precede scheduling. When these two are done simultaneously they generally introduce elements of scheduling prematurely, thus clouding the picture of the plan and severely limiting its flexibility. The planning steps most commonly followed in the NCF are listed in appendix G.
a. Project Analysis. Everyone concerned should know precisely what the project is, its start and end points, external factors such as schedule dates and requirements of others, and availability of resources such as men and equipment.
b. Activity List Development. All projects consist of separate but interrelated operations. In network analysis these are called "activities." The first stage in applying this technique is to obtain a list of all activities which constitute the project to be scheduled. This list is obtained in various ways: study of manufacturers' specifications; bills of materials and technical drawings; modification of a previous application; worksheets for a previous project;
and a joint discussion by those persons who sufficiently understand the project. The last method should be employed even if a tentative list has been obtained by other means. These are no specific definitions as to what constitutes an "activity," and it is largely a matter for individual interpretation according to the requirement of a particular project. A useful guideline is to ask three questions about any particular activity in the network. (These are simply useful pointers, not infallible rules).
(1) Would the activity normally be regarded as continuous from start to finish?
This does not mean that it may not prove expedient to split the activity later to facilitate scheduling. The key here is the word "normally."
(2) Will the required resources remain constant throughout the duration of the activity? In certain circumstances this may not be achievable but facilitates control if it can be done.
(3) Is the amount of work involved small enough to allow a reliable duration estimate to be assigned to the activity?
No attempt should be made to minimize the number of activities in a network by leaving out those considered to be unimportant and to have considerable float. They can easily be forgotten and their omission could lead to an entirely false analysis. A good rule is, don't plan a project in any more detail than is necessary to properly manage the scheduling of the work during the construction phase. The reason for this is to avoid being overwhelmed by a lot of detailed items of work that will change faster then the planner has time to adjust or plan for. For most NCF projects, detailed activities should not be less than one day in duration (preferably not less than three days). One further point to remember; there may be space limitations in the computer for activity descriptions. Each activity should be given a fairly concise title which will identify it in the computer printout.
ACTIVITY
c. Network Construction. A "network" is used to represent any sequencing of priorities among the activities that constitute a project. This sequencing is determined by two types of dependency.
(1) "Hard" dependencies are those which are based upon the physical characteristics of the job such as the necessity to place a foundation before building walls. A hard dependency is normally inflexible.
(2) "Soft" dependencies are those which are based upon practical considerations of policy and may be changed if circumstances demand. For example, the decision to start at the north end of a building rather than the south.
5. PRECEDENCE DIAGRAMING. In constructing networks the NCF uses Precedence Diagraming. A Precedence Network derives its name from the fact that each activity is identified in a computer program in terms of its immediately preceding and succeeding activities.
Precedence Diagraming does not require the use of dummy activities, is easier to draw, and has greater applications and advantages when Networks are put into the computer.
a. Activities and Events Representation. An activity in a Precedence Diagram is represented by a rectangular box and is identified by an activity number. A typical activity block is shown in figure 5-1. The left side of the activity box represents the start of the activity and the right side represents the completion. Lines linking the boxes are called "logic connectors" and the general direction of flow is indicated by arrowheads.
1
2
3
The rule that governs the drawing of a network is that the start of an activity must be linked to the ends of all completed activities before that start may take place. Activities that take place at the same time are not linked in any way. In figure 5-2 both activity 2 and activity 3 start as soon as activity 1 is completed. Activity 4 requires the completion of both activities 2 and 3 before it may start. Figure 5-2 shows a finish to start Logic Relationship. This is the most common relationship used in the NCF.
FIGURE 5-2. Precedence Diagram
b. Delay Representation. In certain cases there may be a delay or lag between the finish of one activity and the start of another. In this case the lag may be indicated on the connector itself incased in parenthesis as has been done in figure 5-3. Here, activity 3 may start as soon as activity 1 is completed, but activity 2 must wait two days. The "lag" is stated in the basic time units of the project, in which case the word "days" can be omitted.
1 2
3 (1)
1
(1)
c. Parallel Activities Representation. Some activities may parallel other which can be achieved in precedence diagrams without increasing the number of activities. For instance, it is possible to start laying a long pipeline before excavations are complete. This is known as a start-to-start Logic connection, as shown in figure 5-4. Activity 3 cannot start until one day after activity 1 has started.
FIGURE 5-4. Start-to-Start Logic Connection
It is also possible to start an activity independently, but not to complete it before another activity is completed. This is known as a finish-to-finish logic connection, as shown in figure 5-5. Activity 3 cannot be completed until one day after activity 1 is completed.
Care should be taken in using either of these logic connections. If possible, the Finish-to-Start Connection should be used for all NCF planning.
6. PRECEDENCE NETWORK CALCULATIONS. Precedence networks are the primary project scheduling tools used in the NCF. The scheduling of any construction activity is dependent on the duration of the construction activities that precede it. The forward pass, the backward pass, and float calculations are the basis to form the construction schedule.
a. Definitions
(1) The Forward Pass determines the Early Start and Early Finish of an activity.
The earliest date that an activity can be started and finished based on the durations of activities that precede it. It determines the total project duration based on the sum of the activity durations along the longest path through the project.
Calculations:
Early Start + Duration = Early Finish
Early Finish + Lag (if any) = Early Start (of next activity)
NOTE: With two or more predecessors, use the LARGER of the two Early Finish + Lag dates.
(2) The Backward Pass determines Late Start and Late Finish of an activity. The latest that each activity can start and finish without delaying the total project duration calculated on the Forward Pass.
Calculations:
Late Finish - Duration = Late Start
Late Start - Lag (if any) = Late Finish (preceding activity) NOTE: With two or more follow-on activities, use the SMALLER of the Late Start - Lag
dates.
(3) The Critical Path is those activities whose early start/early finish and late start/late finish dates match. The critical activities form a critical path from the first activity to the last.
(4) Total Float is the total number of days an activity can be delayed without delaying the completion date of the project, and allows a crew leader or project supervisor the discretion to start an activity anywhere between the early and late start dates without delaying the completion date of the project.
Larger #
Total Float = Late Start - Early Start or Late Finish- Early Finish NOTE: The total float for critical activities is always zero since the Early and Late Start
and Finish dates are the same. Therefore, Total Float is calculated for non-critical activities only.
(5) Free Float is the number of days an activity can be delayed without delaying the next activity from its Early Start or the number of days of Float that can be used without taking Float from another activity.
All or a portion of the Total Float may be shared with follow-on activities. If Float is not shared with a follow-on activity, it is free. Using Free Float does not take Float away from follow-on activities nor does it delay the next activity from its Early Start date.
Calculations:
Free Float = Early Start (next activity) - Lag (if any) - Early Finish NOTE: With two or more follow-on activities use the smaller of the early start - lag.
Early Start + Duration = Early Finish
*Early Finish + Lag (if any) = Early Start (next activity) Use largest of the (Early Finish + Lag) with two or mor e predecessors
FIGURE 5-6. The Forward Pass
Late Finish - Duration = Late Start
* Late Start - Lag (if any) = Late Finish (preceding activity) Use smallest of the (late Start - Lag) with two or more follo w on activities.
FIGURE 5-7. The Backward Pass
2010 3
12 15
(2)
2020 4
14 18
Total Float = Late Start - Early Start or Late Finish - Early Finish
* Free Float = Early Start (next activity) - Lag - Early Finish NOTE: With two or more follow on activities use the smaller o f
Early Start - Lag
FIGURE 5-8. Floats
Equations shown for the following Logic Connections are the ones that change from Finish-to-Start Logic Connection. All other equations remain the same.
2010 3 12
14
15
2 0 17
2020 4
14
16 20
18
2
(2)
Forward Pass: Early Finish + Lag = Early Finish (next activity) Backwards Pass: Late Finish - Lag = Late Finish (preceding activity) Free Float = Early Finish (next activity) - Lag - Early Finish
FIGURE 5-10. Finish-to-Finish
NOTE: ACTIVITIES IN THE NETWORK ARE NUMBERED FROM LEFT TO RIGHT AND FROM TOP TO BOTTOM. WHEN ACTIVITIES ARE ADDED OR DELETED IN
7. COMPUTER USE IN THE NCF. The usefulness of the computer as a construction