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Lap length or overlapping length is the length at which two bars of a column are spliced or overlapped in order to maintain the continuity of the bars throughout the length of the column. This method of overlapping two reinforcement bars is called lap splicing. Lapping of reinforcement bars facilitates the secure transfer of loads from one member to another.
The maximum length of reinforcement bars available in the market is 12 m. When it is necessary to lay reinforcement for a structure greater than 12 m, two or more reinforcement bars are joined to achieve the required length. This connection is performed by lapping or splicing method.
Lapping is either staggered, welded, or provided through couplers, as shown in Figure-1. Lap splicing is provided on the lapping zone of the columns and the lapping length is dependent on the grade of the concrete and the steel reinforcement.
This article explains the lap length of reinforcement in columns, the location of lap splicing, and the important points to remember while performing lap splicing.
Lapping Zone of Columns as per IS 13920: 1983
As per Indian standard codes, the columns are divided into three zones as shown in Figure-2 below. The zones are formed by considering the moment diagram of the column under lateral load.
As shown in Figure-2, due to the action of load, the column is subjected to a maximum moment at the ends and zero moment at the center. Hence, the top and bottom portion of the column subjected to maximum moment is categorized as Zone-A, and the portion at which the moment is reducing to zero is categorized as Zone-B. Therefore, the less critical section of the column, Zone-B, forms the lapping zone of the column.
As shown in Figure-3, if the clear distance of the column is hc, then Zone-A has a length of hc/4. Then the height of Zone-B is given by (hc- (h/4 + hc/4)) = hc/2.
These zones are classified in order to provide special confining reinforcement as a part of ductile detailing of R.C.C columns.
Lap Splices for Columns
The important considerations on length, location, and position of lap splices as per IS 456-2000, clause.220.127.116.11 are explained below.
- Lapping is not used for bars greater than 36 mm. For larger bars, connections can be made either using welding, couplers, or welding with helical reinforcement.
- Lapping is not provided on the tension zones (Zone-A).
- The general lap length for column reinforcement is the greater of development length Ld and 30d, where ‘d’ is the diameter of the reinforcement bar. This value is considered if the structural designer does not provide a lap length value.
- The lateral reinforcement or the links are provided closely in the lap length region. As per the ductile reinforcement concept, closely spaced special confined reinforcement is also provided at a distance lo from the top and bottom of the column, as shown in Figure-3. As per IS 13920-1993, clause.7.4.1, ‘lo’ is hc/6 or 450 mm (minimum value is taken).
- When bars of two different diameters are to be lapped, the lap length is calculated for the bar with the smaller diameter.
- The lapping must not be performed at the column-beam junction because the area experiences maximum stress.
- The lapping must be provided alternatively so that the lapping of adjacent rebars does not come in a sequence.
- The rebars are staggered and lapped correctly so that they maintain the clear cover of the column.
- The lapping length provided must not be less than 75 mm.
As shown in Figure-4, the reinforcement bars are spliced by an offset in order to maintain the cover of the column throughout its depth. This bending of bars is also called joggle. This is given by SP 34: 1987, "Handbook on Concrete Reinforcement and Detailing."
The crank length is the bend provided to the reinforcement to maintain the clear cover (Figure-4). Generally, a maximum crank slope of 1:10 and a minimum length of crank of 300 mm is provided.
Crank length = (d1+d2+5)*10 mm
d1 is the diameter of the smaller bar and d2 is the diameter of the longest bar.
Lap length or overlapping length is the length at which two bars of a column are spliced or overlapped in order to maintain the continuity of the bars throughout the length of the column. This method of overlapping two reinforcement bars is called lap splicing.
The maximum length of reinforcement bars available in the market is 12 m. When it is necessary to lay reinforcement for a structure greater than 12 m, two or more bars are connected to achieve the required length. This connection is performed by lapping. Lapping of reinforcement facilitates secure transfer of loads from one bar to another bar.
The crank length is the bend provided to the reinforcement to maintain the clear cover of the concrete structure. Generally, a maximum slope of 1:10 and a minimum length of crank of 300 mm is provided. It is also determined by the formula:
Crack length = (d1 + d2 + 5) x 10 mm
Where d1 and d2 are the diameters of smaller and larger rebar used to lap.
When two bars of different diameter are lapped, the lap length corresponding to smallest diameter bar is determined and lapped.