Development Lengths

 

Development length?

It is a term we hear in RCC subject in engineering while we talk about rebar. 

So what actually is development length?

It is very important for a fresh site engineer to know about the development length. The definition of development length is “A development length can be defined as the amount of reinforcement(bar) length needed to be embedded or projected into the column to establish the desired bond strength between the concrete and steel .” This definition can easily be found in google. What we need to learn is how to calculate the development length.

Development length is denoted by Ld.

Development length depends upon following:-

1)      Diameter of rebar

2)      Grade of steel

3)      Grade of concrete

As per IS 456:2000 clause 26.2.1

Development length= (Día of bar *grade of steel) /( 4* Design bond stress (ζbd) )                                                 

*Value of Design bond stress is given on code which differs from the grade of concrete.  Clause 26.2.1                                                                 

  For e.g. For M20 concrete, Design bond stress = 1.2.

        Sometimes on-site we have two different grades of concrete in beam and column. Then we get confused about which grade to consider while calculating the development length. In this condition, we need to see on which member the development length is fully acting.

                                Fig:- Development Ld provided between beam and column

In above case, the development length is completely acting in the column. So we need to consider the grade of concrete of the column.