LAPPING OF DIFFERENT SIZE OF REINFORCEMENT,PROBABLE REASONS

When lapping the rebars of different diameters, the standard practice is to provide the lap calculated on the based on the smaller diameter.

This is pretty much the general practice but the reason for the same is not easy to find.

Before we dive into the explanation for the above, we have to understand an important implication in RCC design that we normally do not pay much of the attention.

All the rebars are designed for a stress not more than 0.87fy irrespective of their diameter. This implies that for a given tensile force, one can provide less number of larger diameter bars or more number of smaller diameter bars.

Now, consider a lap location where we are lapping two bars of different diameters. The purpose of using a smaller diameter is to achieve an economical design as the tensile force at that location is at a level, which can be resisted by a smaller diameter bar.

As shown in the figure below, a larger diameter bar is required to resist a tension of 300kN or more. However, as the force varies along the length of the element and falls to a value, say 150kN, a smaller diameter bar can used to accommodate this force. If we still go ahead and provide the larger diameter bar throughout, the resulting design will not be an economical one.Also the purpose of steel is show warn the people when the structure is going to collapse so in this way it will first bend and then collapse otherwise in case of more steel, it will crack suddenly without any warning

Now, at the lap location, as the force is 150kN and reduces further, the smaller diameter bar would experience more stress (closer to the design value of 0.87fy) as compared to the larger diameter bar. As the smaller diameter bar is more critical at this location, it would be sufficient to provide a lap length calculated based on smaller diameter so that the required force is transferred.

Remember, one can safely provide the lap length based on the larger diameter but it would be uneconomical as the lap length is directly proportional to the bar diameter.