Rolling margin of steel, both structural and reinforcement steel, are of immense importance when it comes to construction management. In this article we will see in-depth what is the significance of rolling margin of steel and how to test it at the field. We will also see how to do the calculation step-by-step.

**Content:**

- Definition of Rolling Margin of Steel
- Why Rolling Margin of Steel is Important?
- What is the IS Code for Rolling Margin of Steel?
- How to Determine the Rolling Margin of Steel?
- Rolling Margin Test for Reinforcement Steel or Rebar
- Rolling Margin Test for Structural Steel Sections
- Interpretation of the Rolling Margin Test Result
- Allowable rolling margin of Steel
- Rolling margin of steel example
- Download Excel Format for Rolling Margin of steel Calculation

## Definition of Rolling Margin

Rolling margin can be defined as the difference of the actual weight of a steel section with respect to its theoretical weight. It is the margin allowed as per the standards maintaining the actual weight within a certain percentage of its theoretical weight. This is true for both reinforcement steel as well as structural steel.

## Why Rolling Margin is Important?

Thermo-mechanically treated (TMT) steel bars are used in construction as **reinforcement steel**. The primary function of the TMT bars is to endure the tension in the RCC member, called as the ‘tension steel’, but often the use is designed as ‘compression steel’ also in order to reduce or limit the size of the RC member.

For a TMT bar used as reinforcement (tension or compression both) the area of the steel is important for a particular section. The Designers prepare the design and drawing accordingly, and when Site Engineers places the reinforcement bars at site they do it as per the drawing, where the diameter of the bar, number of bars, and length are mentioned. Hence, if the cross-section of the TMT bar is significantly less or more than what it is supposed to be, there is a chance the RC section becoming under-reinforced or over-reinforced. This is the significance of ‘Rolling margin’, which is a measurement of the theoretical (what it should be) and actual (what it is in reality) weight of the TMT bars i.e. reinforcement steel.

The same is the case for** structural steel** also. In the drawings the Designers mention the particular section and the length. If the sectional weight varies it will be inadequate for the design.

## What is the IS Code for Rolling Margin of TMT bars and Structural Sections?

**A) For Reinforcement Steel (TMT Bars)**

**IS 1786 (2008**) is the reference guide for **High strength Deformed Steel Bars and Wires for Concrete Reinforcement**. We get all the required information for checking and testing the rolling margin of reinforcement steel from this IS code (IS 1786-2008) alone.

**B) For Structural Steel**

IS 808 (1989) is the reference of weight of standard structural steel sections. Hence we can use this Indian Standard code (IS 808-1989) for a reference to find out the rolling margin of structural steel.

## How to Determine the Rolling Margin?

So, to find out the rolling margin of a steel section, structural or reinforcement steel, we need to find out the two things:

- Theoretical weight of the section
- Actual weight of the section

Once we get both of the above data, we need to find out the difference and percentage variation. Let’s see it step-by-step.

## Rolling Margin Test for Reinforcement Steel or Rebar

What is the rolling margin as per is 1786?

First, let us go through the IS 1786 to see what it offers. As we have already discussed, theoretical weight of various diameter of TMT bars (reinforcement steel) are available in this IS Code. The reinforcement steel TMT bars of grades Fe 415, Fe 415D, Fe 500, Fe 500D, Fe 550, Fe 550D and Fe 600 falls within the purview of IS 1786, and in practice also there is no other diameter TMT bars are in use in India.

The nominal sizes of TMT bars/wires as per the IS 1786 are 4 mm, 5 mm, 6 mm, 8 mm, 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, 28 mm, 32 mm, 36 mm, 40 mm. In general, we use TMT bars starting from 8 mm and up to 32 mm diameter, and in special cases we also use 36 mm and 40 mm diameter steel bars for heavy construction works.

## Calculation of Steel Rolling Margin

Rolling margin of steel calculation is done in the following way. The values for the nominal cross-sectional area of the above diameter TMT bars is governed by the formula in IS 1786:

**A = w / 0.00785L**

Where, A = Gross cross-sectional area, in mm2

L = length measured to a precision of +- 0.5% in meter

w = mass weighed to a precision of +-0.5% in Kg

From the above we get,

w = 0.00785 L A

or, w = 0.00785 L. (PI/4 . d2), where d = diameter of the TMT bar, PI = 3.1428

or, w = 0.00617 . L d2

or, w = L . d2 / 162

For, 1 meter length of TMT bar, L = 1

So, **w = d2 / 162**

Now, we put d = 8, 10, 12, 16, 20, 25, 28, 32 (Corresponding to 8 mm, 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, 28 mm, 32 mm diameter TMT reinforcement bars) and get the following table (Refer to table-1 of IS 1786). This is the theoretical weight or theoretical mass of the respective diameter of steel bars.

Now, to test the rolling margin of steel, we need to get a cut-length of 1 meter of a specific diameter of steel. The length of the samples should be at least 0.5 m or 500 mm. For example, let us take the cut-length of 16 mm diameter TMT bar and weigh it. We get actual weight of the cut-length steel as 1.56 Kg.

Now, theoretical weight of the steel bar of 16 mm diameter TMT bar from the above table is, w2 = 1.58 Kg per meter.

Actual weight of the 1 meter cut-length, w1 = 1.56 Kg.

So, the difference of theoretical and actual weight of the reinforcement steel = (w2 – w1)

= (1.56 – 1.58) Kg

= -0.02 Kg

So, the percentage difference = (w1 – w2) / w1 = 0.02 / 1.58 = –**1.27%**

From the above we can say that the rolling margin of the 16 mm diameter bar in this example is **-1.27%**

## Rolling Margin Test for Structural Steel Sections

Refer to IS 808, we get the theoretical weight of all the standard steel sections. Length of the samples should be at least 0.5 m or 500 mm.

For example, from table-2 of IS 808 the theoretical weight of ISMB-100 is 8.9 Kg per meter.

Now, if we get the actual weight of a 1-meter length piece of ISMB-100 as 9.2 Kg,

w1 = 8.9 Kg, w2 = 9.2 Kg

So, Rolling margin of ISMB-100 = (w2-w1)/w1 = (9.2 – 8.9) / 8.9 = **3.37%**

From the result we can say that the rolling margin of the ISMB-100 section in use is **+3.37%**

## Interpretation of the Rolling Margin Test Result

When this percentage is negative, the section is called **under-rolled**. This means the actual weight of the TMT bar is less than the theoretical weight of the steel. The significance of under-rolled section is, if you place this section at site as per drawing you are actually placing less weight, or, less sectional area of steel. This is profitable for the contractors, but if beyond the permissible limit it may be detrimental from the strength point of view. An under-rolled steel section beyond permissible limit is a weaker section and we must avoid it.

When this percentage is positive, the section is called **over-rolled**. This means the actual weight of the TMT bar is more than the theoretical weight of the steel. The significance of over-rolled section is, if you place this section at site as per drawing you are actually placing more weight, or, more sectional area of steel. This is loss-making for the contractors, and if the percentage is beyond the permissible limit it will create a over-reinforced section which is not desirable from strength point of view. An over-rolled steel section beyond permissible limit is also not desirable and we must avoid it.

## Allowable Rolling Margin of Steel

The table-2 of IS 1786 depicts the permissible limit of the Rolling Margin for TMT bars.

As per the tolerance above, deviation in weight of reinforcement bars of different diameters according to IS-1786 is as below:

**4 mm to 10 mm diameter: ** From -7% to +7% for a batch

**12 mm to 16 mm dia**meter: From -5% to +5% for a batch

**20 mm to 40mm dia**meter: From -3% to +3% for a batch

Please note that for individual samples plus side tolerance is not applicable. Rolling margin formula is applicable to both batch and individual samples.

## Rolling margin of Steel Example

**Problem**: In a concrete beam, Designer has given 6 numbers of 20 mm diameter TMT bars (length = 5 meter each) as the main steel. Site Engineer finds that the rolling margin of the 20 mm diameter steel delivered at site is +3%. What is the significance of the same?

**Solution**: As per Table-1 of IS 1786, total theoretical weight of main steel in the beam as per Designer = 6 x 2.469 x 5 = 74.07 Kg.

Since the rolling margin is +3%, the actual weight of the steel is more than the theoretical weight.

Actual weight of steel provided as per drawing (6 nos 20 mm diameter TMT bars @ 5 meter length) = 74.07 + 74.07 x 0.03 = 76.29 Kg

So, there will be a loss of the contracting agency = 76.29 – 74.07 = 2.22 Kg of steel multiplied by the rate of TMT bar.

On the other hand, as per the design, the area of steel at cross-section of the beam = 6 x 314.2 = 1885 sq.mm

Actual area provided = 1885 + 1885 x 3% = 1942 sq.mm, which is more than the required. Hence then no problem from technical point of view.

**Download Excel Format for Rolling Margin of steel Calculation here**

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