# Ihab Saad – Network Constraints

The speakers discuss network constraints and how they can affect project scheduling and planning. These constraints are represented on the network and can lead to irregularities. The types of constraints that can be inserted into a network, including starting at a certain date, will affect the calculations of forward and backward pass. Shading of the triangle will affect the calculations of late start and late finish, and the most rigid type of constraints are based on the shading of the triangle.
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Music, hello and welcome to another class in construction

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management. 324,

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construction planning and scheduling. And today, we're going

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to start talking about network constraints. So far we have

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learned about how to break down the project into work

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groups, and break down these work groups into activities through the

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work breakdown structure. We learned about how to estimate the

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duration of an activity which is going to be through the equation Q

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over P. We learned about how to sequence the activities in a

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logical relationship and in logical network through either ADM

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or PDM, and especially we're gonna focus on PDM. We're not gonna

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the new topic, which is network constraints. What are network

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constraints and what is their effect on the network calculations

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so far, just to give you a quick review. When we tried to calculate

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the total float for an activity, we could do it either from the

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beginning of the activity or from the end. So from the start side,

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it used to be late start minus early start, or from the finish to

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be late finish minus early finish.

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And we usually ended up getting the same number from either side.

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We also used to find that the network should have at least one

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critical path connecting all the activities, all the critical

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activities, from the beginning of the network till the end, and that

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was the longest path in the network. Today, once we start

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talking about network constraints, and later on, when we see a

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numerical example on adding these constraints to the network, we're

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going to find that they're going to break some rules. So for

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example, we might end up having an activity that's not critical, or

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non critical, it might be even half critical. Or we might end up

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having two different floats for the activity, one calculated from

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the beginning and one calculated from the end, or we might end up

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having a half critical path, or a an incomplete critical path. All

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of these are irregularities in the network that might occur due to

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the introduction of these constraints. So let's go ahead and

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see what are these constraints, how are they represented on the

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network, and what is their effect on the network calculations.

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be imposed on the network schedule, usually in the form of

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milestones or constraints. So for example, if I have to finish a

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certain part of the project at a certain date, if, for example, I

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have a mandatory completion for the substructure work that

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includes the foundations and includes all the earth work and

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includes all the underground utilities for the project. If I

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have a deadline by which I have to finish that package of work

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because it might be affected by weather or any other reason, in

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this case, I might say that this milestone has to be completed by a

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certain date, or it has to be completed no later than a certain

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date, as we have defined milestones previously. Milestones

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are zero duration activities. They are not dummy activities. They are

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just zero duration activities denoting the start of an event

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which, in this case, could be a start milestone, or the completion

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of an event which would be considered a Finnish milestone.

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Flags are in a similar way. It could be a start flag or a finish

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flag. These are checkpoints inserted in the schedule to make

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sure the sought progress is achieved and we comply by certain

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dates that are specified in the project contract. Since these are

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zero duration activities, they do not affect the forward or the

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backward pass calculations. So we're not adding any numbers,

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we're not subtracting any numbers, therefore the calculations are

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going to go as usual.

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Constraints. On the other hand, are another type of controls that

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may be either a natural or an artificial constraint imposed on

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the network restricting an activity, such as, for example,

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constraint could be, you cannot start earlier than a certain date,

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and it's called Start no earlier than constraint. Or it could be

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you cannot finish before a certain date, if, for example, we are

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going to wait for an external inspection, we cannot finish the

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activity until the inspection is complete. So we cannot finish

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before a certain date, or we cannot start later than a.

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Certain date again, if it's going to be impacted by weather, we

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cannot start later than that, and we cannot finish later than a

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certain date. Also for the same reason,

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the more the most rigid type of constraint is the one that binds

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me both ways. So starting exactly on a certain date, not before

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that, not after that, but exactly on or finishing exactly on a

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certain date again, not before, not after, but exactly finishing

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on a certain date. So these six types of constraints start no

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earlier than start, no later than finish, no earlier than finish no

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later than start on and finish on. Are types of constraints that can

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be inserted in the network, and they might have some effect on the

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calculations, not always, but in many cases, they would have an

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effect on the calculations. Some of them will have an immediate

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effect on the calculations, as we're going to see in a moment.

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How are we going to represent these constraints, at least

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graphically, they are represented by an inverted triangle,

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inverted equilateral triangle like this one here,

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with all three sides equal, and that triangle is going to be split

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in the middle by a vertical line,

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which splits it into two identical halves.

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Now the shaded half represents where the constraint is going to

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be applied. Remember that the network flows from left to right.

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So on the left side, as we're moving, if I hit the constraint,

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that means it's going to affect my forward pass calculations. And the

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forward pass calculations are basically the early dates. So if

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it's an early date constraint, the shaded part is going to be on the

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left side of the triangle, or the dark side, the dark part is going

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to be on the left side of the triangle. If, on the other hand,

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the shaded or the dark side is on the right side, that means it's

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going to affect my backward pass calculations. Is not going to have

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any impact on the forward pass. It's only going to affect my

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backward pass calculations, including late start and late

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finish. Now think about it as a one way valve, for example, or

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slowest gate that you might have learned about in mechanical or

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electrical works. So a one way valve allows for the flow in one

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direction and blocks it in the opposite direction. So if we have

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an early date constraint, it's going to affect the flow in the

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forward pass, is going to block it or have a check on it in the

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forward pass, whereas in the backward pass, it has no effect on

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the calculations whatsoever. The opposite can be said for the late

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date constraint, when we are doing the forward pass, it's going to

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allow our motion to proceed and our numbers to proceed without any

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modification, whereas in the backward pass is going to have

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that check, and it might affect these numbers.

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What if? Now, so looking at this constraint, for example, here it

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shows that the left side is shaded. If it is put on the Start

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side of the activity, it would mean start no earlier than if it's

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put on the finish side of the activity, it would mean finish no

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earlier than this one the late date constraint again, if on the

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Start side, it would say start no later than and if put on the

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finish side, it would mean finish no later than.

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Question, now, what if both sides are shaded? What if the whole

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triangle is darkened or filled? What would that mean? It means

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that in the forward pass, if it's on the Start side, it says Start

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no earlier than,

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and in the backward pass, it would mean

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start no later than. So what does that mean if it says Start no

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earlier than, for example, day 45 start no earlier than day 45 start

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no later than day 45 what does that leave? It leaves only day 45

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which means you have to start exactly on day 45 and that

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becomes, as we say, the on constraint, or the absolute

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constraint, the most rigid type of constraints that affects both

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forward and backward. Pass calculations,

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so again, here, as we can see, if the whole triangle is shaded, this

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is the most strict and rigid type of constraints, which is called,

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if it's on the Start side, it's called start on. If it's on the

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finish side, it would be called finish on, also called an absolute

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constraint, as it affects both.

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