Crank Bar (Bent-up Bar)

A crank bar (bent-up bar) is a main reinforcing bar that is bent diagonally upward — usually at 45° (or 30°/60°) — near the supports of a beam or slab. The bend places steel where it is needed: in the mid-span the bar sits at the bottom to resist sagging moment, then 'cranks' up over the support to resist the hogging (negative) moment there, and the inclined portion simultaneously contributes to shear resistance like an inclined stirrup.

In continuous beams/slabs the crank typically starts at about L/5 to L/7 from the support face. The bend deduction and the inclined-length increase are computed in the bar bending schedule per IS 2502 (for a 45° crank, extra length ≈ 0.42 × crank height per bend). Modern practice often favours separate straight top/bottom bars + closed stirrups (especially for ductile/seismic detailing per IS 13920, which discourages bent-up bars as shear reinforcement), but crank bars remain common in routine slab/beam detailing and are a standard BBS shape code.