Calculation of reinforcement lap length in concrete structures
Reinforcement lap length is considered as the one of the most important topics in the reinforcement. While arranging the steel in Reinforced concrete structure, when the desired length of single bar is curtailed, the lapping of two bars side by side is performed to retain the desired design length. As a substitute, the mechanical couplers are provided.
Lapping means the overlaying of two bars side by side up to the design length. Normally, the stock length of steel bars is restricted to 12m for delivering the steel bars to the construction site easily. As for instance, suppose, it is required to construct a 100ft tall column, but it will be unavailable practically. Therefore, the bars are cut each second story.
Then the tension forces should be transmitted from one bar to the other bar at the position of cutoff of the bar. Therefore, the second bar should be provided narrowly to the first bar and overlapping is performed. This amount of overlapping among two bars is known as “lap length”.
Generally lapping is achieved where there is limited bending stress. Normally, if both bars contain the same width, the lap length is 50d, that means 50 times the bar diameter.
Lap length in tension:
The lap length along with anchorage value of hooks should be as follow :-
- For flexural tension – Ld or 30d whichever is greater is taken.
- For direct tension – 2Ld or 30d whichever is greater is taken.
The straight bar lapping length must not be below 15d or 20 cm.
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Lap Length in Compression:
The lap length is same as the development length measured in compression but not below 24d.
For Different Diameter Bars:
If the bars of different diameters required to be spliced, the lap length is measured based on the smaller diameter bar.
For bars with a diameter greater than 36 mm, lap splices should not be catered for. In such situations, consideration should be given to welding. But if in some conditions welding is also not attainable, lapping is permitted for bars greater than 36 mm in diameter. But along with lapping, the lapped bars should be supported with additional 6 mm spirals.
Lap length for concrete of 1:2:4 Nominal mix:
The Lapping length in tension (MS bar – mild steel bar) together with anchorage value should be 58d. So eliminating the anchorage value, the lap length becomes = 58d – 2*9d = 40d. (Where 9d denotes hook allowance up to 25 mm and k=2)
The voltage lapping length (MS bar – mild steel bar) should be 58d along with anchorage value. Unless the anchorage factor is eliminated, the lap length will be = 58d - 2*9d = 40d. (Where 9d refers to a maximum capacity of 25 mm and k=2)
Lap length for M20 concrete:
- Columns – 45d
- Beams – 60d
- Slabs – 60d
This means that a minimum lap of 45*20 = 900 mm is required if it is required to lap column bars with a diameter of 20 mm.