When the smaller dia bar is lapped with the larger dia bar, lap length or development length should be calculated based on the diameter of smaller dia bar.
Let’s take an example for this case,
There is a continuous beam which has two members. First member has a longitudinal bar of 25 dia, and second member has a longitudinal bar of 16 dia. Now while lapping, we continue the 25 dia bar into the second member and extend upto the length ‘Ld’ calculated considering the diameter as 16mm. As there is no requirement for 25 dia bar to be extended where the beam is designed as 16 dia.
Here are some ‘Ld’ values for info
I. Lap length for bars in bending tension should not be less than ‘Ld’ or 30d
2. Lap length for bars in direct tension(water tanks) should not be less than 2Ld or 30d
3. Lap length for bars in compression should not be less than ‘Ld’ or 24d
According to IS456:2000
When bars of different are to be spliced, the lap length shall be calculated on the basis of diameter of smaller bar.(page 45)
Guideline for calculating lap length
- for bar greater than 36mm dia welding should be preffered. If welding is not possible than spiral of 6mm bar provided at lap.
- lap length including anchorage value of hooks for bars in flexural tension shall be greater of these two
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- development length (by eqn given in IS 456, section 26.2.1)
- 30*dia of bar
- for direct tension lap length will be greater of these two
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- 2*development length
- 30*dia of bar
- there are few more thing you need to consider like cover, you can check it in IS 456
