Hook length and Bond length for stirrups

We generally consider length of hook = 10 D (for bars having diameter greater than or equals to 8mm)

Where d → Diameter of stirrup

Is code recommends, Minimum hook length = 75 mm

Bend Length

The bend length defines “the length you will need to add to the actual leg lengths of the part in order to get the flat pattern cut to a correct size”.

For  45 degree bend, length of reinforced bar  increase by 1 d

For 90 degree bend, length reinforced bar increase by 2 d

For 135 degree bend, length reinforced bar increase by  3d

For 180 degree bend, length reinforced bar increase by 1d

Example:- 

Q:- Calculate the cutting length of the stirrups given in fig 1. Clear cover of beam is 25 mm

Ans:-

Given data:

Clear cover = 25 mm

Depth of beam = 450 mm

Width of beam = 300 mm

Diameter of stirrup =8 mm

Cutting length of stirrup = 2a+2b + (2 x Hook length) – Bend deduction

Where, 

a= Width of beam – (2 x clear cover) – ( 2 x Half of diameter of bar)

b= Depth of beam – (2 x clear cover) – ( 2 x Half of diameter of bar)

Therefore, 

a= 300 – (2 x 25) – (2 x 4) = 242 mm

b= 450 – (2 x 25) – (2 x 4) = 392 mm

Length of hook (Development length) = 10 d = 10×8 = 80 mm

Bend deductions: 

90 degree bent –> 3 no’s 

For 90 degree bend, length reinforced bar increase by 2 d

i.e., Bend length = 3 x (2×8) = 3×16 = 48 mm

135 degree bent –> 2 no’s 

For 135 degree bend, length reinforced bar increase by  3d

i.e., Bend length = 2 x (3×8) = 2 x 24 = 48 mm

∴ Cutting length of stirrup = (2×242) + (2×392) + (2×80) – (48+48) = 1428 – 96 = 1332 mm

Why hook is provided in stirrups

Hook is provided in stirrups for the following reasons:

1. To prevent from buckling of column.
2. The main requirement for safety against bond failure is to provide a sufficient extension of the length of the bar beyond the point where the steel is required to develop its yield stress and this length must be at least equal to its development length. However, if the actual available length is inadequate for full development, special anchorages must be provided, such as cogs or hooks or mechanical end plates.
3. Hooks are provided for to resist seismic movement.
4. To prevent concrete from splitting outward.
5. It  prevent slippage of steel from the concrete.  
6. To Keep longitudinal steel bars in position and  hold steel tightly.