OVERLAPPING OF STEEL BARS IN DIFFERENT CONCRETE MIX

The proper overlapping of steel bars in the different concrete mix is very important because in R.C.C construction works proper lapping of steel bars is very essential.

Before going to calculate the lapping of the steel lets clear the following basic concepts.

Why over Lapping of steel is important?

The overlapping of the steel is very important in R.C.C works because splicing of steel is used to transfer all the stresses from one bar to another bar. The length of the lapping is different in different concrete mixes because different concrete having a different crushing strength.

At the site basically, we receive a steel bar at the site having a length 6 to 12 meter. So if we have a length more then 12 m. We must overlap the steel bar so the splice length of the steel bar depends on the,

  1. The diameter of the steel bar going to use.
  2. Amount of Stresses developed in the bars.
  3. And what type of concrete is going to use? (M15, M20 etc.)

Suppose we have a column having a height 200 ft. So at the side, we don’t have a 200 ft long one single bar we must overlap the bar to achieve the total length of the column.

OVERLAPPING RESTRICTION:

  1. The lapping is not provided above 36 mm diameter bars because those diameter bars doesn’t transfer the stresses from one bar to another bar and also the alignment of the column bar is also affected by providing the lap on these bars.
  2. The lapping of the steel bars also not provided in high shear force zones and it should be provided at that zone where shear force will be minimum.

LAPPING OF DIFFERENT CONCRETE MIX:

Basically, we have three types of zone Compression zone Neutral zone and Tension zone. In different zones, we provide different splice length of the steel bars. In R.C.C construction we mainly use 3 types of concrete mix M15, M20, and M25.

IN M 15 CONCRETE MIX:

In M 15 we use the concrete mix 1 : 2: 4 means 1 part of cement, 2 part of sand and 4 part of aggregates.

Tension zone        Compression Zone.

Fe 250 – 55 ∅              Fe 250 – 45 ∅

Fe 415 – 57 ∅               Fe 415 – 47 ∅

Fe 500 – 68 ∅              Fe 500 – 57 ∅

Tension zone develops at the bottom of the Beam, Slab etc and the compression zone developed at the top. In the tension zone, steel bars resist against elongation etc.

IN M 20 CONCRETE MIX:

In M 20 we use the concrete mix 1: 1.5 : 3 means 1 part of cement, 1.5 part of sand and 3 part of aggregates.

Tension zone        Compression Zone.

Fe 250 – 46 ∅              Fe 250 – 37 ∅

Fe 415 – 47 ∅               Fe 415 – 38 ∅

Fe 500 – 57 ∅              Fe 500 – 46 ∅

IN M 25 CONCRETE MIX:

In M 25 we use the concrete mix 1 : 1: 2 means 1 part of cement, 1 part of sand and 2 part of aggregates.

Tension zone        Compression Zone.

Fe 250 – 39 ∅              Fe 250 – 32 ∅

Fe 415 – 41 ∅               Fe 415 – 33 ∅

Fe 500 – 49 ∅              Fe 500 – 39 ∅

Where ∅ is the diameter of the bar. Suppose we are using a steel bar Fe 500 and concert mix M25. The diameter of the bar will be 10 mm. By using the formula, the splice length of the steel bar will be 49 x 10 = 490 mm and if the bar is in tension zone and 39 x 10 = 390 mm if the bar is in the compression zone.

Note: These all formulas are commonly used at the site but also concern your site engineer before going to use these formulas.

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