1. Project Scheduling 4
The Network Diagram
Prepared by Sebghatullah Karimi
(Junior Lecturer of Kabul Polytechnic University)
1392
Kardan University
Civil Engineering Faculty
2. Explain how the duration is determined for an
activity in a network schedule.
Perform network calculations to determine the
project duration, as well as the early start, early
finish, late start, and late finish times for each
activity.
Identify the critical path for a network schedule.
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3. Duration – The time (usually defined in days) required
for a particular project activity to be performed.
Daily Output – The number of units of work that a
specified construction crew can accomplish in a typical
workday.
Labor-Hours – The number of hours required for one
worker to construct one unit of a task.
Early Start Time (ES) – The earliest possible time that
an activity can start based on the logic and durations
identified in the network.
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4. Early Finish Time (EF) – The earliest possible time that
an activity can finish based on the logic and durations
identified on the network.
Early finish = Early start + Duration.
Late Finish Time (LF) – The latest possible time that an
activity can finish without extending the completion
date of the project.
Late Start Time (LS) – The latest possible time that an
activity can start without extending the completion
date of the project.
Late start = Late finish - Duration.
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5. Float – The additional time that an activity can use beyond its
normal duration and not extend the completion date of the
project.
Float = Late finish – Early finish
or
Float = Late start – Early start
Free float – The amount of time that an activity can be delayed
without taking float away from any other activity.
Free float = Early start (of succeeding activity) – Early finish (of
activity in question)
Critical path – The longest path through the network (the path
with the largest total of all activity durations). The critical path
has zero float. Any delay along this path delays the project.
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8. Once the development of the network diagram
for a project is complete, completion of the
project schedule is accomplished by:
1. Determining the duration of each individual
activity in the network and,
2. Performing network calculations to
determine the overall project and activity
start and completion dates.
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9. Ideally, activity durations should be determined
in conjunction with the development of the
estimate for the project.
Estimate Line Items Schedule Activity
1. Metal Door Frames 1. Install Metal Frames, Doors, and
Hardware
2. Metal Doors
3. Door Hinges
4. Locksets
5. Panic Devices
6. Thresholds
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10. The method used to determine the duration of an activity depends
on the type of activity.
1. Production Activities: Activity duration can be determined by
talking to the appropriate subcontractor, researching past
projects of a similar nature, or using reference books like those
provided by R.S. Means.
2. Procurement Activities: Activity duration can be determined by
talking to the particular vendor. Consistent with the project's
contract documents, time must also be allowed for the
preparation of submittals and submittal reviews.
3. Administrative Activities: Activity duration can be determined
by consulting with the particular agency that provides the
service. Owners, designers, or construction professionals who
have worked on past projects in the area can also be consulted,
as they have experience with the times and procedures
involved in these administrative steps.
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11. In the R.S. Means construction cost data
references, activity durations can be
determined from the crew, crew output, and
labor-hours columns.
The Daily Output column identifies how many
units of work the crew can accomplish in a
typical workday. Therefore, if your project will
use this same crew configuration:
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)dayperunitsworkin(OutputDaily
)unitsworkin(Quantity
)daysin(Duration =
13. The Labor-Hours describes how long it takes one
worker to construct one unit of a task. This factor is
particularly useful if you are planning on
accomplishing an activity with a crew size different
from the standard crew that R.S. Means suggests.
The units for this column are given in labor-hours, so
if you are working in days, as is normal, you must
divide the sum of the work quantity times the labor-
hours by 8 (assuming an 8-hour work day).
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dayperhours
)unitworkperhoursin(LaborHours)unitsworkin(Quantity
)daysin(Duration
8
×
=
14. Network calculations are used to determine how long the project will
take, when each activity can start and finish, and which activities
can be delayed without affecting the overall completion date of
the project.
Early start (ES)
Early finish (EF)
Late finish (LF)
Late start (LS)
All of the start, finish, and float information necessary to manage a
project is recorded on the activity node, as shown in Figure 10.3
of Gould.
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16. The first network calculation is called the forward pass, because it
starts at the first node and moves forward through the network,
from left to right. The calculation begins by assigning zero as the
early start time in the first node, and with the early finish time
equal to the early start time plus duration. The early start time of
the activity with only one precedent equals the early finish time of
the preceding activity. (See Figure 10.4 in Gould.) If a node has
more than one predecessor, the early start time equals the largest
early finish time of the preceding activities. (See Figure 10.5.)
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18. After the early start and finish dates are established and
the project completion date is found, the forward pass
for the project is complete. These times identify the
earliest possible time that each activity can start and
finish as well as the earliest possible time that the
project can be finished.
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19. The backward pass is conducted after the completion of the
forward pass. The backward pass cannot be done until the
project duration is known. The backward pass begins with the
last activity, with the late finish time for the last activity equal
to the project duration. The backward pass is completed by
working backward (from finish to start), subtracting each
activity's duration from the late finish time to calculate the late
start time. The late finish time of an activity with only one
successor is equal to the late start time of the successor. (See
Figure 10.7 in Gould.)
In the case where a node has multiple successor activities, the
late finish time of an activity equals the smallest late start
times of all the succeeding activities. (See Figure 10.8.)
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21. The backward pass provides the information
necessary to calculate the late start and finish
times that define the latest time that an
activity can start or finish and not delay the
scheduled completion time for the project.
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22. As can be seen in Figure 10.10 in Gould, an activity
can be done either early or late, and the difference
between the late start time and early start time is
equal to the difference between the late finish
time and the early finish time. This difference is
called float. Float is extra time that occurs in most
network schedules. Float can be calculated using
either of the following two equations:
Float = Late finish – Early finish
Float = Late start – Early start
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23. Float may be zero for some activities. An activity that
has non-zero float can be delayed without delaying
the project completion date. The float represents
the maximum possible activity delay that will not
delay the overall project.
Although any one activity may use its float without
delaying the project, a delay may occur if two or
more activities along any given path through the
network use their float. This is the case because
the use of one activity’s float may reduce the float
of a succeeding activity.
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24. Free float is another type of float that can be included in a network.
The formula for calculating free float is as follows:
Free float = ES (of succeeding activity) – EF (of activity in question)
Free float is useful to the project manager because it defines the
amount of time that an activity can be delayed without taking float
away from any other activity.
In general, float provides flexibility. By using float, the project team
can shift activities around to use people, equipment, and space
more efficiently. This flexibility can often save money.
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25. The longest path through the network (that is, the path with the
largest total of all activity durations) is called the critical path. The
critical path has zero float. Any delay along this path delays the
project. It is possible for a network schedule to have more than
one critical path. It is important for the project manager to identify
these paths and examine them closely, because control of the time
along these paths defines the duration of the project. If the
duration of the project is too long, activities that lie on the critical
path must be shortened, either by planning them differently, be
accomplishing them more rapidly, or by eliminating them.
If the critical path is shortened, more critical paths are usually
created, and the probability of the project being delayed due to
some unplanned circumstance increases. It is important for
management to study both the critical and the almost-critical
paths closely to anticipate and plan for all possible problems that
might occur. The fact that unforeseen problems do occur is why
float is so important. Float provides the project team with the
ability to respond to the many unplanned circumstance that occur
on any project.
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