Ihab Saad – Network Constraints
AI: Summary ©
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.
AI: Summary ©
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.