1.4 with a_l/z = 1.25. Lengths are rounded up to 25 mm. The schedule uses SANS 282 shape codes.

3. Cover, bond and anchorage

Cover, again. The nominal cover on the drawing is the minimum cover (the largest of bond, durability and 10 mm) plus an allowance for fixing tolerance, Δc_dev. The calculators use 10 mm; an annex may allow less where the fixing is checked.

Bond strength. How hard the concrete can grip a ribbed bar depends on the concrete’s tensile strength and on where the bar sits when it is cast: bars near the top of a deep pour sit in weaker, settled concrete, and get η₁ = 0.7.

Anchorage (8.4.3, 8.4.4). The basic length is the length of bond that develops the bar’s stress σ_sd. The factors α₁ to α₅ can shorten it for bends, cover and links; the calculator takes them all as 1, which is the safe side.

Bars at a simple support (9.2.1.4). Near the support the bottom bars still carry a force from the shear truss: F_E = V_Ed a_l/z. They must be anchored for it beyond the face of the support, and at least a quarter of the span steel must reach the support.

4. Bends, laps, spacing and cut-offs

Bends. A bar bent round too small a former crushes the concrete inside the bend and may crack. The Eurocode sets the smallest former by bar size: four bar diameters up to 16 mm, seven above. A tighter bend earns no anchorage in the calculator’s checks, so on its Eurocode 2 route the beam calculator bends each bar round the larger of the SANS 282 former and the Table 8.1N one (Y20: 140 mm, not 120).

Laps. Where a bar is too short for the run, it overlaps the next. The lap length is the basic anchorage length times α₆, which depends on how many of the bars are lapped at the same place.

Staggering laps. Lapping all the bars at one section weakens it, so the Eurocode asks for longer laps then. Stagger the laps (centres at least 1.3l₀ apart) and fewer are lapped within each length.

Spacing between bars. The concrete must get between the bars and grip them. Lapped bars may touch, but the clear gap between laps and the next bars still applies (8.7.2).

Curtailment (9.2.1.3). A bar may stop only where the bars left can carry the moment curve shifted by a_l = z cot θ/2 for the shear truss, and then it runs on by its anchorage length.

5. The schedule

The bending schedule. The calculators and the project board schedule bars with SANS 282 shape codes, on the Eurocode route too: they are the shapes South African benders work to. Elsewhere you may use another standard’s codes; the dimensions are the same idea. Where a SANS 282 standard bend is tighter than Table 8.1N (bars over 16 mm), call up the larger former.

From design to site. Each calculator can send its bars to the project schedule; the project board puts them on sheets with one schedule and the quantities; STATIX 2D details and schedules inside AutoCAD. The board needs an account; its PDF sheets, order list and cutting list are part of the calculator packs.