Updated March 4th, 2018
Companies often use Software for estimating the project cost. They more often do this for biding purpose.
But we are construction professionals and working on field level. So we can’t always use software for estimating materials for the small part of a building. Sometimes it is impossible to construct the whole part of a building at a time because of the critical condition of the project.
Let me clear this.
Suppose, there are 30 numbers of isolated footing in a building. Whatever the reason, sometimes we can not make the all 30 numbers of footing ready for casting at a time. At that situation, we just make 3 or 4 numbers of footing ready for the casting. And it is not economical to cast this small quantity of concrete with readymix. So we always prefer machine mix concrete for this purpose.
For constructing footing of a building you have to have plan before starting the task. You have to estimate the materials for footing. You have to make purchase requisition for materials and ensure the required materials are available on the project.
The question is, what is isolated footing and what are the materials required for a isolated footing?
Isolated footing is a shallow type footing. It normally holds one column on it. To know more about the types of footing read the following post –
Various Types of Foundation
Materials Required for Isolated Footing
For constructing a isolated footing following materials are required –
- Shutter Material
- MS Rod
- Binding Wire
- Cover Block and Chair
Shutter Material for Isolated Footing
Most of the time wooden shutter materials are used for the isolated footing. Sometimes steel shutter materials are also used. We will use wooden shutter for this purpose, at least in this post.
How to Estimate Wood for Shuttering Isolated Footing?
See the above picture. The length of footing is 5 feet, width is 4 feet and height is 1 feet. To make the shutter for side-A of the footing you’ll need 4 feet long and 15 inches height wooden plank.
Our footing height is 1 feet. Then why we use 15 inches height wooden plank? Because we will fix the shutter below 3 inches from the bottom of footing.
Then the required wooden shutter for side-A is,
4′ x 15″ = 5 sft (12″ = 1 feet, sft= square feet)
We’ll use 1.5″ thick wooden plank.
So the volume of the wood is,
5 sft x 1.5″ = 0.63 cft (cft= cubic feet)
Wood is always measured as cubic feet.
For the side-C we’ll require the same quantity of wood as side-A.
Now, lets calculate the wood for side-B.
Length of wood for side-B is, 5′-3″. But our footing length is 5 feet then why we’ll require 5′-3″ long shutter? Because we’ll join this shutter with the side-A’s and side-C’s shutter. The thickness of side-A and Side-C shutter is,
1.5″ + 1.5″ = 3″
Length of side-B shutter is 5′-3″ or 63″
Width of side-B shutter is 1′-3″ or 15″
Thickness of shutter is 1.5″
volume of side-B shutter is,
63″ x 15″ x 1.5″ = 0.82 cft (cft= cubic feet).
Side-D also require the same quantity of wood as side-B.
So far, we have found –
For side-A = 0.63 cft
For side-B = 0.82 cft
For side-C=side-A=0.63 cft
For side-D=side-B=0.82 cft
Total wood= A+B+C+D= 2.92 cft
We will require 2.92 cft wood for making shutter for the isolated footing. If you don’t find required size of wood then you’ll need some 2″ x 1.5″ size wooden plank for making shutter.
We estimated required wood for one isolated footing. If you have 30 isolated footing of different sizes calculate the required wood for each one using same technique we applied here. Total them all and make purchase requisition for the required wood.
Now we’ll estimate the MS rod for the isolated footing.
How to Estimate MS Rod for Isolated Footing?
See the above picture. There is a instruction for rod placement. That is 12mmØ[email protected]″c/c both way.
It means 12mm diameter bar should be placed at the distance of 4″ centre to centre.
The length of our isolated footing is 5 feet. The clear cover of reinforcement is 3 inches for one side. For both side it is 6 inches. So we have to place rod for 5′ – 6″=4.5′.
Requires number of rods are, 4.5′ divided by 4″.
4.5’/4″= 13.63 numbers.
Say 14 nos.
You have to add one more number of rod for starting point.
So total required number of rod is 14+1= 15 nos.
Now we have to calculate the length of rod. We’ll place the rod along short-side (4′ side).
So the length will be
4′ – clear cover of both side + hook (90 degree bend) length of both side.
= 4′ – 2×3″ + 2×6″
= 4′ – 6″ + 12″ (12″ = 1 feet)
So total length of all 15 numbers of bar is
15 x 4.5′
= 67.5 feet
Now we’ll calculate the rod for long-side of the isolated footing.
By following the above method,
Number of rods,
= 3.5’/4″ + 1
Say 12 nos
Length of rod,
= 4.5′ + 1′
Total length of rods for long-side of isolated footing,
= 5.5′ x 12
= 66 feet
Total rods for the isolated footing = 67.5′ + 66′
= 133.5 feet.
Say 134 feet
You have to add 10% more rod with the estimated rod during ordering for purchase.
You may not order rod as running feet. So you have to convert them in kilogram or ton.
Oh. One more thing, you have to order rod for short column also. Because rebar of short column should be placed before casting footing.
I’ll write about estimating rod for column some other day here in this blog. So subscribe our Email newsletter to get the update when it happens.
Summary of MS Rod Calculation
For rods along short-side
Numbers of rod= 4.5’/4″ +1= 14.64 say, 15 nos.
Length of rod= 4′ – 6″ + 1’= 4.5′
Length of total rods=15 x 4.5=67.5′.
For rods along long-side
Numbers of rod= 3.5’/4″ + 1= 11.61 say, 12 nos.
Length of rod=5′ – 6″ + 1′ = 5.5′.
Length of total rods=12 x 5.5=66′
Total rod for the footing= 67.5′ + 66’=133.5′
You’ll need a 10mm Ø bar for binding the main rod of the footing. The length of the binder is,
2 x 4.5 + 2x 3.5= 16′.
So, total required rod is,
- 10mm Ø bar= 16′, and
- 12mm Ø bar= 133.5′
Binding Wire for Isolated Footing
For binding rods you need binding wire. We need 9 kg to 13 kg binding wire for 1 ton of rod. You can estimate average 10 kg binding wire per ton of rod. So estimate total binding wire depending on required rod.
Cover Block and Chair for Isolated Footing
To keep the rod in place and maintaining concrete clear cover you should provide either cover block or chair for bottom and cover block for side. Cover block should be placed at 2 feet distance.
You’ll need 8 numbers of cover block for sides and 5 numbers of cover block/chair for bottom of the Isolated footing.
Concrete for Isolated Footing
The size of our isolated footing,
Length – 5 feet
Width – 4 feet
Height – 1 feet.
Volume of concrete for the footing is,
5 x 4 x 1
= 20 cubic feet.
If you want to estimate ingredients for the footing first know the ratio of concrete suggested in the structural drawing. Then read the following post.
I’ve just shown the estimating technique for one isolated footing here. You may have many footings of different sizes in your project.
Follow the same technique to estimate materials for all the isolated footings of your project and sum up them to order materials for purchase.
Some symbols and terms I used here in this post
‘– Foot symbol
“ – Inch symbol
Ø – Diameter symbol
1′ = 12″
Sft – Square feet
Cft – Cubic feet
Nos – Numbers
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