The reinforcement lap length is one of the important topics. When the size of a single bar is reduced, two bars are lapped side by side to maintain the desired design. Mechanical couplers can be used as a replacement.
Lapping is the process of overlaying two bars together up to their design length. Stock lengths of steel bars are normally limited to 12m to make it easier to deliver the steel bars to the construction site. Imagine, for example, that a 100ft high column is needed, but the steel bars are not available. The bars are, therefore, cut every second story.
The tension forces are then transmitted from the first bar to the second bar at the cut-off position. The second bar is then provided close to the first and overlapped. The “lap length” is the amount of overlap between two bars.
Lapping is usually achieved when there is little bending stress. If both bars have the same width, then the lap length will be 50d. This is equal to 50 times the diameter of the bar.
The length of the lap in tension
The anchorage value and lap length of hooks are as follows:
- If the flexural tension is greater than 30d, then Ld is used.
- Direct pressure is 2Ld or 30, whichever is greater.
The length of the straight bar must not be less than 15d or 20cm.
The Lap Length of Compression
The lap length must be the same as that of the compression length, but it cannot be less than 24d.
To Order Different Diameter Bars
The lap length will be measured using the bar with the smaller diameter if the bars are to be spliced.
Lap Splices
Lap splices are not recommended for bars larger than 36 mm in diameter. In these situations, welding should be considered. If welding is not possible in certain conditions, then lapping of bars larger than 36 mm is allowed. Lapping is permitted, but the bars must be reinforced with 6 mm spirals in addition to lapping.
Concrete of Nominal Mixture:
The lap length (MS bar, mild steel bar), together with the anchorage value, should be 58d. The lap length is = 58d + 2*9d =40d. The hook allowance is up to 25mm in 9d and k=2.
Along with the anchorage value, the voltage lapping length should be 58d. The lap length = 58d + 2*9d =40d unless the anchorage factor has been eliminated. (Where 9d is a maximum capacity of 25 mm, and k=2)
The lap length of M20 concrete
- Columns – 45d
- Beams 60d
- Slabs 60d
If you want to lap a column bar with a 20 mm diameter, then a minimum lap length of 45*20= 900 mm will be required.