Crank Length of Rebars

Описание: What is cranking of rebars?
Cranking means bending reinforcement bars (usually in slabs) at a slope instead of keeping them completely straight. We do this mainly near supports to handle negative bending moments — the stresses that occur on the top side of the slab above beams or walls. By lifting some bars up towards the top surface at supports, we place steel exactly where tension forces are highest, improving strength and economy.

2. What is crank length?
Crank length is the extra bar length required to make that slope.
When a bar is bent diagonally, it naturally becomes a bit longer than if it were kept straight.
That extra diagonal portion — from the point where it starts to rise to where it levels out again — is called the crank length.

3. Why do we provide 0.42d crank length and which code says that?
The value 0.42d comes from geometry for a 45° crank angle, which is the standard recommended in IS 2502 – Code of Practice for Bending and Fixing of Bars for Concrete Reinforcement.
d = effective depth of the slab (from top surface to the center of bottom reinforcement).
When you bend at 45°, the horizontal projection of the diagonal works out to roughly 0.42 × d extra length.
This ensures the bar rises exactly to the correct position at the support for proper anchorage and stress resistance.

Why not 0.5d or 0.2d?
We don’t choose crank length randomly — it’s tied to geometry and practical bending.
If we used 0.5d – the slope would be steeper than necessary, making bars longer, harder to bend, and wasting steel without adding benefit.
If we used 0.2d – the slope would be too shallow, and the bar wouldn’t reach the top reinforcement zone in time, reducing its ability to resist negative moments.
0.42d is the sweet spot – matches the 45° standard bend, easy for workers to fabricate, fits space in the slab, and meets strength requirements without waste.

💬 Comment below if you’d like me to post the step-by-step derivation for 0.42d so you can see exactly how the number comes from simple geometry.

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