BBS Basics: Lapping, bend, development and hook length calculation -lceted LCETED INSTITUTE FOR CIVIL ENGINEERS

## Mar 18, 2022

In this article, we explained lap length, bend length and development length and hook length calculation

LAP LENGTH

How to Calculate Lap length?

The lap length is the overlap of the two rods, which provides the required design length. In the RCC configuration, lapping is done if the length of a bar is not sufficient to create the design length.

Lap length is the length provided to connect two rebars to each other to securely transfer loads from one bar to another and instead provide mechanical couplers

Suppose we want to build a building 15 m high. But 15m single bar is not available in the market. The maximum length of steel available in the market is usually 12m – 40feet, so you need to connect two bars of 12m to get a 15m bar.

Each over-lap length = 50d (Practically)

Therefore, d = dia of bar

(over-lap length means Joining to the steel bar with extra length of Steel bar)

Extra Length = Over-Lap Length = Lapping Length

Theoretical Standards for Over-lap length:-

1. For Compressive member (vertical member) Standard over-lap length = 40d (Vertical over-lap)

2. For Tensile member (Horizontal member) Standard over-lap length = 50d (Horizontal over-lap)

Problem-1: Find the length of steel bars including the over-lap length from the given beam below figure

Given data:

Beam = Horizontal Member = 50d

Over-lap length Length of steel bar = 50d

We will use 12mm, 16mm and 20mm dia bars for the calculation

Length = 6m

Solution: Length of steel bar = Total length + over-lap length Length of steel bar = L + 50D

1. Length of bar, if dia of bar = 12mm

12mm = 0.012m (12/1000 = 0.012)

= L + 50D = 6.0m + 50 x 0.012m = 6.60m

2. Length of bar, if dia of bar = 16mm

16mm = 0.016m (16/1000 = 0.016)

= L + 50D = 6.0m + 50 x 0.016m = 6.80m

3. Length of bar, if dia of bar = 20mm

20mm = 0.020m (20/1000 = 0.020)

= L + 50D = 6.0m + 50 x 0.020m = 7.00m

Problem-2: Find the length of steel bars including the over-lap length from the given Column below figure

Given data:

Column = Vertical Member = 40d

Over-lap length Length of steel bar = 40d

We will use 12mm, 16mm and 20mm dia bars for the calculation

Length = 20’

Solution: Length of steel bar = Total length + over-lap length Length of steel bar = L + 40D

1. Length of bar, if dia of bar = 12mm

12mm = 0.0393 feet

L + 40D = 20' + 40 x 0.0393' = 21.57200 feet (21 feet 655⁄64 inches)

2. Length of bar, if dia of bar = 16mm

16mm = 0.0524 feet

= L + 40D = 20' + 50 x 0.0524' = 22.62 feet (22 feet 77⁄16 inches)

3. Length of bar, if dia of bar = 20mm

16mm = 0.0656 feet

= L + 40D = 20' + 40 x 0.0656' = 22.62400 feet (22 feet 731⁄64 inches)

BEND LENGTH AND DEVELOPMENT LENGTH

How to Calculate Bend Length and development of Bar?

When we bend the steel bar, the length of the bar increases slightly as it is stretched in the bending area (see image below). The length expansion depends on the quality of the steel and the size of the bend.

The length of bar increases when the increasing degree of bending increases and it decreases with higher quality steel. (Fe250, Fe450, Fe500)

For 45° bend = 1d

For 90° bend = 2d

For 135° Bend = 3d

For 180° bend = 4

Bend Length calculation:

Each Bend length = 16d (Practically) (development length)

Therefore d = Dia of bar

Problem -1:

Find the length of steel bars including Bend length from given the figure

Given data:

Bend length = 16d

Length = 3m

We will use 12mm, 16mm and 20mm dia bars for the calculation

Solution: Length of steel bar = Total length + Bend length Length of steel bar = L + 16D

1. Length of bar, if dia of bar = 12mm

12mm = 0.012m (12/1000 = 0.012)

= L + 50D = 3.0m + 16 x 0.012m = 3.192m

2. Length of bar, if dia of bar = 16mm

16mm = 0.016m (16/1000 = 0.016)

= L + 50D = 3.0m + 16 x 0.016m = 3.256m

3. Length of bar, if dia of bar = 20mm

20mm = 0.020m (20/1000 = 0.020)

= L + 50D = 3.0m + 16 x 0.020m = 3.32m

Problem -2:

Find the length of steel bars including Bend length from given the figure

Given data:

Bend length = 16d

Length = 9’

We will use 12mm, 16mm and 20mm dia bars for the calculation

Solution: Length of steel bar = Total length + Bend length Length of steel bar = L + 16D

1. Length of bar, if dia of bar = 12mm

12mm = 0.012m (12/1000 = 0.012)

= L + 16D = 9’ + 16 x 0.0393' = 9.6288 Ft

2. Length of bar, if dia of bar = 16mm

16mm = 0.016m (16/1000 = 0.016)

= L + 16D = 9' + 16 x 0.0524' = 9.8384 feet (9 feet 101⁄16 inches)

3. Length of bar, if dia of bar = 20mm

20mm = 0.020m (20/1000 = 0.020)

= L + 16D = 8.416' + 16 x 0.0656' = 9.4656 feet (9 feet 519⁄32 inches)

Lapping zone

Hook Length

How to calculate hook length for stirrups

What is Hook Length?

The hook is the extra length left at the rod joining corner of a stirrup so that the stirrup retains its shape.

Each Hook length = 9D

Therefore, d = dia of bar

d = Diameter of stirrup

Total length of steel bar including Hook lengths = L + 9D + 9D = L+18D

Problem-1:

Find the length of Stirrup from the given figure if dia of Stirrup = 6mm and 8mm

Given data:

Length - 0.30m

Length of Stirrup or tie = L + 18D

Solution: Ã˜ = 6mm & 8mm for Stirrup or Tie

(i). Length of Stirrup or tie = L + 18D

Therefore, 'L' = ( L + B ) x 2

= (0.30+0.40) x 2 = 1.40m

Length of Stirrup or tie = 1.40 + 18 x 0 .006 = 1.508m

(ii). Length of Stirrup or tie = L + 18D

Therefore, 'L' = ( L + B ) x 2

= (0.30+0.40) x 2 = 1.40m

Length of Stirrup or tie = 1.40 + 18 x 0 .008 = 1.544m

Problem-2:

Find the length of Stirrup from the given figure if dia of Stirrup = 6mm and 8mm

Given data:

Length – 9”

6mm = 0.236”

8mm = 0.314”

Total length (L) = Therefore 'L' = (L + B) x 2 = (9" + 12") x 2 = 42"

Length of Stirrup or tie = L + 18D

Solution: Ã˜ = 6mm & 8mm for Stirrup or Tie

Length of Stirrup or tie = L + 18D

Therefore, 'L' = (L + B) x 2

= 42"+18 x 0.236"

= 46.248"

= 46.248/12 = 3.854 Ft

Length of Stirrup or tie = L + 18D

Therefore, 'L' = (L + B) x 2

= 42"+18 x 0.314” = 47.652”

= 47.652/12 = 3.971 Ft

FAQ

Why hook is provided in stirrups?

1.  Hook is provided in stirrups for the following reasons:

2.  To avoid buckling of the column.

3.  The main requirement for protection against bond failure is that the steel provides a good extension of the bar length beyond where it is required to create its yield stress, and this length must be at least equal to its growth length. However, if the actual available length is not sufficient for total growth, special anchors such as cogs or hooks or mechanical end plates should be provided.

4.  It resist seismic movement.

5.  To prevent the concrete from cracking externally.

6.  it prevents the steel from slipping from the concrete.

7.  Hold the steel tight by positioning the long steel rods.

What is Lap Length?

The lap length is the overlap of the two rods, which provides the required design length. In the RCC configuration, lapping is done if the length of a bar is not sufficient to create the design length.

What Is Bend Length?

The bend deduction is the difference between the bend allowance and twice the outside setback

The bend allowance is the arc length of the bend as measured along the neutral axis of the material you are using.

What Is the Lapping Length of Rebar?

1. For Compressive member (vertical member) Standard over-lap length = 40d (Vertical over-lap)

2. For Tensile member (Horizontal member) Standard over-lap length = 50d (Horizontal over-lap)