# Ihab Saad – Precedence Diagramming Method PDM

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Music,

welcome to another class in construction management, 324,

construction planning and scheduling. And today we're going

to start discussing another scheduling technique, which is

activity on node, also known as precedence diagramming method, so

either known as AON activity on node or presence Diagramming

Method, PDM, last time we discussed ADM activity on arrow,

or AOA activity on arrow, or ADM Arrow Diagramming Method, and we

learned about the network flow from left to right. And we learned

about the not the nodes that represent the start event and the

finish event of the activity, and the line connecting these two

nodes representing the activity itself. And we learned about some

of the difficulties representing activities that start at the same

time finish at the same time, and having a common predecessor or a

common successor. And we had to resort to using the dummy

activities in this case. And another problem with ADM is that

it only allowed for one type of relationship linking the

activities, which is commonly known as finish to start, where

the successor has to start, only after the predecessor was

complete. While this is not always the logic connecting the

activities in the construction industry, we had to look for

another alternative, especially also with the dummy activities not

being the most intuitive thing to understand. We needed to look at a

better way of representing activities. And that basically was

the reason why activity on node or presence Diagramming Method

started.

So we're going to discuss what's the activity on node network.

We're going to discuss the network calculations for AON or PDM

networks. We're going to look at the presence diagrams, and we're

going to look the precedence diagram calculations, which are

going to be pretty much the same as the ones in Arrow Diagramming

Method.

So this PDM, or presence Diagramming Method, also known as

activity on node network, instead of the activity being represented

by two nodes with an arrow connecting them, so all of these

three entities represented the activity the activity, we're going

to consolidate all the information about the activity into one box,

and that box is going to be the activity itself, rather than the

line. The lines, in this case, that you can see on the screen,

represent the relationship between the activities. So it uses the

node or box to represent the activity, as opposed to the arrow

used with the activity. On arrow or AOA network, the arrows

represent logical relationships, and their length does not reflect

any special meaning. So again, this is not drawn to scale. The

length of all the arrows are pretty much the same, so it does

not represent any special significance. And the good

advantage that we have in presence diagramming method is that we do

not have any dummy activities. So when you look, for example, at a

Gantt chart or a bar chart that has five bars representing five

activities, the translation is going to be one to one. Each bar

is going to be translated into one activity. We could not do that in

Arrow Diagramming Method, because, in case you had some activities

overlapping, or you had some lag between the activities, you had to

introduce another activity to represent that lag, or you had to

break the activity into more than two sections to represent the

overlap. So here in presence diagramming method, it's much

easier that every bar is represented by a box in this case.

So what we have here, for example, is a box representing the

activity, showing the ID of the activity, number 10, the name of

the activity mobilized, and the duration of the activity, followed

by another successor, or actually two successors, number 20 and

number 30 begin construction and side fencing two days and three

days respectively. And here it shows the relationship between the

activity. Looking at the box, we're going to find out that the

box has basically two vertical lines, the left side and the right

side. Since we already agreed that the network is always going to

flow from left to right. We're going to call this left side the

start side of the activity, whereas the right side is going to

be the finish side of the activity. Now looking at the line

connecting two consecutive activities, like 10 and 20 or 10

and 30, we're going to find that it starts from the right side of

the activity, which is called the finish side and ends at the left

side of its successor, which is called the start side. Therefore

this relationship is going to be called Fs, or finish to start,

because it starts from the finish side and ends at the start side of

the successor. And here we have the number zero.

Two which shows that there's no lag between these two activities,

which means activity 20 is going to start as soon as activity 10 is

complete. Now whether we add this zero or not is basically

redundant. So if we had zero, then it means it starts immediately. If

we do not add anything, it should be understood as exactly the same

thing. So the relationship between 10 and 30 is exactly the same as

between 10 and 20. It's still finished to start with zero lag.

We did not have to add that because, again, this should be

understood from the flow of the network. If, however, we add any

positive number or negative number. Here we're going to see

what's the significance of these numbers in a couple of slides.

The network should start with one node and end with one node, as we

have done before.

As usual, it flows from left to right, therefore the arrowhead is

redundant. Activities can only be linked either from the start side

or from from the finish side, or sometimes from both. We're going

to see that in a few minutes, we can never connect the activities

from the middle of the box. That would be a big mistake to connect

the activities from anywhere other than the start side or the finish

side. Of

the concept of legs and leads. Legs and leads are pretty much the

same thing, by the way, legs and leads if you're looking, for

example, at two

cars in a race car, and one of them is ahead of the other by,

let's say, a couple of yards, or whatever, in a very fast NASCAR

racing for example, you can say that the car number one is leading

car number two by two yards. Or you can say that car number two is

lagging by two yards behind car number one. So basically, the lead

and the lag are exactly the same thing, depending on it only varies

depending on where are you looking from. If you're looking from the

one that's ahead, you say we are leaving. Or if you're looking from

the one you're that's behind, you say we are lagging. So a lag is a

minimum waiting period between a start or an end finish

of an activity and the start or end of its of its successor. So

for example, a large concrete slab or the rebar can start after the

start of four work, but not necessarily wait for its

completion. I have a large concrete slab for a warehouse, for

example, a slab on grade, and the activities are going to be

basically four work for the sides of that slab, or even if it's a

suspended slab, not necessarily a slab on grade, if it's a suspended

slab, then we have the formwork, and then we place the rebar, we

place the mechanical electrical inserts, and then we place the

concrete. Now if it's a large slab, you don't have to wait for

the whole formwork to be done to start working on either the rebar

or the mechanical electrical inserts. You can start a little

bit after the start of the formwork, when you have enough

work to do. So you don't have to wait until the formwork is

complete. Therefore, in this case, we say that the rebar is going to

start after the start of the formwork, not necessarily after

the completion of the formwork. It does not have to wait until the

completion of the formwork. So in this case, the relationship is

going to be a start to start, but there's going to be some lag. You

cannot start on the same day. You have to not to have enough buffer.

You have to have enough work done on the formwork in order to be

able to start the rebar. So the arrow networks cannot accommodate

a lag, and this is the main reason for falling out of favor,

especially in the construction industry, a lead is the same as

the lag looked at from the opposite side. And overlap, on the

other hand, is a negative lag. So think about it for a second if you

say that activity number two starts three days before the

completion of activity number one, which means there's going to be an

overlap of three days between the durations of these two activities.

So as if we are moving in the opposite direction, therefore is

going to be a negative lag. Lag is usually going to be a positive

number. Overlap is going to be a negative number.

So when we look at the node diagram drawing, nodes should be

drawn as squares or as rectangles. Basically do not connect the nodes

from the top or the bottom. So this is wrong. And this is wrong

because again, we mentioned that the relationship is going to start

either from the start or the finish of the activity. So this is

somewhere in the middle, which does not mean mean anything,

especially that the boxes are of the same size, and the location of

the line does not represent any scale.

So connect size only. The left side represents the start side and

the right side represents the end or the finish side.

Left to right, we're going to start moving from right to left.

Instead of adding durations, we're going to subtract duration. Now

we're going to have another element, which is the element of

lags and overlaps. The lag, as we said, is going to be a positive

number, so it's going to be added as we move forward. And the lag,

which used to be a positive number added in the forward pass is going

to be subtracted in the backward pass. On the other hand, the

overlap, which we mentioned, is going to be a negative number, so

the overlap is going to be subtracted in the forward pass,

and when we reverse go in the backward pass, we're going to add

the overlaps again. Don't worry about that. We're going to see a

numerical example on network calculations, which is going to

illustrate this issue, and it's going to be extremely

straightforward. So the early start of an activity is equal to

the maximum of the early finishes of all of its predecessors, plus

any lag or overlap that's going to be affected in and the early

finish of an activity is equal to its early start plus its duration

in the backward backward pass

running from right to left. It's used to determine the late finish

and the late start of each activity. Late finish of the

activity is equal to the minimum of the early finishes of all of

its predecessors minus lag of overlap. Late start of the

activity is equal to late finish minus duration. Again, exactly the

same rules that we used for Arrow Diagramming Method.

Now again, we're going to be faced with the issues of floats, as we

learned last time we had total float and free float and we talked

about something called interfering float, which we're not going to

use. So here we're going to focus primarily on total float and free

float, the definition is exactly the same. The total float is the

amount of time by which a non critical activity can be delayed

without delaying the whole project. Whereas the free float,

we're going to change the last couple of words, it's the amount

of time by which a non critical activity can be delayed without

delaying its immediate successor. We're not looking to the end of

the project. We're just looking for the immediate successor. And

as we agreed last time, also the free float is a subset of the

total float, which means the free float can never exceed the total

float. Remember that quite well, because this is one of the common

mistakes that I usually see on assignments and on exams, someone

giving a total float of three and a free float of five that can

never happen. The maximum of the free float can be equal to the

total float of the activity. The minimum for the free float, it can

be equal to zero. The free float can never be a negative value

again. Remember that, because this is another common mistake. So the

free float has boundaries. The lowest one is zero, the highest

one is equal to the total float of the activity. Activities whose

total float is zero are on the critical path. And if an activity

is on the critical path, by default, its total float is zero,

and if the total float is zero, then definitely the free float is

also going to be equal to zero, because it cannot exceed the total

float, and it cannot be a negative value. Therefore the only value

left is zero.

To calculate the total float, the total float is equal to the late

finish minus the early finish, or the late start, minus the early

start. So again, you calculate it from either side of the activity,

late minus early for the same side, late start minus early

start, or late finish minus early finish.

Now for the free float, it's little bit more complex when it

comes to PDM, because we may have different types of relationships,

we may have lags and overlaps and so on and so forth. So I invented

a method to calculate the free float, and it's called the sad

method to calculate the free float. And if you follow it,

you're going to find that it's the simplest and easiest way to

calculate the free float of an activity.

The free float of an activity is equal to the total float of that

activity minus the largest total float of any of its immediate

successors. Listen again. It's equal to the total float of the

activity minus the largest of the total floats of any of its

immediate successors. So if we have an activity, having a total

float of five,

and its immediate successors have total floats of five, three and

six.

So in this case, according to the Saad method.

So the free float of this activity is equal to five minus the largest

total float of five, three and six, which is six, five minus six

is negative one. But we just said that the free float cannot be

negative. In case you get a negative value, put the free float

equal to zero. So another example, if an activity has two immediate

successors. The first activity has five days of total float. Its

immediate successors have floats of three and two. Then the free

float of this activity is equal to five minus the larger of the three

and two, which is three. So five minus three, that gives two days

of free float for that activity.

So there's a general

understanding on how to draw the activities in the network, and

it's something like this box here, where we divide it primarily into

seven components, seven compartments. In the middle here,

we're going to have the activity ID or its description. What is

that activity about? And in the middle, at the bottom, we're going

to have the duration, which is going to be given or calculated

primarily. And then, based on that, we're going to perform our

calculations. We're going to calculate the early start plus

duration gives the early finish, the late finish minus duration

gives the late start, late start, minus early start, or late finish,

minus early finish, is going to give the total float. So the only

number that we start with is the duration, and based on the

duration which we acquire from Q over P, remember that very simple

equation that we said is always going to be with us, Q divided by

P, Q, the amount of work to be done divided by P, the lowest

production rate of any of the resources involved in that

activity. So q over P gives the duration, and from that, we can

calculate all the other dates for that activity, depending on the

relationship linking this activity to other activities.

President's network, some people would would like to make a

distinction between

Aon and precedence. I do not usually make the distinction. I

consider them the same thing. So precedence network have these four

types of relationships, finish to start, start to start, finish to

finish and start to finish.

So besides relationship types, each relationship can be

accompanied by a lag or overlap value. So if it's a finish to

start with lag, meaning that the successor is going to start three

days after the completion of the predecessor. Examples for that. We

have placed the concrete for a slab,

and the next activity is to remove the four more for that slab. Of

course, we cannot remove the four more. As soon as we have finished

placing the concrete, we have to wait for this concrete to have

setting, initial setting, and part of the permanent setting

dependent, dependent on the code, depending on the span, depending

on the concrete mix, depending on the weather and other conditions

and so on. So we have to wait, for example, let's say a week after

the concrete has gained enough strength before being able to

remove the formal so in this case, we're going to say that removal of

four work is gonna lag seven days after the completion of placing

that concrete slab. The lag values indicate the amount of delay

between the two elements of the relationship described by the

relationship type. So if it's a start to start, the start of the

successor is gonna lag by certain number of days after the start of

the predecessor. If it's a finish to finish the completion of the

second activity is going to lag a certain number of days after the

completion of the first one, and so on and so forth.

Two ways to present different relationships. So if we have a

just an arrow spanning between the end of an activity and the start

of the other one. It would be a finish to start.

However, if we want to designate the start to start, we can draw it

from the start of the activity to the start of the successor, or use

the traditional finish to start designation and put on it the two

letters representing the connecting ends of the activity,

SS for start to start, or FF, for finish to finish. Honestly, I do

not prefer this however you might see it in some books or art or

articles or publications. I do not use this one. So forget about this

one. We're not gonna use it to designate start to start. This is

the one that we're going to use which has less confusion, it shows

that the relationship is from the start of the predecessor to the

start of the successor. And usually, in most cases here, we're

going to have a positive number, which represents a lag.

And similarly, for finish to finish is going to be from the.

Finish of one activity here to the finish of its immediate successor,

and so on.

Now, since we talked about

contiguous activities, which we meant that these are activities

that once started, cannot stop until the activity is complete,

the other option is called interruptible activities, which

are activities that can be interrupted, paused for a certain

number of days and then resumed at a later date and completed at the

end. So we have two different types of calculations. One of them

is going to be based on a contiguous activity assumption,

and the other one is going to be based on an interruptible activity

assumption. We're going to look at both examples in a numerical

example that we're going to solve in another lecture. So the

continuous the contiguous activities cannot be interrupted.

And once started, they must continue until they are finished,

for the finish to start relationship in the forward pass,

the early finish is equal to the early start plus the duration

and the early start of the successor activity is going to be

the maximum of all predecessor dates, which might be the largest

early finish of the previous activities, plus any lag or

overlap values between the two activities that's going to appear

on the arrow, or the relationship between the activities

in the backward pass. Again, the backward pass determines, or

provides the late dates, late start and late finish for the

activity. It begins at the last activity on the network moving

backwards, so the late finish of the activity is going to be the

minimum of all the successors, late start plus lag or overlap.

And the late start is going to be the late finish minus the

duration, because we move from right to left, subtract

in case of start to start again, it's exactly the same concept.

We're going to look at the largest number coming to the start of

this, this activity, whether it's coming coming from the immediate

predecessors, whether it has lags or overlaps and so on. We are

going to factor that into our calculation.

In some cases, we might have something called a dangling

activity or open ended activity. This is not a good thing to have

in a network, but it might happen in some projects. To give you an

example, something like landscaping activities. If we're

talking about a five story building, when can we start the

landscaping activities? Well, we can start them as soon as the

enclosure of the building is complete, so that we do not have

any heavy equipment on the outside of the building. We still have a

lot of work to be done inside the building, all the interior

finishing and so on and so forth, but we can start the landscaping

at that point. Now, when does the landscaping need to be finished?

It needs to be finished by the end of the project. If the landscape

is going to take two months, and the interior finishing is going to

take five months, if we start the landscaping, once the enclosure is

complete, it's going to start, and then it's going to be done in two

months. It still have, has three months of total float until the

end of the project, because we are still working on the inside. So in

this case, we say that this is an open ended activity, or dangling

activity.

A dangling activity has either no predecessors or no successors.

Should be avoided, as they reflect the false amount of float. Again,

it would appear in this case that the landscaping has a huge amount

of total float, but we can tie it to other activities on the

construction site. The dates may vary depending on whether the

activity duration is contiguous or interruptible. We're going to see

that again in the numerical example.

In case of finish to finish, the finish of one activity controls

the finish of another its successor, and it's used to relate

to activities that are done in parallel or may have lags. So

again, same calculations would apply a

a start to finish. As I mentioned before, it's very rarely used, and

it's usually linked linking one non construction activity, like a

marketing campaign, for example, to some construction activities.

It's used to identify activities whose starts are related to their

successors finish

difficult to identify a pair of such activities in construction.

Again, do not worry about that, because we're not going to have

many activities like that in in our calculations,

as we discussed before, one of the fatal loops.

In any network, one of the fatal errors in any network is the

existence of loops. A loop is activities going back and forth in

an unending fashion, so two or more activities linked in a

circular manner. You say that activity two is a successor to

activity one, and activity one is a successor to activity two. So it

keeps the calculations keep going in a circle, and that's a fatal

error. The software, if we use the software, is going to give you an

error that you cannot operate in a loop. It usually can be found in

relationships where the arrow turns backward. It's a fatal

mistake that should be avoided at all costs, and represents a faulty

logic. So this is basically a an introduction about presence

diagramming methods or

activity on node. We are going to see a in another lecture or

another example. We're going to see a numerical example on how to

draw them and how to make the calculations, and what's the mean

difference between ADM and PDM See you in another lecture. You.