Calculation of Lap Length in RCC and Definition of Development Length

Lap Length in RCC

Generally, Steel TMT found on the length of (11.88-12) m Lap is provided when you require to provide tension reinforcement but the length of the bar is not sufficient to continue through the entire member, as we know that consider, columns are typically 30m tall. Neither can we have a 30m long bar, nor is it easy or possible to fix that much long bar at a site, so we tend to cut bars. Now, at the location of joint we want the tension forces to transfer from one piece of bar to another lower piece of bar. So for that we tend to provide second bar near to the one that is discontinued and the amount of overlapping between the two bars is called a lap length.

 

Steel Lap

Lap length is the most important term in the reinforcement work or steel work. This is usually confused with another important term called development length. In this topic, the lap lengths of bars is discussed. During the placement of steel in Reinforced concrete Cement (RCC), if the required length of single bar is fall small. To get the desired design length, lapping of two bars side by side is done. An alternative to this is to make mechanical couples.

   

Lap in Beam

Lap in Column

Lap Length in Tension Zone:

The lap length including anchorage value of hooks shall be

·         Flexural tension – Ld or 30d whichever is greater is considered.

·         Direct tension – 2Ld or 30d whichever is greater is considered.

The straight length of lapping of bars shall not be less than 15d or 20cm.

Lap Length in Compression Zone:

The lap length is equal to the development length calculated in compression but not >24d.

Lap Length for Different Structure

·                     Columns – 50d

·                     Beams – 60d

·                     Slabs – 60d

Where, d= Diameter of Bar

If we need to lap 12 mm diameter column bars, a minimum lap of 45 * 12 =540 mm is provided.

Development length

When you have to develop the grip between the steel and concrete makes in a continuous structure than we have to provide reinforcement bar of some length under the another member, if we do not provide some length of bar in another member, than the bar does not have any restrain and we know that if a member does not have any restrain than it does not resist any force. So that we have to provide development length.

Development Length

Let us consider the above cantilever beam, this length (CD) is called “Development length”.