What is the rolling margin of steel?
According to IS-1786
(2008), the percentage difference between the theoretical and actual weight
of a steel bar per meter length must be within the permissible range. The
margin that allows maintaining the section diameter of steel rods must be
within a certain percentage of the weight of the steel produced, called the
rolling margin of steel to its standard weight. In simple words, The actual
unit weight of a steel bar from its standard or theoretical unit weight
percentage deviation of allowable margin is called the rolling margin of
steel.
How do I check the rolling margin in steel?| How to calculate the rolling margin in steel?
We understand the rolling margin with help of example calculation. Before starting the calculation of the rolling margin of steel we want to know about the important point which is
What is the allowable rolling margin of steel as per IS code?
IS-1786 (2008) sets a standard for calculating the rolling edge of steel in percentage, maintaining the safety of buildings, completing construction within a specified time, and controlling the loss of estimated project cost.
According to the IS code tolerance for reinforcement below
According to the
tolerance above the reinforcement weight. The allowable deviation in weight of
reinforcement bars of different diameters according to IS-1786 is as follows.
4 mm to 10 mm dia of a bar, The Batch
weight of Up and Down 7%, Individual weight of Up and Down 8%.
12 mm dia bar to 16 mm dia bar
reinforcement, The Batch weight of Up and Down 5%, individual weight
of Up and Down 6%.
20 mm dia bar to 40mm dia bar
reinforcement, The Batch weight of Up and Down 3%, individual weight
of Up and Down 4%.
8mm to 10mm | ±7% |
12mm to 16mm | ± 5% |
20mm and above | ± 3% |
Ok, During the billing calculation work we take the only standard unit weight of steel. But the weight of steel rod may be overweight or underweight steel
Overweight Steel:
When the weight of the steel is greater than the standard weight of the steel it is called overweight steel. Overweight steel bar contractors face the problem of loss.
1. Invisible loss to the contractor.
2. The steel manufacturing company will make a profit.
Underweight Steel:
If the weight of the steel is less than the standard weight of the steel then the weight is called lightweight steel. Profit is available if the weight of the steel contractor is less.
1. The invisible profit of the contractor.
2. Loss to the steel manufacturing company.
Now, look at the calculation…
Suppose the site requires 100 meters of steel bar. Divide government division pay by unit weight of weight. But contractors pay companies that produce steel based on the net gross weight of the steel. The problem arises when the unit weight of steel is greater than the weight of steel produced by the steel company.
The standard unit weight of steel is calculated using this formula D2/162. With the help of the formula, we calculate the weight of the steel per meter.
For 16 mm diameter, the unit weight per running meter is 162/162 = 1.58 Kg/Meter.
Assume that the unit weight of 16mm diameter steel coming from the base is 5% more than the unit weight. It is necessary to calculate the rolling margin.
Now the actual weight of steel per running meter is the sum of the standard unit weight of steel and 5 % of the standard unit weight of steel, therefore (1.58 + (5/100) x 1.58) = 1.659 Kg/Meter.
The total weight of 100 m steel bar at standard unit weight is 100 x 1.58 = 158 kg.
Now we bring 158 kg of steel to the site. But the actual steel weight is 1.659 kg per running meter. According to the actual unit weight of steel, the length of steel coming to the site is 158/1.659 = 95.238 meters.
The actual loss of length is 100 meters - 95.238 meters = 4.762 meters. Now the loss of the contractor is caused by 4.762 meters in length or 4.762 x 1.58 = 7.523 kg of steel.
This level is called steel overweight. The contractor always incurs a loss when purchasing overweight steel. This is known as an invisible loss. Profit is available if the weight of the steel contractor is less.
Do not carry overweight or underweight steel on construction sites. The overweight or underweight steel level is determined with the help of the limit value given in the table above.
Why it is
necessary to calculate the rolling margin of steel?
When purchasing steel rods for construction work, the weight of the rods obtained at the site will always vary from the theoretical weight.
The constant weight of the steel bar is calculated based on the theoretical diameter and density of the particular bar. But in the practical, in the production of steel rods, it is difficult to obtain the correct bar diameter and density. It may be produced in lightweight, standard weight, or maybe overweight.
Therefore construction steel purchases are always based on weight, the length of overweight bars will vary from the theoretical length, which will ultimately be less than the bar length we need.
This shortage of rebar affects the overall estimated cost of the project, making it more expensive to purchase additional rebar to complete the construction project.
likewise, if the purchased steel rods fall into the lightweight section, it will affect the design strength and stability of the building. The section percentage of lightweight steel wire used in the construction project will be less than the design requirement, which may exceed safety standards. Therefore, to prevent such mistakes, it is necessary to calculate the steel rolling edge.
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