Some useful guidelines on steel bar overlapping in raft foundation
In construction work, steel bar over lapping plays an important role. In this useful construction video tutorial, you will learn how to place lapping in exact position in raft foundation. You will also learn the required quantity of lapping, errors in lapping and how to fix them.
While arranging steel in Reinforced concrete structure, if the desired length of single bar can’t be attained, the lapping of two bars side by side is executed.
Then, it is required to transmit the tension forces from one bar to the other bar at the position of discontinuation of the bar. Therefore, the second bar is retained narrowly to the first bar and overlapping is executed. This amount of overlapping among two bars is known as “lap length”. Lapping is generally provided where least bending stress exists. Usually, lap length is 50d i.e. 50 times the bar diameter, if both bars contain identical diameter.
Lap length in tension:
The lap length along with anchorage value is as follow :-
For flexural tension – Ld or 30d whichever is larger.
For direct tension – 2Ld or 30d whichever is larger.
The straight length of lapping of bars should not be under 15d or 20cm.
Lap Length in Compression:
The lap length equivalent to the development length is measured in compression but not under 24d.
For Different Diameter Bars:
When the bars with several diameters should be spliced, the lap length is measured based on the smaller diameter bar.
Lap splices are not recommended for the bars which contain diameter larger than 36 mm. Under such situation, welding is required. But if welding can’t be done, then lapping should be provided for the bars greater than 36 mm diameter. But together with lapping, extra spirals of 6 mm should be arranged around the lapped bars.
Lap length for concrete of 1:2:4 Nominal mix:
The Lapping length in tension (MS bar – mild steel bar) along with anchorage value is 58d. So, if the anchorage value is deducted, the lap length = 58d – 2*9d = 40d. (Where 9d = hook allowance up to 25 mm and k=2)
Lap length for M20 concrete:
Columns – 45d
Beams – 60d
Slabs – 60d
It presumes that if it is required to lap 20 mm diameter column bars, a minimum lap of 45 * 20 = 900 mm should be arranged.
To get more details, go through the following civil engineering video tutorial.