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.
- 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 length (In lap splices and lap welding)
- Using spirals or closely-spaced stirrups around the length of the stirrups.
Type of Lapping Method:
- The lapping of bars (lap splice)
- Welding of bars (welded splice)
- Mechanical connection.
What is Development Length?
- The development length is defined as the length of the bar required on either side of the section under consideration. To develop the required stress in steel at that section through a bond.
- The calculated tension or compression in any bar at any section shall be developed on the peach side -of the section by an appropriate development length or end anchorage or by a combination thereof. as per IS code 456-2000, Page-42, Cl. 26.2
- The preceding discussion suggests this concept of development length of a reinforcing bar.
- The development length is defined as the length of embedment necessary into developing the full tensile strength of this bar, controlled by either pullout or splitting.
- With reference to as per below fig, the moment, and therefore the steel stress, is evidently maximum in point a (neglecting the weight of the beam) and zero at the supports.
- If the bar stress is f, at a, then the total tension force Abl, must be transferred from the bar to this concrete in the distance 1 by bond forces.
- To fully develop this strength of the bar, the distance one must be at least equal to the development length of this bar, established by tests.
- In the beam of as per above fig, if the actual length /is equal to or greater than the development length Id, no premature bond failure will occur.
- The beam will fail at bending or shear rather than by bond failure. This will be so even when, in the vicinity of cracks, local slip may have occurred over small regions along this beam.
- It is seen that the main requirement for safety against bond failure is this: the length of the bar, from any point of given steel stress (L or at most f) to it’s nearby free end, must be at least equal to its development length.
- If this requirement is satisfied, this magnitude of the nominal flexural bond force along the beam, as given by Eq. (as below), is of only secondary importance, since the integrity of this member is ensured even in this face of possible minor local bond failures.
U = V/JD
- However, if this actual available length is inadequate for full development, special anchorage, such as by hooks, must be provided.
Also, read: Trapezoidal Footing Formula with Calculation
Difference Between Lap Length and Development Length
|Sr.No.||Development Length||Lap Length|
|1||Length of reinforcement bar required to transfer load or stress developed in a reinforcement bar into the concrete.||Length of reinforcement bar required to completely transfer load or stress from one bar to another bar|
|2||It is denoted by Ld||It minuteness the continuity in the structure|
|3||It is always provide at the end of beam, column, or slab||It is provided when the length of bars ends.|
|4||It is also useful for providing grip & support & doesn't allow slipping||It is transferred to continuity and also transfers stees from one bar to another bar.|
|5||This is bound between concrete and reinforcement.||This is a bond between reinforcement and reinforcement.|
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