Lap length is provided to overlap two rebars in the structural elements such as Columns, Beam and Slab. Lap length is often confused with Development Length. But in fact, both terminologies are different and used in different locations.
During the construction process, rebar length may not be sufficient to cover the length of structure length, in this case, we overlap do rebars to get the desired results, the length of two rebars lapping each other is called lap length.
How To Calculate Lap Length?
Let us assume that we are calculating overlapping for a beam, the beam is facing forces acting on the top surface is called compression (downwards), and the bottom surface faces forces in tension (upwards). The calculation is different for both zones.
Lap Length in Compression:
The rebar overlapping length in compression is calculated as per ACI 12.16.1 as under;
ls = 0.0005fy (d) ; [ For fy less than or equal to 60,000psi]
ls = (0.0009fy – 24)d; [ For fy > 60,000 psi]
A minimum overlapping length of 12” is required for rebar in compression. If reinforcement bars of different sizes are overlapping as compression bars then the lap length must be bigger in the following cases.
Compression rebar length of smallest dimension or compression development length of the biggest dimension bar.
Lap length in Tension:
To calculate the lap in tension, we use the ACI Code 12.15, according to the code, the acting force in tension is transferred from rebars to the concrete and turn back from concrete to the reinforcement. This results in a continuous line of reinforcement.
Consequently, the rebar lap over shows diverse interaction on various factors such as the strength of concrete, grade of steel, location, rebar size, and spacing. According to the ACI 318, the tension bars are classified into two classes, known as Class A and Class B.
Class A Rebar: Lap Length ls = 1.0 ld
Class B Rebar: Lap Length ls = 1.3ld
Where ld is Development Length as per ACI 12.2, a minimum overlap length of 12 inches is required.
Lapping Zone in Different Building Components are as under:
Column Lapping Zone
The column is basically divided into three zones, suppose the column length is L, then we have to distribute the column with (L/4). both ends of the column have a maximum moment at (L/4) length because of lateral forces, so we must avoid lapping there, the ideal zone for the lapping is the center zone. So the rebars can transfer stress smoothly.
All the rebar should be staggered within the center zone as shown in the pictures above. Less than 50 % of the bars should be lapped in the same level, if the value is greater than 50% or more, there is a high chance of collapse and failure.
The stirrups center to center distance must be reduced at the lapping zone as compared to the regular c/c spacing. For example, regular spacing is 150mm then at lapping zone it should not be greater than 125 mm.
Furthermore, if the two bars in the column lapping zone are different sizes then we must follow the smallest diameter bar for lapping. For example, if the lower bar is 20mm and the upper bar is 16mm then we must take the lap length (50d) for 16mm.
50 ( 16mm)= 800 mm
Beam Lapping Zone
The ideal zone for rebar lapping is where the bending moments remain zero for continuous beams or where the values of bending moment cross the x-axis at bending moment diagram or either remain at (L/4) for simply supported beams.
The lapping should be provided on the tension zone as moments will be created with greater tension.
Top reinforcement in the beam is lapped at mid-span since there is no negative bending moment at the mid-span of the beam. For bottom reinforcement, it is lapped alongside the ends because there is no positive moment at the end of the beam (L/4).
Lapping Zone In Slab
There are two types of slabs, One Way slab and two-way slab, if the slab is designed as one way, then the slab functions similar to the beam. It is best to provide lapping bars at the point of the least bending moment or similarly at the point of contra flexure.
Suppose, L is the effective span of the slab. Then practically laps are provided beyond L/5 to L/3 from the support of rebar the bottom reinforcement. The top rebar is short and there are no laps required, however, at no point the overlapping bars shall exceed ⅓ of the total bars.
The two-way slab functions the same but the difference is the reinforcement must be in both directions.