What is the Crank Length of Reinforcement

The crank length of reinforcement is the distance between the end of the bar and the point where it bends or turns perpendicular to the main reinforcement. It is commonly used in construction to provide additional reinforcement and improve the structural integrity of concrete elements.

The crank length plays a crucial role in preventing cracks and enhancing the load-bearing capacity of concrete members. By adding this bend, it redistributes the stress and helps in resisting the bending moments that occur during the lifespan of the structure.

The crank length of reinforcement is typically determined by the following factors:

• The type of concrete element being reinforced
• The magnitude of the bending moments that the element is expected to experience
• The diameter of the reinforcement bar
• The clear cover requirements

For example, in a reinforced concrete slab, the crank length of the top reinforcement bars is typically 0.42 times the effective depth of the slab. This is because the top bars are responsible for resisting the negative bending moments that occur at the supports of the slab.

In a reinforced concrete column, the crank length of the longitudinal reinforcement bars is typically equal to the diameter of the bar plus the clear cover requirement. This is because the longitudinal bars are responsible for resisting the axial compression loads that the column experiences.

It is important to note that the crank length of reinforcement should not be less than 300 mm. This is to ensure that the bar has sufficient anchorage in the concrete.

Here are some of the benefits of using cranked reinforcement:

• Improved resistance to cracking
• Increased load-bearing capacity
• Reduced reinforcement congestion
• Better aesthetics

Cranked reinforcement is commonly used in a variety of concrete elements, including slabs, beams, columns, and footings. It is an effective way to improve the structural performance and durability of concrete structures. Crank Length of Reinforcement

Calculating the cutting length of the crank bar in a slab / Cutting length of bent-up bar in a slab.

• Clear span between the columns = 3500mm.
• Slab thickness = 150 mm.
• Clear cover = 25mm.
• Rebar diameter d = 12mm.
• Development length Ld = 40d

If sometimes development length is not given in the drawing, you can assume it as 40d for the slab.

Cutting length of the crank bar Crank Length of Reinforcement
= {clear span +( 2nos. × development length )+(2 nos.× extra crank length) – [2nos.× 90° bend + 4nos.× 45° bend]}

= {3500mm +( 2×40d) +( 2 × 0.42D ) – [2 nos.× 2d + 4nos.× 1d ]}

Note:

1. As the crank bar is bent at 45°, the resulting slope length will be 0.42D extra as it represents the hypotenuse of a triangle.
2. There are 2nos. of 90° bend & 4 nos of 45° bend as shown in the below diagram.

= {3500mm + (2 × 40 × 12mm ) + ( 2 ×0.42D ) – [2nos. ×2 ×12mm + 4nos.× 1×12mm.]}

Here, D = slab thickness – ( top & bottom clear cover + bar diameter )

• = 150mm – (2nos × 25 mm. +12mm )
• = 150mm – 62 mm
• D = 88 mm.

Total Cutting length

={ 3500 mm + 960mm +(2 × 0.42 × 88mm) - [ 48mm + 48mm ]}

= {3500 mm + 960mm + 73.92 mm – 96mm}

= 4533.92 mm – 96 mm

=4437.92 say 4438 mm.