In this article, I will discuss the differences between development length and lap length. Let’s get started.
Development length is the length of bar required for transferring the stress into concrete.
In simple words, the quantity of the rebar length that is actually required to be embedded into the concrete to achieve the desired bond strength between concrete and steel by producing required stress for the steel in that area.
The formula for development is given below:
Development length (Ld) = d x σs/τbd
d = Diameter of the bar.
σs = Stress in the bar at the section considered as design load.
τbd = Design bond stress.
Lap length is the overlapping length of two bars side by side which gives required design length. In RCC structure if the length of a bar is not sufficiently available to make design length, lapping is done.
Suppose we need to construct a building of 20 m height. But there is no 20 m single bar available in the market. The maximum length of rebar available in the market is usually 12 m, so we need to join two bars of 12 m to get 20 m bar.
The lap length varies from member to member.
Lap length for tension members = 40d
Lap length for compression members = 50d.
Where, d = Diameter of bars.
In the below image you can see some amount of rebar is left for future construction. This extra rebar will be needed for tying bars of column. This extra length of rebar is called lap length.