Ihab Saad – Network Constraints

Music, hello and welcome to another class in construction

management. 324,

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

to start talking about network constraints. So far we have

learned about how to break down the project into work

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

work breakdown structure. We learned about how to estimate the

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

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

logical relationship and in logical network through either ADM

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

discuss ADM anymore. So today, we're gonna start talking about

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

constraints and what is their effect on the network calculations

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

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

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

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

be late finish minus early finish.

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

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

critical path connecting all the activities, all the critical

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

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

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

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

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

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

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

having two different floats for the activity, one calculated from

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

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

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

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

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

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

We talked about milestones and about flags, so some controls may

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

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

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

have a mandatory completion for the substructure work that

includes the foundations and includes all the earth work and

includes all the underground utilities for the project. If I

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

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

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

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

date, as we have defined milestones previously. Milestones

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

just zero duration activities denoting the start of an event

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

of an event which would be considered a Finnish milestone.

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

flag. These are checkpoints inserted in the schedule to make

sure the sought progress is achieved and we comply by certain

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

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

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

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

going to go as usual.

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

may be either a natural or an artificial constraint imposed on

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

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

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

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

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

activity until the inspection is complete. So we cannot finish

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

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

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

certain date. Also for the same reason,

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

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

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

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

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

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

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

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

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

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

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

How are we going to represent these constraints, at least

graphically, they are represented by an inverted triangle,

inverted equilateral triangle like this one here,

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

in the middle by a vertical line,

which splits it into two identical halves.

Now the shaded half represents where the constraint is going to

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

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

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

forward pass calculations are basically the early dates. So if

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

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

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

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

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

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

backward pass calculations, including late start and late

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

slowest gate that you might have learned about in mechanical or

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

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

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

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

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

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

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

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

modification, whereas in the backward pass is going to have

that check, and it might affect these numbers.

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

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

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

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

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

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

finish side, it would mean finish no later than.

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

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

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

no earlier than,

and in the backward pass, it would mean

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

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

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

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

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

constraint, the most rigid type of constraints that affects both

forward and backward. Pass calculations,

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

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

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

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

constraint, as it affects both.