Detailing and the bending schedule

The last step from design to site: cover, spacing, anchorage, laps, bar shapes and the SANS 282 bending schedule. This lesson details the lesson 1 beam and the lesson 7 base and schedules them, by hand and in STATIX.

1. Learning outcomes

After this lesson you can:

choose the cover for durability and fire, and check bar spacing;
work out anchorage and lap lengths;
read and draw the common SANS 282 shape codes and work out cutting lengths;
fill in a bending schedule, with masses;
send bars from the STATIX calculators to the project schedule, the project board and STATIX 2D.

2. Before you start

The designs from lesson 1 (beam), lesson 3 (links) and lesson 7 (base).
The RC beam calculator and the project schedule.
A pocket calculator and about 60 minutes.

3. From design to site

Why detailing matters. A design only becomes a building through the drawings and the bending schedule. The schedule (SANS 282) tells the yard what to cut and bend and tells the fixer what goes where, bar mark by bar mark.

4. Cover

Cover is measured to the outermost steel, usually the link. It must be enough for durability (SANS 2001-CC1, Table 3: by exposure and concrete class), for fire, and at least the bar size and the aggregate size. Concrete cast against soil needs 75 mm.

STATIX takes the durability cover from SANS 2001-CC1 Table 3, by exposure (moderate to extreme) and concrete class, and the fire cover by rating; use the larger. Members cast against soil get 75 mm.

5. Bar spacing

Bar spacing for concreting (SANS 10100-1, 4.11.8.1). The clear gap between bars must let the stone and the poker vibrator through: at least the aggregate size plus 5 mm and at least the bar size; between layers at least 2/3 of the aggregate size. Lesson 4 gave the upper limit for cracking.

6. Anchorage and laps

Anchorage (SANS 10100-1, 4.11.6). A bar can only reach its full force if enough of it is buried in concrete. The concrete grips the ribs with a bond stress f_bu (Table 24, by concrete grade); the length needed is l_b = 0.87 f_y φ / (4 f_bu).

tension: l_b = 0.87 f_y φ / (4 f_bu) compression: l_b = f_yc φ / (4 f_bu) (f_yc as in lesson 5; compression f_bu is higher) STATIX rounds up to 25 mm

Top bars bond less well. Fresh concrete settles and bleeds; water and air gather under bars near the top of a deep pour, so the grip is weaker. For top bars in members over 300 mm deep STATIX reduces the bond stress to 0.7 f_bu, so they need longer anchorages and laps.

Laps (SANS 10100-1, 4.11.6.6). Where bars are too short they overlap. A tension lap is at least the anchorage length, times 1.4 for a top-cast bar or a corner bar with cover less than 2φ, or bars closer than max(75 mm, 6φ), and times 2.0 when two of these apply. Compression laps are 1.25 times the compression anchorage. Stagger laps where you can, away from the highest stresses.

Laps are at least the larger of 15φ and 300 mm.

7. Shapes and cutting lengths

The common shape codes of SANS 282, as STATIX uses them, with their dimension letters. A to E are outside dimensions of the bent bar; the schedule lists the shape code and the letters, and the yard bends to them. STATIX also handles cranked bars (038, 41) and a custom shape (99).

Bends are made round a former. Too tight a bend cracks the bar or crushes the concrete inside the bend, so the code sets a minimum radius. Because the letters on the schedule are outside dimensions, the bar's real length is a little shorter than their sum; SANS 282 Table 1 sets the deductions (r/2 + d for the one bend of an L-bar), and its Figures 2 and 3 set the standard radius: 3d for high yield (Y) bars and 2d for mild steel (R) bars.

20 straight: A 37 L-bar: A + B − r/2 − d 38 U-bar: A + B + C − r − 2d (A and C the legs, B the long side) 60 link: 2A + 2B + 2N − 1.5r − 3d (N = the hook allowance: 100 mm for Y6 to Y12) r = 3d for Y bars (2d for R); then round the cutting length up to 50 mm (SANS 282 cl. 3.5)

Link hooks. Shape 60 links close with two 135° hooks (SANS 282 tabulates their length as the allowance N) that turn into the concrete core, so the link stays closed even if the cover spalls. Hooks bent at 90° into the cover open up and the link stops working.

8. The bending schedule

The bending schedule (SANS 282): one line per bar mark, with the member, the type and size (Y16 is a high-yield 16 mm bar), how many members and bars, the cutting length, the shape code and its dimensions. STATIX adds the mass. Lengths are rounded up to 50 mm, dimensions to 5 mm.

Mass per metre is πφ²/4 × 7 850 kg/m³: for example 0.617 kg/m for Y10, 1.579 for Y16 and 2.466 for Y20.

9. Detailing with STATIX

The STATIX route from design to site. Each concrete calculator has an “Add to project schedule” button; the project schedule collects the bars of all members; the project board keeps the sheets, schedules and quantities together; and STATIX 2D draws the details in AutoCAD.