Length of Hook’s in Stirrups
We generally consider the length of hook = 10 D (for bars having a diameter greater than or equals to 8mm)
Where d → Diameter of the stirrup
Is code recommends, Minimum hook length = 75 mm
Bend Length
The bending 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 3 d
For 180 degree bend, length reinforced bar increase by 1 d
Example:-
Q:- Calculate the cutting length of the stirrups given in fig 1. Clear cover of the beam is 25 mm
Ans:-
Given data:
Clear cover = 25 mm
Depth of beam = 450 mm
Width of beam = 300 mm
The diameter of stirrup =8 mm
Cutting length of stirrup = 2a+2b + (2 x Hook length) – Bend deduction
Where,
a= Width of the beam – (2 x clear cover) – ( 2 x Half of the diameter of the bar)
b= Depth of beam – (2 x clear cover) – ( 2 x Half of the diameter of the 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
The hook is provided in stirrups for the following reasons:
1- To prevent from buckling of the 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 endplates.
3- Hooks are provided to resist seismic movement.
4- To prevent concrete from splitting outward.
5- It prevents slippage of steel from the concrete.
6- To Keep longitudinal steel bars in position and hold steel tightly.
For more information watch this video
