During placing the steel for
R.C. structure if the required length of bar is not available then lapping i.e.
placing two bars side by side for a designed length is done. This designed
length is the development length for the particular group or type of steel.
The lap length including
anchorage value of hooks in tension shall be L4 where L4
is the development length in tension. The straight length of the lap shall not
be less than 15 d or 20 cm.
The lap length in compression
shall be equal to the development length in compression but not less than 24 d.
Where two bars of two different
are to be spliced, the lap length shall be calculated on the basis of diameter
of the smaller bar. Lap splices shall be considered as staggered if the centre
to centre distance of the splices is not less than 1.3 times the lap length
calculated and described above.
For ordinary concrete 1:2:4
nominal mix the lap length for tension with plain Grade-1 M.S. bar and below is
the development length 58d including anchorage value. So excluding anchorage
the lap length = 58 – 2 x 9 d = 40d. 9 d is the value for hook allowance for
bars up to 25 mm with a value of K = 2. For HYSD bars the lap length with 1:2:4
concrete is 68D including anchorage. So excluding anchorage the lap length = 68
d – 2 x 13d = 42d. 13d is the hook allowance for deformed bars up to 25 mm dia.
Having the value of K = 4.
