What is Lap Length?
The lap length is the length provided to overlap two rebars in order to safely transfer load from one bar to another bar and alternative to this is to provide mechanical couplers.
Lap length also known as lap splices. Suppose we need to construct the building of 20-metre height but there is no 20metre single bar available in the market. The maximum length of Steel bar available in the market is usually 12 metre.
Why it is provided?
Suppose we need to construct the building of 30 metre height but there is no 20 metre single bar available in the market. The maximum length of Steel bar available in the market is usually 12 metre.
It is because of the transportation problem and manufacturing difficulty so we need to join three bars of 12 metre to get 30 metre bar.
What will happen if we don’t provide lap length?
If we don’t provide lap length then the load transfer mechanism will fail which eventually lead to failure of structure also if we provide less lap length then the required then the reinforcement bar split and Crack can be developed in the concrete.
How to calculate lap length?
The calculation for tension zone and compression zone are different. Let us take the case of the beam when the beam is subjected to forces in a building the bottom portion of beam experiences tension and top portion of beam experiences compression so, first we will discuss about tension zone.
In, tension zone, there are two cases
- one flexural tension
- direct tension
For flexural tension, the lap length shall be Ld that is development length or 30d whichever is Greater is considered. Where d is the diameter of the bar. Generally, development length is 41d where d is the diameter of the bar. To know more about development length – What is Development Length?
For direct tension, the lap length should be 2 Ld or 30d whichever is greater is considered. In this case, the straight length of the lapping bar shall not be less than 15d or 20cm.
Lap length in compression
In the case of compression, the lap length is equal to the development length calculated in compression but not less than 24d.
What are the general rules for lap length?
For the different diameter of bars
When the bars of different diameters are to be spliced the lap length is calculated considering the smaller diameter bars.
Suppose you are constructing a column, from bottom 20 mm diameter bar is coming and from here 16 mm diameter bar has to be spliced then for calculating lap length 16 mm diameter should be considered and not 20 mm.
If the diameter of the bar is more than 36 mm then lapping should not be done instead of lapping, this bar should be welded but when welding is not possible then lapping can be permitted for bars larger than 36 mm but in this case, additional spiral shall be provided around the lapped bar.
Lapping should be done in a staggered manner. These laps should not be given at the same level to avoid buckling. The stirrup shall be closely spaced in the lapping portion it’s because when we provide lapping in concrete member the strength of member slightly reduces. Hence, we need to provide more numbers of stirrups in this portion.
In case of bundled bars, lap splices of bundle bars shall be made by splicing one bar at a time. Such individual splices within a bundle shall be staggered. In this image you can see some amount of rebar is left for future construction with extra rebar will be needed for tying bars of the column. This extra length of rebar is also called as lap length.
This is the column. L is the length of the column. In the case of column the tension zone is located at L/4 distance from both ends of the column. This zone experiences tension so here we should not provide lapping. The bending moment at the middle portion of the column is zero it means the middle portion of the column is least stressed. Hence, lapping should be provided in the mid-section of the column so that transfer of stresses from bar to bar happen smoothly in this region.
Whereas in case of the beam as I already explained before the top portion of beam experiences compression and bottom portion experience tension. So, the top reinforcement in the beam is left at midspan. As the beam does not experience any negative moment at midspan so lapping is great in this region.
In the case of bottom reinforcement the lapping is provided near the ends of the beam or L/4 distance from column face but should not be in the midpoint of the beam and one last point The lapping should not be provided at joints.
I hope now you understood the concept of Lap Length. If you find this article helpful please don’t forget to share it. If you want to add any information which i have missed in this article you can mention in the comment section.
Finally thanks! for reading the article.