What is Lap Length?
Lap Length is required when bars placed short of their required length (due to nonavailability of longer bars) need to be extended.
- Lap Length is also required when the bar diameter has to be changed along the length (as is sometimes done in columns).Branding
The purpose of ‘Lap’ is to transfer the axial force effectively from the terminating bar into the connecting bar with this same line of action in the junction.
This invariably introduces stress concentrations at this surrounding concrete. These effects should be minimized by
- Using proper splicing techniques.
- Keeping these lapping locations away from sections with high flexural/shear stresses. and
- Staggering the locations of splicing at the individual bars of a group (as typically in a column).
When splicing in such situations becomes unavoidable, special precautions need to be employed, such as
- Increasing the length of the lap (In lap splices and lap welding)
- Using spirals or closely-spaced stirrups around the length of the stirrups.
Lap Length As Per IS 456
Generally, development length is 41d where d is the diameter of the bar. For direct tension, the lap length should be 2 Ld or 30d whichever is greater is considered. In this case, the straight length of the lapping bar shall not be less than 15d or 20cm.
Also, read: What Is Lintel | Type of Lintel
Type of Lapping Method:
- Lapping of bars (lap splice)
- Welding of bars (welded splice)
- Mechanical connection.
1. Lap Splices
Lap splices are achieved by overlapping this bars over a certain length, thereby enabling this transfer of axial force from the terminating bar into the connecting bar through the mechanism of anchorage (development) bond with the surrounding concrete (As per below fig)
A) Lap splice action through development bond
The splitting and cracking behavior observed in lap splice tests are found to be similar to those in anchorage bond tests (As per below fig).
B) Use of spirals in lap splices for large diameter bars
The Lap splices are usually not permitted for very large dia. Bars (Ø > 36 mm), for which welded splices are recommended. However, where welding is not practicable, the Code (IS Code 456-200 Page No- 45, Cl. 220.127.116.11a) permits lap splices with additional spirals around the lapped bars (As per below fig).
C) Staggered splicing of bars
It is desirable to bend the bars slightly (particularly large diameter bars) near the splice location in order to ensure a collinear transfer of force (without eccentricity), as shown above, fig A.
The Code specifies that the straight length of the lap should not be less than 15Ø or 200 mm.
As the force transfer is through development bond, the lap length should at least be equal to the development length Ld.
The Code (IS Code 456-200 Page No- 45, Cl. 18.104.22.168c) specifies a lap length of 2Ld in situations where the member is subjected to direct tension.
In no case should the lap length be less than 30Ø under flexural or direct tension and 24φ under compression.
When bars of two different diameters are to be spliced, the lap length should be calculated on the basis of the smaller diameter.
Splices in tension members shall be enclosed in spirals made of bars not less than 6 mm diameter with the pitch, not more than 10mm.
In the revised Code, some additional clauses have been incorporated (IS Code 456-200 Page No- 45, Cl. 22.214.171.124c) to account for the reduction in bond strength with regard to rebars located near the top region.
When lapping of tension reinforcement is required in the top of a beam (usually near a continuous support location or a beam-column junction), and the clear cover is less than twice the diameter of the lapped bar, the lapped length should be increased by a factor of 1.4.
If the rebar is required into turn around a corner (as in an exterior beam-column junction), the lapped length should be increased by a factor of 2.0.
This factor can be limited to 1.4 in the case of corner bars when the clear cover on top is adequate, but this side cover (to the vertical face) is less than twice this dia. of the lapped bar.
When more than one bar requires splicing, care must be taken to ensure that the splicing is staggered, with a minimum (center-to-center) separation of 1.3 times the lap length, as indicated in as per above fig (c).
It is also desirable to provide (extra) transverse ties (especially in columns), connecting the various longitudinal bars in the spliced region.
In the case of bundled bars, the lap length should be calculated considering the increased Ld, and the individual splices within a bundle should be staggered.
2&3. Welded Splices and Mechanical Connections
Welded splices and mechanical connections are particularly suitable for large diameter bars.
This results in reduced consumption of reinforcing steel. It is desirable to subject such splices to tension tests in order to ensure the adequacy of strength
Welding of cold-worked bars needs special precautions owing to the possibility of a loss in strength on account of welding heat.
The Code (IS Code 456-200 Page No- 45, Cl. 126.96.36.199) recommends that the design strength of a welded splice should, in general, be limited to 80 percent of the design strength of the bar for tension splices.
Butt welding of bars is generally adopted in welded splices. The bars to be spliced should be of the same diameter.
An additional two or three symmetrically positioned small diameter lap bars may also be provided (Especially when the bars are subjected to tension) and fillet welded to the main bars.
Even in the case of ‘lap splices,’ lap welding (at intervals of 5φ) may be resorted to in order to reduce the lap length. End-bearing splices are permitted by the Code (IS Code 456-200 Page No- 45, Cl. 188.8.131.52) for bars subject to compression.
This involves square cutting the ends of this bars and welding the bar ends to suitable bearing plates that are embedded within the concrete cover.
Bars Bundled in Contact of Reinforcement
The development length of each bar of bundled bars shall be that for the individual bar, increased by 10 percent for two bars at contact, 20 % for three bars in contact, and 33 percent for four bars in contact. (Is Code 456:200 Page-43, Cl-184.108.40.206)
Most Important Point of Lap / Maximum Manual Lapping
Lap splices shall not be used for bars larger than 36 mm; for larger diameters, bars may be welded; in cases where welding-is not practicable, lapping of bars larger than 36 mm may be permitted, in which case additional spirals.(Is Code 456:200 Page-45, Cl-220.127.116.11)
Lap Length of Column & I.M.Point
(Is Code 456:200 Page-48, Cl-26.5.3)
- The bars shall not be less than 12 mm in dia.
- The minimum number of longitudinal bars provided at a column shall be fou in rectangular columns and six in circular columns.
- The spacing of longitudinal bars measured along the periphery of the column shall not exceed 300 mm.
- Lap length for Columns – 45d.
Lap length for Beams & I.M.Point
(Is Code 456:200 Page-48, Cl-26.5.2)
- The diameter of reinforcing bars shall not exceed one-eighth of the total thickness of the slab.
- The mild steel reinforcement in either direction at slabs shall not be less than 0.15 percent of the total cross-sectional area. However, this value may be reduced to 0.12 percent when high strength deformed bars or welded wire fabric are used.
- Lap length for Slabs -60d.
Lap Length for Slabs & I.M.Point
(Is Code 456:200 Page-46, Cl-26.5.1)
- Where the depth of the web at a beam exceeds 750 mm, side face reinforcement shall be provided along with the two faces.
- The total area of such reinforcement shall be not less than 0.1 percent of the web area and shall be distributed equally on two faces at a spacing not exceeding 300 mm or web thickness, whichever is less.
- The transverse reinforcement in beams shall be taken around the outer-most tension and compression bars. In T-beams and I-beams, such reinforcement shall pass around longitudinal bars located close to the outer face of the flange.
- Lap length for Beams – 60d.