Kumar IronUpdated
In this guide
Drawing first, thumb rule second
The structural drawing gives the lap length for your job. It depends on the grade of steel, the grade of concrete, the bar size and whether the bar is in tension or compression. Always follow the drawing and the engineer.
The figures here are a rule of thumb, to help you plan material and to spot a lap that is clearly too short.
Development length and lap length
Development length is the length of bar needed in concrete to grip it and carry its full stress. IS 456 works it out from the bar stress and the bond strength of the concrete. For deformed Fe500 bars in M20 to M25 concrete, this comes to roughly 48 to 57 times the bar diameter.
IS 456 asks that a tension lap be at least the development length and not less than 30 times the diameter. For compression, the lap can be shorter, but not less than 24 times the diameter. That is why the usual site habit is 40d to 50d for most bars, and longer for tension zones with weak concrete. Higher grades of concrete need a shorter lap, and a ductile steel with higher yield needs a longer one.
Lap length by diameter
Try it: Column and beam steel calculator
Total steel: 41.3 kg (0.041 tonnes) for 1 beam, including 5% wastage
| Item | Quantity | Kg per member |
|---|---|---|
| Main bars 16 mm | 4 x 4.00 m | 25.3 |
| Stirrups 8 mm | 27 x 1.320 m | 14.1 |
| Per member, no wastage | 39.4 | |
| Per member, +5% | 41.3 |
| Bar diameter | 40d | 50d |
|---|---|---|
| 8 mm | 320 | 400 |
| 10 mm | 400 | 500 |
| 12 mm | 480 | 600 |
| 16 mm | 640 | 800 |
| 20 mm | 800 | 1000 |
| 25 mm | 1000 | 1250 |
| 32 mm | 1280 | 1600 |
Add lap length into your steel quantity. A long wall or slab can add several percent to the weight, which our steel calculator helps you plan.
Where to lap and where not to
- Slab bottom bars: lap near the supports, where the bending is least. Slab top bars: lap near mid span.
- Columns: lap just above the floor level, in the middle third of the storey height, not at the beam joint.
- Do not lap at the point of maximum stress, such as the middle of a beam at the bottom.
- Beams: avoid lapping more than a limited share of the bars at one section. IS 456 says not more than half the bars should be spliced at one place in flexure.
Stagger the laps
Never put every lap at the same section. If all bars are spliced in a line, you get a plane of weakness, and the concrete there is crowded with steel, which makes it hard to compact.
Stagger the laps along the length, so that neighbouring laps are at least about one lap length apart. Keep bars in a lap touching and tie with binding wire at the ends and the middle.
Hooks and bends at the ends
Where a bar ends in a support, it is usually anchored with a hook or a bent end. IS 456 treats a 180 degree hook as an anchorage of 16 times the bar diameter, and a 90 degree bend as 8 times, and the standard hook has a straight tail of at least 4 times the diameter. Bend radius and bending rules follow IS 2502 and are covered in our guide on bending and welding bars.
The detail for each location is on the drawing, so follow that rather than rules of thumb.
Frequently asked questions
- Is 40d or 50d the right lap length?
- It depends on bar grade, concrete grade and whether the bar is in tension or compression. Follow the drawing, and use 40d to 50d only as a rough planning figure.
- What is the lap for 12 mm bars?
- About 480 to 600 mm on the rule of thumb, and about 50 times the diameter for tension zones in Fe500.
- Can I lap bars with welding?
- Normally no. Use a proper lap with binding wire, unless the engineer calls for a welded joint.
- Do I count the lap when ordering steel?
- Yes. Add the laps, hooks and wastage to your quantity.