Kumar IronUpdated
Why slabs have crank bars
In the middle of a slab span, the bottom is in tension, so steel is needed at the bottom. Near a support that continues into the next slab or is held by a beam, the top goes into tension instead. Cranking alternate main bars up near the supports moves steel to the top where it is needed there, without adding a separate set of bars.
Cranked bars also help resist shear near the supports. Many engineers today prefer straight bottom bars with separate top extra bars because they are easier to place. Both methods work. Follow your drawing.
Where the bar is cranked
- Usually alternate main bars are cranked, and the rest run straight along the bottom.
- A common detailing practice is to crank at about one seventh of the span from the support for simply supported edges, and about one quarter of the span for continuous edges.
- The crank is usually at 45 degrees.
- Your structural drawing overrides any thumb rule.
Where 0.42d comes from
Take d as the vertical height of the crank: the distance between the bottom and top bar positions. A simple way to estimate it is slab thickness minus top cover minus bottom cover minus one bar diameter.
At 45 degrees, the sloping part of the bar is d ÷ sin 45° = 1.414d long, while it covers a horizontal distance of only d. So each crank adds 1.414d minus d = 0.414d of extra bar, which is rounded to 0.42d. A bar cranked at both ends gets 2 × 0.42d extra.
Worked example
| Step | Value |
|---|---|
| Slab thickness | 125 mm |
| Top and bottom cover | 20 mm each |
| Bar diameter | 10 mm |
| d = 125 − 20 − 20 − 10 | 75 mm |
| Extra per crank = 0.42 × 75 | about 31.5 mm |
| Clear span + bearing (example) | 3,600 mm |
| Add two cranks = 2 × 31.5 | 63 mm |
| Cutting length before hooks and bend deductions | about 3,663 mm |
Then adjust for hooks or development length at the ends and the bend deductions your bar bending schedule uses. The crank only adds a few centimetres per bar, but across a whole slab it adds up.
Frequently asked questions
- Why is 0.42d added for a crank bar?
- At a 45 degree crank, the sloping length is 1.414 times the crank height d, so each crank adds about 0.414d, rounded to 0.42d, compared with a straight bar.
- Are crank bars compulsory in slabs?
- No. Many designs use straight bottom bars plus separate top extra bars at supports. Use whichever your structural drawing shows.
- What is d in the crank bar formula?
- The vertical height of the crank, roughly slab thickness minus top and bottom cover and one bar diameter.