Accounting for Wastage and Overlaps in BBS

In Bar Bending Schedules (BBS), accounting for wastage and overlaps is a crucial aspect of ensuring that the correct amount of steel reinforcement is ordered, fabricated, and used in construction projects. Wastage and overlaps can occur during the cutting, bending, and handling of reinforcement bars, and if not properly accounted for, they can lead to material shortages or excess steel being ordered. This article discusses how to account for wastage and overlaps in BBS with practical examples.

What is Wastage in BBS?

Wastage refers to the extra steel that is either cut off or discarded during the fabrication and installation process. This can occur due to incorrect measurements, mistakes during cutting or bending, or due to the requirements for hook lengths and bends that add extra length to the bars. Wastage also includes pieces of bars that are left unused due to irregular shapes or lengths after cutting to the specified size.

What is Overlap in BBS?

Overlap, also known as lap length, is the length of steel bars that are overlapped to provide continuity and strength at the joints of reinforcement bars. Overlaps are used to join two bars together to form a continuous reinforcement throughout the concrete element. The overlap length depends on the type of bar, the diameter of the bar, and the grade of concrete. It is essential to consider the lap length in the BBS to ensure that the bars are joined correctly and that no structural weakness occurs.

Accounting for Wastage in BBS

Wastage is usually accounted for as a percentage of the total amount of steel required for the project. The percentage of wastage can vary depending on the complexity of the project, the type of reinforcement, and the fabrication process. Common wastage percentages range from 1% to 5% of the total steel quantity.

Formula for Wastage:

Wastage = (Total Quantity of Steel × Wastage Percentage)

For example, if a construction project requires 1000 kg of steel and the wastage percentage is assumed to be 2%, then the wastage amount will be:

Wastage = 1000 kg × 0.02 = 20 kg

Therefore, in this case, 20 kg of steel should be added to the total steel requirement to account for wastage.

Example of Wastage Calculation:

Suppose the total steel requirement for a project is 5000 kg, and the wastage percentage is 3%. The wastage would be:

Wastage = 5000 kg × 0.03 = 150 kg

Thus, an additional 150 kg of steel should be ordered to account for wastage.

Accounting for Overlap in BBS

Overlap is typically accounted for by adding extra length to the bars to account for the lap joint. The amount of overlap is determined based on factors such as the type of reinforcement bar, its diameter, and the grade of concrete used in the project. The overlap length is typically specified in the structural design, and it is essential to incorporate this in the BBS to ensure proper bonding of the bars.

Formula for Overlap:

Overlap Length = (Lap Length) × (Number of Bars to be Joined)

In some cases, the lap length for a bar can be specified as a minimum value (e.g., 40 times the diameter of the bar) or as a fixed length. For example, if the lap length is 40 times the diameter of the bar and the bar diameter is 12 mm, the overlap length will be:

Overlap Length = 40 × 12 mm = 480 mm

Example of Overlap Calculation:

Consider a scenario where a project requires overlapping of two bars, each with a diameter of 16 mm, and the specified lap length is 50 times the bar diameter. The lap length calculation would be:

Overlap Length = 50 × 16 mm = 800 mm

This means an additional 800 mm of steel will be used for the overlap to join the two bars together. If multiple bars are to be overlapped, this value is multiplied by the number of bars.

How to Account for Wastage and Overlaps in BBS

To properly account for wastage and overlaps in BBS, the following steps are typically followed:

1. Step 1: Calculate the Total Steel Quantity
   Begin by calculating the total steel required for the project without considering wastage or overlaps. This is usually done by the structural engineer or the quantity surveyor.
2. Step 2: Add Wastage
   Add the wastage percentage to the total steel quantity. For example, if the total steel requirement is 5000 kg and the wastage percentage is 3%, add 150 kg of steel to account for wastage.
3. Step 3: Add Overlap Lengths
   Determine the overlap lengths required for each bar and add this to the total cutting length. For example, if the overlap length for a bar is 800 mm, this length should be added to the total length of bars required in the BBS.
4. Step 4: Adjust for Multiple Bars
   If multiple bars are to be overlapped, multiply the overlap length by the number of bars being joined. Ensure that the total length of bars in the BBS is adjusted accordingly.

Example of Final Calculation:

Suppose a project requires 1000 bars with a diameter of 16 mm. The total length of the bars required is 2000 meters. The lap length is 50 times the bar diameter, or 800 mm per bar. If wastage is calculated at 3%, the final calculation would be:

• Wastage: 3% of 2000 meters = 60 meters of steel.
• Overlap: 800 mm per bar × 1000 bars = 800 meters of overlap.
• Total Steel Required: 2000 meters (initial length) + 60 meters (wastage) + 800 meters (overlap) = 2860 meters.

Conclusion

Accounting for wastage and overlaps is an essential part of preparing Bar Bending Schedules (BBS). Properly considering these factors ensures that the right amount of steel is ordered and used, preventing material shortages or wastage. By calculating wastage as a percentage of the total steel requirement and adding overlap lengths based on the structural design, engineers can accurately estimate the required reinforcement for the project.