ESTIMATION AND ANALYSIS

Namaskar and welcome to this series of lectures under the course Principles of Construction

Management and today, we will talk about estimating quantities from a drawing.

Basically, if you look at this simple example, let us say, a boundary wall. Now, our objective

is to calculate what is the cost of this simple boundary wall. So, once we understand the

principles involved and the method of doing it, you can extend it to more complicated

projects. If you look at this boundary wall, what are the components involved. . There

are these piers which we can see or pillars; then there is this wall which is there in

between these pillars; we have these barbed wires which are running through the pillars

above a certain height. So, from the ground level to this level, we have the wall and then there are these barbed wires which are running at a certain distance.

Obviously, we must imagine that the wall does not stop here. There is some kind of a foundation

which has been dug below the ground. So, this gives us an idea as to what goes on in the

construction of this simple boundary wall. So, now in order to estimate its cost, we should know what the cost is related to. The

first thing that comes to mind is (a) to list what are the different items of work involved,

then, (b) what is the quantity of the different items involved, and finally, (c) what is the

unit rate of the different items. So, what we will do today is look at (a) and (b) of

this outline first, and, then come back to (c) which is the unit rates after a small

break. Basically, what we are trying to say here is the following. That if there are items A, B, C and D, first of all, we have to make

a list of these items. What are the items involved, then, what is the quantity of these

items? Maybe 100 square meters, 150 square meters, 300 cubic meters, 50 running meters,

and so on. So, there has to be the square meters, square meters, cubic meters, may be

meters, and so on, and then finally, we will have to have, what is the unit rate for A.

So, for example, A might cost 10 rupees, B might cost 20, C might cost 25, and so on.

So, once we have this picture, then, we will be able to figure out what is the total cost

of this project. So, now, let us move on and try to take a look at using appropriate drawings, and, trying

to find out the items and the quantities of the various items for this boundary wall.

Now, here is the boundary wall that we talk about. I have included this picture just to

give you an illustration as to how this picture which is not to scale and which is just a

photograph, as a matter of fact, gets translated to what is called the ‘plan’ view. So,

this is the plan view of the boundary wall that we are going to do the quantity estimation

for. And what it says is that there is a 230 mm thick boundary wall, has reinforced concrete

pillars, placed at 2.5 meters center to center. So, these are the pillars that we are talking

about. In between here, we have the brickwork. So, what is happening here is that instead

of these kind of panels being used in the wall, we are using brickwork and the length

of the opening is 5 meters. So, we have a 20 meter center to center distance here, a

10 meter center to center distance on the other side and we have a 5 meter opening which

could possibly be used to install a gate and so on, at some point in time. So, the gate is not really part of our discussion today.

So, now let us look at more drawings related to this to get an idea of what actually the

work entails. Here are the details of the wall. So, this is a 230 thick wall which we

are talking about. There is a ground level. We must remember that these dimensions are all in millimeters and the drawing is, obviously, not to scale. We also say that there is a

15 mm thick plaster in 1:6 mortar. So, there is a mortar which is specified and the thickness

of a plaster which we apply on the brickwork, whether it is on both sides or one side that

remains to be seen, that depends on what the kind of instructions we have. Then, there is this brick work in 1:3 mortar. So this is the brickwork in the boundary wall

and in the bed joints, we will use 1:3 mortar and in the plaster we will use 1:6 and this

wall which is 230 thick as far as the portion above the surface and a small portion here

which is below the surface is concerned, it is supported by a 400 thick here and a 500

thick here and a 600 thick support or an offset for different heights. This entire brick structure

is placed on a 900 wide PCC which is Plain Cement Concrete made with 1:4:8.

So, some of these terminologies are taken as prerequisites to this course. We would

expect that you have done some basic course in concrete construction, of concrete materials before you come to this course on management, and I would leave it at that.

Now, continuing from here, if we look at the details of the RC pillar and the foundations.

So, now, as far as the RC pillar is concerned, the best place to see would be this section

A-A which is given here. So, this is 230 by 300 thick in M25 concrete and we come back to this. So, the pillar is

a reinforced concrete pillar which has 6 of 16 mm bars and then, there are stirrups which

are 8 mm diameter placed at 175 center to center. So, this is how this drawing looks

like. So, now this pillar which is basically a small column and is cast into this footing which is also made with M25 concrete and measures

800 by 800 as given here. As far as the reinforcement of the footing is concerned, it is given to

be 12 millimeter bars placed at 100 mm center to center both ways, and this is the same

PCC that we talked about, and we see here that the offset from PCC to the RCC is 50

mm. The thickness of this footing is given is 250. So basically, the idea of going through this drawing in some detail is to take you through

the steps of how to read the relevant details from the drawing. So, it is basically simple

exercise in reading the drawing. Please understand that since it is not to scale which is obvious

from the fact that this point here where the top of the footing is shown as 875mm and the

top of the pillar here is at 1.8 meter from this Zero. So, this distance is given as 1.8

meter, whereas this distance is shown to be 875mm. So obviously, this drawing is not to

scale. Now, as far as the RC pillar is concerned, there is another small addition to this which is this ISA 100×100×8. This is the angle

with holes here to support the barbed wires which are going to run all along the boundary

wall. So, this angle projects 400mm and the drawing does not show the embedment, but let

us read something else. So, moving on from this embedment, there is a section A which

we have already done. Now, there is this detail C, which we can see here. It shows how a stirrup

should be bent. It tells you the angle, it tells you the length and, all these kind of details, which will become very relevant, when we talk about it at a later point in

this discussion. So, moving forward let us take a look at some additional notes. Excavation is to be calculated

allowing 200 millimeters on both sides beyond the minimum width required. So, what it really

means is that suppose, there was PCC which had a width of 900 somewhere. Now, what we

are allowing here through this provision is that the excavation should or could be carried

out which is 200 millimeters beyond this point, in order to facilitate the construction work.

So, when you are calculating the earthwork quantities, as we will at a later point in time, this 200 millimeter is the extra width which we are allowing. As mentioned, this

allowance is made to facilitate the construction. All concrete is in M25 grade concrete, concrete

pillars are also to be plastered, RC pillars are supported on isolated footings measuring

800 by 800 square footings, there is a clear cover of 40 mm wherever required. Those of

you who have done courses in reinforced concrete design would know that whenever we have a

column, or, we have a beam, and, we provide steel reinforcement in it, there is this minimum

distance which has to be provided for, in order to protect the reinforcement from the

environment and also ensure appropriate structural action, and, this cover has been specified

to be 40 mm in this case. Carrying on with the notes, this picture is there just to illustrate that this is what

we are talking about. The original and the final grounds on both sides of the boundary wall are the same. So basically, what we are saying is that there is a ground here, we

are going to make an excavation here to make PCC or whatever it is, and finally there will be the wall that will come out, and, this part of the wall has been con, so, if the

wall has been constructed, so, this part will be what is called ‘backfill’ and the ground

level restored to the original level which is the same on both sides of the boundary

walls. So, this is our boundary wall to begin with, and, also at the end of the construction

operation, the ground levels are the same on both sides. The plaster on both sides of the wall extends a 100 millimeters below ground level. So,

if we have the wall, which is something like this, and, the ground level is somewhere here,

then what we are saying is that the plaster which is applied here, does not end here, but extends 100 millimeters below the ground level. So, this distance here is given to

be a 100mm, and, this is done on both sides of the boundary wall. The angle which is ISA

100×100×8 is embedded 150 millimeters in the center of the RC pillar, and, extends

400 millimeters above the top of pillar to allow the placing of barbed wires. So, the depth of embedment is given here in the note and can be included in the drawing

as well. So, this is given to be a 150 millimeters. The barbed wires are placed at 100 mm center

to center in the angle in four layers. So, if we have a 400 mm length, we are talking

about four layers of barbed wires being placed with one layer being at the top.

So, having completed that, now let us try to see if we have understood the sequence

of operations, and, the kind of operations involved. We should be able to identify the

quantities which are involved in the construction of this boundary wall. We have the plan, we

have the RC footing details, and, we have gone through the wall details. So, once we

know all this, the items involved would be excavation. Excavation should normally include

removal of earth and possibly backfilling. So, this is my ground level originally. We

are going to have to remove certain amount of soil in order to be able to carry out this

construction and then, this part here will need to be backfilled. So, this entire exercise

is what is normally called excavation. Then we have what is called the PCC, that is, this is the PCC that is the plain cement concrete which will have to be placed at the

bottom of the RC footing as well as the brick wall. A question that I am leaving for you

is, why do we need to have PCC? Why we cannot cast an RC footing or for that matter, just

start the brickwork also on ground? That is a question which I am leaving for you to answer.

Of course, below the PCC we have normal ground, and how this ground is to be handled is something

which we will see in the next class. And moving on with our discussion today, we have the brickwork which is in the wall here, and, in the offset portion below the ground,

we have the concrete work which means we are covering the concrete in the pillars, as well as in the foundation. We are plastering which we talked about. Plastering is in the wall

extending a 100 millimeters below the wall on both sides. Then, we have shuttering which

is required to be able to place the concrete and we have the length of barbed wires. So,

the barbed wires have to be placed here. After the barbed wires, there is reinforcement work

because we know that our column as well as our footing is reinforced with steel.

And finally, we have structural steel which is providing these angles. So, this here essentially

is a list of the items that are required to be completed in order to be able to construct

this boundary wall. The next step is to calculate what is the

quantity of each of these items? Now, when we start calculating quantities, here is a very standard format that we use.

This standard format helps us write the item, how many numbers of that item are required,

the length, breadth and height, or, whatever the quantities are, and finally, calculate maybe the square meters, or, the cubic meters, or, whatever for this particular item, and,

that unit has to be specifically mentioned here. Then in the remarks columns, we can always show the details of the working as we will see in some of our subsequent slides.

So, now the first item that we go about calculating, let us say, is the earthwork involved as far

as this particular boundary wall is concerned. Now, in the earthwork we are talking about the earthwork for this portion here. So, along this portion, we will have to do what is more

or less like a trench. So, we will have to create a trench which will be like this all

along the length of this boundary wall. So, there is only one length which is involved,

i.e., overall length let us say we are talking about is 60 meter. So, 60 meter basically means 20 here, 20 here, 10 here and 10 here. Please remember that we have taken the entire

60m here. The breadth is taken as 1.3 which is coming from the 900mm plus 200mm and 200mm

which is the extra width which has been allowed as we mentioned in a previous discussion so that makes it 1.3 (Breadth: 0.9+0.2+0.2=1.3). The height of excavation is 1.2m, this I am

leaving to you to read from the drawings that have already been presented, and, we will get a quantity like this, which is in cubic meters.

So, basically what it says is that 93.6 cubic meters of earthwork is involved, but now what

happens to the earthwork that should not be accounted for. For that we will deduct, we will show it as a deduction. And how much is the deduction? If the breadth and the height

does not change, but the length is not 5 meters as is shown here because here will be a pillar,

and, once we have the pillar, the excavation will extend a little bit beyond the boundary

wall. And this extension here, and, the similar extension here will have to be excavated,

and, only this portion here will not be excavated. How much is this length? Though I am leaving

to you to read, please confirm that it is indeed 3.7 meters. Now, if that is what happens then 5.77 cubic meters of earthwork will not happen and therefore,

the total quantity of earthwork required is about 87.83 cubic meters. So, if we go through

this kind of an exercise in detail with a certain amount of precision, we know exactly

the kind of quantities which are involved. More accurate this exercise is, the more accurate

will be our cost estimates. Moving forward, let us talk of the quantities involved in the RC pillars and footings. So,

I am leaving it to you to confirm from the dimensions, that there will be 23 pillars involved. The size is 0.3×0.23 which is given to you and the height is 2.675m. Now, how

does this 2.675 come? Because the elevation is 1.8m and we are going 0.875m below ground

level. So, from this point to this point, is what we have taken the height of the pillar

(i.e., 1.8m+0.875 = 2.675m). So, if we take the height to be 2.675m, the concrete involved, as far as the pillars, is concerned is 4.25 cubic meters.

Similarly, for the footings the dimensions are 0.8m×0.8m and 250mm thick. So, we have

this quantity here. So, the total amount of RC, which is given to be M25 grade, is 7.93

cubic meters. In this case, there are no deductions. So, as far as PCC is concerned, we can more

or less follow the same dimensions as given for the earthwork, except that the breadth

will be 0.9m here because there is no extra 200 mm to be considered and the height is

75 mm, which is given in the drawing. So, once we have that, we know the quantity of

PCC, but again in PCC as well, there will be a deduction. So, in this case the deduction

will be 4.1m, because the PCC does not extend to the extra 200 mm.

So, this becomes 4.1m, 0.9m, andm 0.075m. So, we deduct this much from here and we get

this number. So, 3.78 cubic meters of PCC in 1:4:8 concrete, this is what is our quantity

of PCC involved in this project. Moving further from here, let us talk of quanty of brickwork. These have been estimated using

22 units, these are the 22 units that we are talking about. That is the unit from, or in

between, one RC pillar to another. Now, this length is 2.2m, the center to center distance

between pillars is given to be 2.5 meters, and, since it is 300 wide, half of that goes

on either side and we get 2.2m as the clear distance for the brickwork, the breadth is

0.23m, and the height is 2m. Now, how this height is 2m is something which I am leaving to you to figure out from the details given in the drawings. So, we basically

get 22.26 cubic meters of brickwork involved which has a width of 230 mm. Then, we have

offsets of 0.4m, 0.5m and 0.6m as given in the drawing, and, this is what we are talking

about 0.4m, 0.5m and 0.6m in the drawing the heights are also taken from here in the drawing and we get these quantities.

So, if we sum them up, we will get a total amount of brickwork involved, but is that

all? That is where we will need to understand that when we do the brickwork here what happens

to the portions here. When we have taken 2.2, we have said that this is the 2.2m. This portion

needs to be deducted, that is, that portion where the brickwork is being done on the foundations,

or, on the footings, that amount has to be deducted. That is exactly what is being done

here, what is being called a double overlap, where the overlap was on both sides, and, then there is a single overlap, these details I am leaving to you to understand on your

own. It is 0.5m, 0.5m, 0.25m, 0.25m, and so on. So, if we do this deduction here, we will

get that the total brickwork involved is 41.49 cubic meters.

Moving further down, we go to plastering. I am not going into the detailed calculations like I did for the previous slides. I am sure you understand how it is done. There is a

long wall, and, there is a short wall, then we will deduct for the opening, that is why we have taken the complete lengths, and, we will calculate the height as 1.9m. 1.9m because

1.8m is the clear distance, which is available to you beyond the ground level, and, it is also required that the plaster extent a 100 millimeters below ground level. So, that is

why it becomes 1.9m. Please remember that in the case of plastering, there is no breadth,

and the thickness of plaster is usually specified with the description of the item for plastering.

That is something which we will see in the next class. So, once we do this kind of a calculation, we find that this boundary wall, measuring

20 meters by 10 meters with a 5 meter opening, 1.8 meters high, we get a total of 210.84

square meters of plastering involved. Now, let us come to shuttering. What is shuttering? Shuttering is the support that is placed,

or, required to be able to cast fresh concrete. So, now, once you are casting a concrete in

a column what we need to do is to prepare what is called the shuttering on all the 4

sides, have the reinforcing bars here, and, then pour the concrete.After this concrete

has hardened, we remove the shuttering. So, that is the principle of it. Of course, in order to support the shuttering, we need to do a lot of other things which is something

which is beyond the scope of this course. We are sticking to estimation of quantities involved. So, since we talked of 23 number of RC pillars,

we have the 23 numbers where a total length of 1.06m for the shuttering is involved having

a height of 2.675m this height of 2675mm, you would recall is the same as the height

of the concrete pillar. And the length is 1.06m because we are talking of the shuttering

being placed on all the 4 sides. Basically, we are looking at the pillar perimeter, which is 2×0.23m and 2×0.3m. Similarly, for the footings, we will require

shuttering along the perimeter of the footing and to a height of 0.25m. So, we get a total

of 83.61 square meters of shuttering involved in this project.

As far as the barbed wire is concerned, we have four lengths of 55 meters and therefore,

we need 220 meters of barbed wire. So, with this, we more or less, complete the description

of all the items that we listed, except the item for structural steel, which is the ISA

100×100×8. I am leaving it as an assignment to you, to be able to calculate the quantity

of the structural steel angle, which is there in the drawing which is this one. So, this

is something which have not covered here in the class. And I would like you to complete that exercise on your own.

Continuing from here, there is another small aspect which we would like to discuss – and that is related to the reinforcement. In fact, reinforcement has not yet been calculated.

What is the total amount of reinforcement in the footings and the column? So, that also is an item which is still not done. But, before we get into the calculation of the quantities

of reinforcement, let us understand and how do we actually calculate the quantity of reinforcement. So, this picture here of this picture here has been shown as an illustration of what

the actual reinforcement, as placed at site, looks like. So, this here is a typical structure

of the reinforcing cage which is very similar to what we have put in our drawing. We have

some bars which are at a certain distance center to center on both sides and we have this pillar or column, which is going to be coming out of the footing. So, basically we

are also talking of something similar to this. Now if we look at this closely, we realize that while these bars are straight, these bars are not straight. They are actually bent

and tied at different places. So, these bars cannot be straight, they need to be bent for

different reasons including being able to have them fixed at that particular location.

So, there is the shape of the bars which is very important. Further, we must remember

that these bars do not come in these lengths. What comes from the factory is long bars which

could be 6 meter, 7 meters, or, 8 meters. In fact, the length of the bars as delivered

to the site is largely governed by the size of the trucks, or the trailers by which the

bars are transported. Those bars have to be cut to size, depending on what is the requirement.

So, we need to determine what is the cut length, for what length, how many bars, and what shape.Once

we understand that, then we are able to calculate the actual quantities involved.All this exercise

happens after the drawings are available. So, making the drawing is perhaps a structural

engineer’s job or a designer’s job, but moving from there is something which has to be done at site. Now, of course, there are software and computer programs, which will

help you do that kind of estimation. The situation here is something a little more complicated

because as you can see, it is a radial placement of reinforcement, and, therefore, this distance

here will become much smaller as we approach the center. So, what designers often do is,

in order to avoid the congestion here, they will curtail some bars at a certain distance.

So, what you will have is a bar going here, the next bar going up to this point, the third bar going here, the next bar being curtailed, and the fourth bar going here, and so on.

So, we should know exactly is the kind of lengths of these bars and here you can see they are bent at one side. I am not so sure if in this picture, it is not clear whether

they are bent on the other side or not. So, these are the kind of details which have to be read from the drawing and appropriate instruction is given so that the bars can be cut and bent.

So, with this background let us try to understand some of these standard shapes that are given

in codes. So, this of course, is a straight bar, here is a hook, this is a simply a bent,

one 90 degree bend, those of you who are familiar with reinforced concrete design would understand where these kind of bars are used. These are the standard stirrups, and, from our detail

C, or whatever it was, in one of these earlier discussions today, we know how this hook has

to be provided. This is a helical reinforcement, which we use in the columns.

Once we have this background, and IS 2502 is the source for this kind of discussion.

You can imagine that this was published in 1963. So, the shapes of bars has not changed.

How to provide those reinforcements, how much to provide, how much not to provide is all right, but this was something which has been understood, the importance of this kind of

thing is understood and what we prepare is what is called the bar bending schedule.

This bar bending schedule gives you for each diameter of the bar, what is the shape, what

is the length and what is the requirement for that kind of steel bar.

Now the bar bending schedule lists the reinforcing bars used in a given RCC work, along with

the details of the bar including the diameter, shape, length of each bend, and straight portion,

the number of each type of bar, and, the total length of bars, in a tabular form. This information

is helpful in estimating the total quantity of steel required in an RC work. And now, let us quickly run through the steel requirement for the RC pillars and footings, as far as

we are concerned in this example. So, the first thing that we take up is the 12 millimeter bars which are to be placed

in the footing. So, this is a 12 millimeter bar which we are talking about. This is to be placed at 100 mm center to center and this is shown to be in this shape with a and b.

So, a is given to be 100mm, b is determined to be 720mm. This 720mm is basically the 800mm

which is the size of the footing minus 2 times the cover. We have given that the cover, in this case, is 40 mm and therefore, this becomes my straight portion. So, please remember that

here we are talking about centerline lengths, not extreme lengths, and, we are going to

ignore any effect that is going to come out of bending. So, if we do that, then we have 100mm and 720mm, and we get a cut length of 920mm, which

is 2 of this plus this, then the total number of such bars required, I am leaving to you to verify, is 322. So, the total length of 12 millimeter bars required for the footing

is 296 meters. This weight which is given in kgs per meter turns out to be 0.888 tones.

So, this standard weight which is given in kgs per meter turns out that 263 kgs of 12

mm diameter bars are required. Now, on this, you might add one percent, or

half a percent, of wastage which will be incurred. You will recall that in one of the lectures

we said that the material is not interchangeable. So, if we have 12 millimeter bars in spare,

those bars may or may not be useful in the pillars. Here we have 16 millimeter bars,

6 of them and we will see how they are placed, but they are in principle not interchangeable.

If you want to interchange them, yes, they will have to be a procedure which you will have to follow. So, coming to the 16 millimeter bars, this distance is given to be 250mm and we can calculate

the length as 2825mm. So, as far as this number 138 is concerned, you know the number of pillars

and the number of bars required for each pillars. So, this gives you the total length of bars required for 16 millimeter bars. Given their unit weight, we know that 670.5 kgs of 16

mm bars will be required. Similarly, if you look at the requirement, as far as stirrups are concerned, you calculate

the length and the breadth from the drawings, find out the cut length, and include the hooks

as is given here. And you get the total number of stirrups, the total length required, from

the unit weight you know 139 kgs of 8 mm bars are required. So, once you do that exercise, the total steel required in the boundary wall construction

is a 1073 kgs. Now on this, you may add a few kgs to account for wastages and so on.

Please remember that unless we have these quantities of the different diameters, it

will be difficult to complete the boundary wall. Now, as far as the reference material is concerned, for this part of the discussion, you can refer

to B N Dutta’s Estimating and Costing in Civil Engineering. This book is in the 25th

revised edition. This is just one of the books and you can read any other literature. Another assignment that I am giving to you is to actually complete the table that okay, for this particular

project, what was the total excavation involved, what is the brickwork involved, what is the plastering involved, and so on. We have calculated all these quantities in

the class today, and, I would like you to complete this table, for your own understanding.

Of course, just to recall that there was an assignment on the structural angles and that

structural steel, in the form of angles to be provided on the RC pillars, should also

be included. With this, we come to close for the discussion today, where, we have actually gone through an exercise of a simple boundary wall, and,

calculated the quantities involved. Thank you.