Rebar Calculation For Slab: A Step By Step Process

Rebar Calculation for slab

This is a step-by-step process of rebar calculation for a slab.

Following the process, you can easily calculate the quantity of steel required for any type of slab.

So, let’s learn the process of…

Rebar Calculation For Slab

Rebar Calculation for slab

In a building construction project, you need to calculate the quantity of reinforcement for purchasing.

You shouldn’t purchase all the required steel for a project at a time. The reason is, it blocks a huge amount of money. It also requires a large place to stack steel.

For these reasons, you should purchase steel whenever you need them.

In my project, I normally order steel for columns, slabs, and beams of a floor only at a time.

You should do so.

That’s why you need to calculate the quantity of steel for a slab.

For that:

Step-1: Get The Slab Layout Plan

A slab layout plan is a drawing that you’ll get in the structural drawing book.

It somehow looks like the image below:

A simple slab layout plan of a building

As you can see, the slab layout plan often doesn’t show any dimensions.

But you’ll need dimensions for calculating steel quantity. You can get dimensions from the column layout drawing.

And, here is an example of a column layout drawing:

Column Layout

Sometimes, you can get the slab dimension from the architectural drawing.

If you find a slab layout plan with dimensions in your architectural drawing book then you don’t need to calculate dimensions from the column layout plan.

In that case, your task will be easy.

An architectural slab layout plan normally looks like this:

Architectural Slab Layout Plan

If you don’t find a slab layout plan with dimensions just calculate the dimensions from the column layout plan and write them with a pencil in your slab layout plan.

Now…

Step-2: Get The Slab Reinforcement Details

The slab reinforcement details are separate drawing sheets where the details of steel placement of slabs are shown.

And you’ll get these drawing sheets in the structural drawing book.

There are various types of slabs used in a building project. And there are also various types of reinforcement designs are used.

For example, you’ll find some one-way slabs where crank bars are used like the image below:

Example of One Way Slab Reinforcement details

You’ll also find two-way slabs with crank bars in both ways like the image below:

Two way slab reinforcement details

And you’ll find some slabs without any crank bars.

What type of slab reinforcement drawing you get from your structural drawing book, don’t worry, the calculation process is the same.

But you need to…

Step-3: Study The Drawing Carefully

Let’s say, the reinforcement details drawing of our example slab is as the image below:

Bottom reinforcement details of a slab

And

Top Reinforcement Details of a Slab

Look, there are no crank bars used in our example slab. All bars are straight.

There are different sizes of bars are used in different slab panels. The spacing of bars is also different.

Considering all these things, you need to decide how you should start calculating rebars for the slab.

In my case, I first mark each slab panel with a letter like the image below:

Slab panels marked with letters.

(Pro Tip: Don’t use pens for marking. Use a pencil. So that you can erase the mark later.)

I’ll show you the rebar calculation process of the slab panel “A”.

Following the process, you can calculate rebars for the rest of the panels.

Step-4: Rebar Calculation Process Of The Slab Panel-A

4.1: Calculate The Main bars

The main reinforcement of the slab panel “A” is marked as (1).

Bottom Reinforcement details of slab panel A

And the rebar details of (1) is 10mmØ @ 8″ c/c.

First, Get The Number Of Required Bars

The formula is,

Number of bars = (length ÷ spacing) + 1

Dimension of the slab panel A

Here:
The length of the panel “A” = 17′-1½″ or 17.12′
The spacing of bars = 8″ or 0.67′

So,

The number of bars = (17.12 ÷ 0.67) + 1

= 26.55

Say, 27 nos.

Next, Get The Length Of A Bar

The length of a bar is,

= Clear span + (2 × beam width)

Here:

Clear span = 8′-5″ or 8.42′
Beam width = 10″ or 0.83′

So,

The length of a bar for panel “A” is,

= 8.42′ + (2 × 0.83′)

= 10.08′

Say, 11′

[NOTE: you might have noticed, I didn’t deduct clear cover from the bar length while calculating the length of a bar. If it was a crank bar, I’d not add the crank length to get the length of a bar. Forget about all these things. You actually don’t need to think about these things while calculating the quantity of steel for an RCC member. You need these things while getting the actual cutting length of a bar, either for preparing the bar bending schedule or for cutting the rod for actual placement.]

Finally, Calculate The Total Length Of Main Bars For The Panel A

The formula is,

= Number of bars × length of a bar

= 27 × 11′

= 297 feet. (10mmØ)

4.2: Calculate The Bottom Distribution Bars (Binder)

The distribution bar is marked as (2).

Details of the distribution bars of the slab

And the reinforcement details of (2) is 10mmØ@10″ c/c.

First, Get The Number Of Required Bars

The formula is,

Number of bars = (width ÷ spacing) + 1

Width of slab panel A

Here:

The width = 8′-5″ or 8.42′
The spacing of bars = 10″ or 0.83′

So,

The number of bars = (8.42 ÷ 0.83) + 1

= 11.14

Say, 12 nos.

Next, Get The Length Of A Bar (bottom distribution bar)

The length of a bar is,

= Clear span + (2 × beam width)

Calculating the length of the slab's distribution bars

Here:

Clear span = 17′-1½″ or 17.12′
Beam width = 10″ or 0.83′

So,

The length of a distribution bar is,

= 17.12′ + (2 × 0.83′)

= 18.78′

Say, 19′

Finally, Calculate The Total Length Of Distribution Bars

The formula is,

= Number of bars × length of a bar

= 12 × 19′

= 228 feet (10mmØ).

4.3: Calculate The Extra Bottom Bars

The extra bottom bars are marked as (3).

Extra bottom bar details of the slab

And the reinforcement details of (3) is 1-12mmØ in between bars.

Now, Get The Number Of Required Bars

The formula is,

Number of bars = (length ÷ spacing) + 1

Calculating extra bottom bars of the slab panel A

Here:

The length = (17′-1½″) – (1′-8″) – (3′-4″) = 12.12′
The spacing of bars = 8″ or 0.67′ [as the spacing of (1) is 8″]

So,

The number of bars = (12.12 ÷ 0.67) + 1

= 19.09

Say, 20 nos.

Next, Get The Length Of A Bar (extra bottom bar)

The length of a bar is,

= Clear span – deduction from both ends

Calculating the length of a extra bottom bar of the slab

Here:

Clear span = 8′-5″ or 8.42
Deduction from one end = 10″ or 0.83′
Deduction from other end= 1′-8″ or 1.67′

So,

The length of a extra bottom bar is,

= 8.42′ – 0.83′ – 1.67′

= 5.92′

Say, 6′

Finally, Calculate The Total Length Of Extra Bottom

The formula is,

= Number of bars × length of a bar

= 20 × 6′

= 180 feet.

4.4: Calculate The Top Bars

Here are the top rebar details of slab panel A:

Top Reinforcement Details of a Slab

Look:

We have top bars on all four sides of the slab panel.

Mark these sides as North, South, East, and West. Like this:

Calculating top reinforcement of the slab panel A

So that you can easily track which side you calculated the reinforcement for.

With that:

First, Get The Number Of Required Bars

The formula is,

Number of bars = (length ÷ spacing) + 1

Get the number of bars on the NORTH side.

North side reinforcement details of a slab panel

Here:

The length = 8′-5″ or 8.42′
The spacing of bars = 6″ or 0.50′

So,

The number of bars = (8.42 ÷ 0.50) + 1

= 17.84

Say, 18 nos.

Next, get the number of bars on the SOUTH side.

Top reinforcement details of south side of a slab panel

As the clear length and spacing of bars are the same as the NORTH side. So the number of bars on the SOUTH side is the same as the NORTH side.

That is:

18 nos.

Now, get the number of bars on the EAST side.

Top reinforcement details at the east side of a slab panel

Here:

The length = 17′-1½″ or 17.12′
The spacing of bars = 6″ or 0.50′

So,

The number of bars = (17.12 ÷ 0.50) + 1

= 35.24

Say, 36 nos.

And,

Finally, get the number of bars on the WEST side.

Top reinforcement details at the west side of a slab panel

As the clear length and spacing of bars are the same as the EAST side. So the number of bars on the WEST side is the same as the EAST side.

That is:

36 nos.

Next, Get The Length Of A Bar

The length of a bar is,

= Extended length + beam width + hook

>For NORTH side

Calculating length of a top bar at the north side of the slab panel

Here:

Extended length = 4′-4″ or 4.33′

Beam width = 10″ or 0.83′

Hook = 12d = 12 × 12 [d is the diameter of the bar which is 12mm]

= 144mm or 5.67″ [25.4mm = 1″]

0r 0.47′

Say, 0.50′

So,

The length of a extra top bar for NORTH side is,

= 4.33′ + 0.83′ + 0.50′

= 5.66′

Say, 6′

>SOUTH side

Calculating the length of a top bar at south side of the slab panel

Here:

Extended length = (4′-4″) + (5′-2″)

= 9′-6″ or 9.5′

Beam width = 10″ or 0.83′

Hook = 12d = 12 × 10 [d is the diameter of the bar which is 10mm]

= 120mm or 4.72″ [25.4mm = 1″]

0r 0.39′

Say, 0.50′

So,

The length of a extra top bar for SOUTH side is,

= 9.5′ + 0.83′ + 0.50′

= 10.83′

Say, 11′

>EAST side

Calculating the length of a top bar at the east side of the slab panel

Here:

Extended length = 2′-1″ or 2.08′

Beam width = 10″ or 0.83′

Hook = 12d = 12 × 12 [d is the diameter of the bar which is 12mm]

= 144mm or 5.67″ [25.4mm = 1″]

0r 0.47′

Say, 0.50′

So,

The length of a extra top bar for EAST side is,

= 2.08′ + 0.83′ + 0.50′

= 3.41′

Say, 4′

>WEST side

Calculating the length of a top bar on the west side of the slab panel

Here:

Extended length = (1′-10″) + (2′-1″)

= 3′-11″ or 3.92′

Beam width = 10″ or 0.83′

Hook = 12d = 12 × 10 [d is the diameter of the bar which is 10mm]

= 120mm or 4.72″ [25.4mm = 1″]

0r 0.39′

Say, 0.50′

So,

The length of a extra top bar for WEST side is,

= 3.92′ + 0.83′ + 0.50′

= 5.25′

Say, 6′

Finally, Calculate The Total Length

The formula is,

= Number of bars × length of a bar

>For NORTH side

= 18 × 6′

= 108′ (12mmØ)

>For SOUTH side

= 18 × 11′

= 198′ (12mmØ)

>For EAST side

= 36 × 4′

= 144′ (10mmØ)

>For WEST side

= 36 × 6′

= 216′ (10mmØ)

4.5 Calculate The Top Distribution Bar

Top distribution bars aren’t shown in our drawing.

But It’s shown in the reinforcement details.

Reinforcement details of our example slab

Wherever the top reinforcement bars, we need to provide distribution bars there.

If it was shown in the drawing, it would look like (yellow color):

Example of distribution bars of a slab panel

With that:

1. Get The Number Of Required Bars

The formula is,

Number of bars = (length ÷ spacing) + 1

>For NORTH side:

If we imagine the distribution bars of NORTH side it’ll look like:

Distribution bars in north side of the slab panel

Here:

The length = 4′-4″ or 4.33′
The spacing of bars = 10″ or 0.83′

So,

The number of bars = (4.33 ÷ 0.83) + 1

= 6.21

Say, 7 nos.

>For SOUTH side:

Imagine the distribution bars of the SOUTH side. it’ll look:

Distribution bars in south side of the slab panel

Here:

The length = 10′-4″ or 10.33′
The spacing of bars = 10″ or 0.83′

So,

The number of bars = (10.33 ÷ 0.83) + 1

= 13.44

Say, 14 nos.

>For EAST side:

The distribution bars of the EAST side will look like this:

Distribution bars in East side of the slab panel

Here:

The length = 2′-1″ or 2.08′
The spacing of bars = 10″ or 0.83′

So,

The number of bars = (2.08 ÷ 0.83) + 1

= 3.50

Say, 4 nos.

>For WEST side:

The distribution bars of the WEST side will look like this:

Distribution bars in West side of the slab panel

Here:

The length = 4′-9″ or 4.75′
The spacing of bars = 10″ or 0.83′

So,

The number of bars = (4.75 ÷ 0.83) + 1

= 6.72

Say, 7 nos.

2. Get The Length Of A Bar

Here, the length of a bar is,

= Clear span of the slab + 2 × beam width.

For:

>NORTH side

The length of a distribution bar is,

= (8′-5″) + 2 × 10″

Calculating the length of a distribution bar at north side of the slab panel

= 10.08′

Say, 11′

>SOUTH side

The length of a distribution bar on the south side will be the same as the north side.

That is 11′.

>EAST side

The length of a distribution bar in east side is,

= (17′-1½″) + 2 × 10″

Calculating the length of a distribution bar at east side of the slab panel

= 18.78′

Say, 19′

>WEST side

The length of a distribution bar in west side is the same as the east side.

That is: 19′.

3. Calculate The Total Length of Top Distribution Bars

The formula is,

= Number of bars × length of a bar

>NORTH side

= 7 × 11′

= 77′ (10mmØ)

>SOUTH side

= 14 × 11′

= 154′ (10mmØ)

>EAST side

= 4 × 19′

= 76′ (10mmØ)

>WEST side

= 7 × 19′

= 133′ (10mmØ)

So far, we have calculated all the bars in slab panel “A”.

Now it’s time to…

4.6: Summarize All The Bars

  • Main bars = 297 feet (10mmØ)
  • Bottom distribution bars = 228 feet (10mmØ)
  • Extra bottom bars = 180 feet (12mmØ)
  • Top bars in north and south side = 108+198= 306 feet (12mmØ)
  • Top bars in east and west side = 144+216= 360 feet (10mmØ)
  • Top distribution bars = 77+154+76+133= 440 feet (10mmØ)

So, the total:

  • 10 mmØ bars = 297+ 228+360+440 = 1325 feet.
  • 12 mmØ bars = 180+ 306 = 486 feet.

As you know, steel is sold as kilogram in the market. So, convert the length into kilograms.

  • 10 mmØ bars = 1325′ × 0.19 = 251.75 kilo. Say, 252 kilogram.
  • 12 mmØ bars = 486 × 0.27 = 131.22 kilo. Say, 132 kilogram.

As you can see, summarising all the bars is a little bit complicated. So I’ve developed a format to make the thing easy. you’ll find instructions on using the format within the format itself.

Download the Slab CalCulation Format from my free resource library (Freebie-5).

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