# Ihab Saad – Precedence Diagramming Method PDM

The speakers discuss the activity on the node network in construction management, including the concept of "active event" and " radiant event" methods. They explain the concept of "active event" and how it will affect the network, including the use of SS for start to finish activities and SS for start to finish activities. They also discuss the calculation of free float and the method used to calculate it.
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Music,

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welcome to another class in construction management, 324,

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construction planning and scheduling. And today we're going

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to start discussing another scheduling technique, which is

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

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either known as AON activity on node or presence Diagramming

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

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

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

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

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

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nodes representing the activity itself. And we learned about some

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of the difficulties representing activities that start at the same

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

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

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

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it only allowed for one type of relationship linking the

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

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the successor has to start, only after the predecessor was

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complete. While this is not always the logic connecting the

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activities in the construction industry, we had to look for

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another alternative, especially also with the dummy activities not

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

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better way of representing activities. And that basically was

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the reason why activity on node or presence Diagramming Method

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started.

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

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

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

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going to look the precedence diagram calculations, which are

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

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Method.

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

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activity on node network, instead of the activity being represented

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

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

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

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

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

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represent the relationship between the activities. So it uses the

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

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

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represent logical relationships, and their length does not reflect

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

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

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not represent any special significance. And the good

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

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

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

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

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

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

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

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

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break the activity into more than two sections to represent the

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

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

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

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

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

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

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number 30 begin construction and side fencing two days and three

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days respectively. And here it shows the relationship between the

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

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

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

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

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

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

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connecting two consecutive activities, like 10 and 20 or 10

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

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

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

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

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

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the successor. And here we have the number zero.

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

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

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

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

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

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

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

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

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

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

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

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

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have done before.

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

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

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

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

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

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

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side. Of

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

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

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example, at two

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

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

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

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

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

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

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

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

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

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minimum waiting period between a start or an end finish

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

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

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start of four work, but not necessarily wait for its

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

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

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

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

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

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place the mechanical electrical inserts, and then we place the

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

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

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or the mechanical electrical inserts. You can start a little

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

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

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

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start after the start of the formwork, not necessarily after

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

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

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

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

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

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

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

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

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

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

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say that activity number two starts three days before the

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

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

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

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

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number. Overlap is going to be a negative number.

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

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

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

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

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

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

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

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the line does not represent any scale.

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

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the right side represents the end or the finish side.

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

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

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

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

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

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

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

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

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

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

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numerical example on network calculations, which is going to

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

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

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

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

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

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in the backward backward pass

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

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

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

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its predecessors minus lag of overlap. Late start of the

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

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same rules that we used for Arrow Diagramming Method.

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

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

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

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

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

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

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without delaying the whole project. Whereas the free float,

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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left is zero.

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

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

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

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

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start, or late finish minus early finish.

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

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

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

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

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

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

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calculate the free float of an activity.

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

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activity minus the largest total float of any of its immediate

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

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

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

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float of five,

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

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six.

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So in this case, according to the Saad method.

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

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

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

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

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

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

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

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

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

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of free float for that activity.

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So there's a general

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

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

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seven components, seven compartments. In the middle here,

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

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

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

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

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

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duration gives the early finish, the late finish minus duration

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

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

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

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

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

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

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production rate of any of the resources involved in that

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

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

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relationship linking this activity to other activities.

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

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distinction between

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

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

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

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finish and start to finish.

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So besides relationship types, each relationship can be

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

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

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

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have placed the concrete for a slab,

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

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

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

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setting, initial setting, and part of the permanent setting

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

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

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

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the concrete has gained enough strength before being able to

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

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

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

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between the two elements of the relationship described by the

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

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

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

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

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

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

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

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

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

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

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

00:24:18 --> 00:24:21

letters representing the connecting ends of the activity,

00:24:21 --> 00:24:27

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

00:24:27 --> 00:24:32

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

00:24:32 --> 00:24:36

00:24:36 --> 00:24:39

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

00:24:39 --> 00:24:45

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

00:24:45 --> 00:24:49

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

00:24:49 --> 00:24:52

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

00:24:52 --> 00:24:55

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

00:24:57 --> 00:24:59

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

00:25:00 --> 00:25:03

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

00:25:04 --> 00:25:04

and so on.

00:25:07 --> 00:25:09

00:25:10 --> 00:25:14

contiguous activities, which we meant that these are activities

00:25:14 --> 00:25:19

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

00:25:19 --> 00:25:23

the other option is called interruptible activities, which

00:25:23 --> 00:25:26

are activities that can be interrupted, paused for a certain

00:25:26 --> 00:25:30

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

00:25:30 --> 00:25:34

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

00:25:34 --> 00:25:37

is going to be based on a contiguous activity assumption,

00:25:38 --> 00:25:41

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

00:25:41 --> 00:25:44

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

00:25:44 --> 00:25:49

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

00:25:49 --> 00:25:52

continuous the contiguous activities cannot be interrupted.

00:25:52 --> 00:25:56

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

00:25:59 --> 00:26:03

for the finish to start relationship in the forward pass,

00:26:03 --> 00:26:07

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

00:26:08 --> 00:26:11

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

00:26:11 --> 00:26:16

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

00:26:16 --> 00:26:20

early finish of the previous activities, plus any lag or

00:26:20 --> 00:26:23

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

00:26:23 --> 00:26:26

on the arrow, or the relationship between the activities

00:26:28 --> 00:26:32

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

00:26:32 --> 00:26:35

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

00:26:35 --> 00:26:39

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

00:26:39 --> 00:26:44

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

00:26:44 --> 00:26:49

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

00:26:50 --> 00:26:53

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

00:26:53 --> 00:26:57

duration, because we move from right to left, subtract

00:27:00 --> 00:27:04

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

00:27:04 --> 00:27:07

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

00:27:07 --> 00:27:11

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

00:27:11 --> 00:27:16

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

00:27:16 --> 00:27:18

going to factor that into our calculation.

00:27:21 --> 00:27:24

In some cases, we might have something called a dangling

00:27:24 --> 00:27:28

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

00:27:28 --> 00:27:31

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

00:27:31 --> 00:27:36

example, something like landscaping activities. If we're

00:27:36 --> 00:27:40

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

00:27:40 --> 00:27:45

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

00:27:45 --> 00:27:49

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

00:27:49 --> 00:27:53

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

00:27:53 --> 00:27:56

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

00:27:56 --> 00:28:01

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

00:28:01 --> 00:28:06

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

00:28:06 --> 00:28:10

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

00:28:10 --> 00:28:13

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

00:28:13 --> 00:28:18

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

00:28:18 --> 00:28:22

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

00:28:22 --> 00:28:25

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

00:28:25 --> 00:28:29

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

00:28:29 --> 00:28:33

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

00:28:33 --> 00:28:33

activity.

00:28:34 --> 00:28:38

A dangling activity has either no predecessors or no successors.

00:28:39 --> 00:28:43

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

00:28:43 --> 00:28:47

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

00:28:47 --> 00:28:51

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

00:28:51 --> 00:28:54

construction site. The dates may vary depending on whether the

00:28:54 --> 00:28:58

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

00:28:58 --> 00:29:00

that again in the numerical example.

00:29:04 --> 00:29:08

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

00:29:08 --> 00:29:11

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

00:29:11 --> 00:29:16

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

00:29:16 --> 00:29:19

again, same calculations would apply a

00:29:24 --> 00:29:28

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

00:29:28 --> 00:29:32

00:29:32 --> 00:29:36

marketing campaign, for example, to some construction activities.

00:29:36 --> 00:29:40

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

00:29:40 --> 00:29:41

successors finish

00:29:44 --> 00:29:47

difficult to identify a pair of such activities in construction.

00:29:48 --> 00:29:51

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

00:29:51 --> 00:29:54

many activities like that in in our calculations,

00:29:57 --> 00:29:59

as we discussed before, one of the fatal loops.

00:30:00 --> 00:30:03

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

00:30:03 --> 00:30:09

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

00:30:09 --> 00:30:13

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

00:30:13 --> 00:30:16

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

00:30:16 --> 00:30:20

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

00:30:20 --> 00:30:23

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

00:30:23 --> 00:30:27

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

00:30:27 --> 00:30:31

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

00:30:31 --> 00:30:34

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

00:30:34 --> 00:30:38

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

00:30:38 --> 00:30:44

logic. So this is basically a an introduction about presence

00:30:44 --> 00:30:46

diagramming methods or

00:30:48 --> 00:30:52

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

00:30:52 --> 00:30:55

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

00:30:55 --> 00:30:58

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

00:30:58 --> 00:31:03

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

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