Project Scheduling 4The Network Diagram  

Prepared by Sebghatullah Karimi(Junior Lecturer of Kabul Polytechnic University)

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Kardan UniversityCivil Engineering Faculty

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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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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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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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 finishor

Float = Late start – Early startFree 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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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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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. Thresholds9

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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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

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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

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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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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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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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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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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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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 finishFloat = Late start – Early start

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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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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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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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