Project Scheduling Technique:
METHOD
DPWH
Basic
Principles
and
Application
Handbook
PRECEDENCE
DIAGRAM
METHOD
Project Scheduling: Precedence Diagram Method 1 | P a g e
TABLE OF CONTENTS
INTRODUCTION ... 2
PRECEDENCE DIAGRAM METHOD (PDM) ... 3
DEFINITION OF TERMS... 3
PDM LOGICAL RELATIONSHIPS ... 6
Finish-to-Start (F-S) relationship ... 6
Start-to-Start (S-S) relationship ... 7
Finish-to-Finish (F-F) relationship ... 8
Start-to-Finish (S-F) relationship ... 8
LAG AND LEAD TIMES ... 9
ANALYZING PDM NETWORK ... 11
Forward Pass Calculation ... 11
For Finish-To-Start Relationship ... 11
For Other Relationships aside from F-S ... 15
Backward Pass Calculation ... 17
For Finish-To-Start Relationship ... 17
For Other Relationships aside from F-S ... 21
THINGS TO REMEMBER ... 24
ANNEX ... 28
Project Scheduling: Precedence Diagram Method 2 | P a g e
INTRODUCTION
In connection with D.O. No. 133, Series of 2015, the adoption of Precedence Diagram Method as Project Scheduling Technique in DPWH Projects, this handbook was made to serve as basis in preparation/evaluation/review of PDM. However, this handbook was prepared in such a way so that it can be used by DPWH and other organizations.
Project Scheduling: Precedence Diagram Method 3 | P a g e
PRECEDENCE DIAGRAM METHOD (PDM)
PDM is a network diagram that shows activities in nodes and usually represented by rectangular/square boxes which are connected by arrows showing the relationships between the activities. This network diagram is also called as Activity on Node (AON) where the nodes are used to designate the activities.
Figure 1 shows the standard layout of PDM and the organization of its attributes, which will be the standard format to be adopted by DPWH for PDM.
DEFINITION OF TERMS
Activity – a single work step with a definable scope of work, an identifiable start and finish requiring time and typically resources to complete
Activity number – numbers used for unique identification which are ordered from lower numbers on the left to the higher numbers on the right to match the logic flow of activities.
Backward Pass – the process of navigating through a network from finish to start for the purpose of calculating the late start and finish times for each activity
Bar Chart – another term for Gantt chart; chart showing the construction time schedule of project activities.
S
TA
R
T
ITEM NO. ACTIVITY NO. DURATION
FI
NIS
H
ITEM DESCRIPTION
EARLY START EARLY FINISH LATE START LATE FINISH
Project Scheduling: Precedence Diagram Method 4 | P a g e Critical Path Method (CPM) – a project management technique that analyzes which activities have the least amount of scheduling flexibility and predicts project duration schedule based on the activities that fall along the "critical path." Activities that lie along the critical path cannot be delayed without delaying the finish time for the entire project.
Dummy Activity – An activity (represented by a dotted line on the arrow network diagram) that indicates that any activity following the dummy cannot be started until the activity or activities preceding the dummy are completed. The dummy does not require any time.
Duration – the length of time required to complete an activity Early Start (ES) – earliest point in time an activity can start Early Finish (EF) – earliest point in time an activity can finish
Finish to Finish – relationship between two activities where the successor activity cannot finish until the predecessor activity has finished
Finish to Start – relationship between two activities where the successor activity cannot start until the predecessor activity has finished
Float – period by which a task can be delayed, brought forward or extended without affecting the schedule; a measure of how much an activity can be delayed without delaying the project completion date. Forward Pass – the process of navigating through a network from start to finish and calculating the early start and finish times for each activity and the minimum time required to complete the project Free Float – the amount of time the start of an activity can be delayed or its duration extended without delaying the early start of following activities
Project Scheduling: Precedence Diagram Method 5 | P a g e Lag – period of time that must pass after the finish of one activity before the following activity may start
Late Finish (LF) – latest point in time an activity can finish and not delay the project
Late Start (LS) – latest point in time an activity can start without delay in the project
Lead – period of time that must pass after the start of one activity before the start of the following activity
Logical Sequence – the sequence of the activities wherein the preceding activity should be started and/or partially or totally completed first before the succeeding activities started in a logical manner
Node – contains the attributes of the activities such as Item Number, Item Description together with Activity name, duration, early start/finish, late start/finish and start and finish side of the node Program Evaluation and Review Technique (PERT) – a graphic representation of a project’s schedule showing the sequence of tasks, which can be performed simultaneously.
Precedence Diagramming Method (PDM) – a method for diagramming construction activities wherein a single node represents an activity which is logically connected to other activities
Predecessor – an activity that must be partially finished or finished before a specified activity can start
Relationships – the logical links between activities used to demonstrate work sequence which is represented by a
Scheduling – the process of determining the timing and specific sequence of tasks in order to carry out the planned construction operations
Project Scheduling: Precedence Diagram Method 6 | P a g e Start to Finish – one activity can only be finished when the next activity has started
Start to Start – two activities that can be started simultaneously Total Float – the maximum amount of time an activity can be delayed from its original early start without delaying the duration of the entire project; calculated by subtracting the early start time and duration from the activity’s late finish time.
PDM LOGICAL RELATIONSHIPS
PDM also shows interdependencies among various activities which is not limited to “Finish-to-Start” (F-S) relationship as compared to PERT/CPM. This network diagram also uses four (4) logical relationships and these are:
Finish-to-Start (F-S) relationship
Finish-to-Start relationship is the most common logical relationship used in PDM. This is also used in PERT/CPM. This kind of relationship denotes that the succeeding activity cannot start until the preceding activity has been completed.
Figure 2. PDM F-S Relationship
Figure 2 shows two activities, Activity A and Activity B, having a Finish-to-Start Relationship. This figure shows that right after Activity A is finished, Activity B can already commence its work.
Project Scheduling: Precedence Diagram Method 7 | P a g e
Start-to-Start (S-S) relationship
Start-to-Start relationship is a logical relationship that shows two activities starting simultaneously. In PERT/CPM, Start-to-Start relationship can be presented by having additional nodes and use of dummy arrows. Figure 3 shows S-S relationship between activities A and B in a PERT/CPM Network Diagram.
Figure 3. PERT/CPM Network S-S Relationship
In PDM, Start-to-Start relationship is introduced to show the relationship between two parallel activities that should be started simultaneously. For example, in Figure 4, both activities A and B can already start their respective works since both activities are using Start-to-Start relationship.
Project Scheduling: Precedence Diagram Method 8 | P a g e
Finish-to-Finish (F-F) relationship
As with Start-to-Start relationship, Finish-to-Finish relationship is used to show the completion of activities simultaneously. As shown in Figure 5, activities A and B are in Finish-to-Finish relationship. This shows that when Activity A is completed, Activity B should also be finished.
Figure 5. PDM F-F Relationship
Start-to-Finish (S-F) relationship
Start-to-Finish relationship is rarely used since there are less confusing interdependencies available in PDM. This relationship indicates that one activity can only be finished when the next activity has started. Figure 6 shows Start-to-Finish relationship between activities A and B. In this figure, Activity B can only be completed as soon as Activity A has started.
Project Scheduling: Precedence Diagram Method 9 | P a g e
Figure 6. PDM S-F Relationship
LAG AND LEAD TIMES
Lag (+) and Lead (-) times are expressed as part of the immediate predecessor notation. Lag time is defined as the delay between two activities, that is, when one activity is completed and there is a waiting period before the succeeding activity starts. In construction industry, specifically in concreting, lag is most commonly used in situations that require material to strengthen before the next activity can be performed. Lag is also used to simplify the network diagram by representing other subactivities related to the main activities (i.e. curing period for concreting, removal of shoring, etc.)
Lead time, on the other hand, is an overlap between the first and second activity, that is, when the second activity will start prior to the completion of the first activity. This condition is used in cases which allow succeeding activities to begin before preceding activities have been completed.
In F-S relationship, lag and lead times are commonly used. As shown in Figure 7a, since activities A and B can work subsequently without any delays, the lag time is equal to zero (0), thus resulting to a total duration of 8 calendar days (c.d.).
Project Scheduling: Precedence Diagram Method 10 | P a g e
Figure 7a. Activities A and B in F-S Relationship
Given that there is a delay between two activites, a lag time is introduced. As shown in Figure 7b, there is a need of 2 days lag between activities A and B which means that Activity B can only start 2 days after Activity A has finished. In this figure, the lag time is equal to 2 c.d. resulting to a total duration of 10 c.d.
Figure 7b. Activities A and B in F-S Relationship with Lag Time
In Figure 7c, there is an overlapping between activities A and B where Activity B can already start 2 days before Activity A finishes. In this case, lead time is being used, which is equivalent to 2 c.d., resulting to a total duration of 6 c.d.
Project Scheduling: Precedence Diagram Method 11 | P a g e
Figure 7c. Activities A and B in F-S Relationship with Lead Time
For S-S, F-F, and S-F relationship, lag time is also used to signify a delay between two activities. Lead time, on the other hand, may be used in these interdependencies to finish the project early and on time but is best to be avoided, if possible, due to difficulties and complexity in analysis of the logic diagram and computation of backward and forward pass.
ANALYZING PDM NETWORK
Forward Pass Calculation
For Finish-To-Start Relationship
a) General Rule in Calculating Early Start & Early Finish dates based on Finish-to-Start relationship without lag and lead time
ES of the Current Activity = EF of Preceding Activity; ES=0 for starting activity
EF = ES + Duration
For Multiple Predecessors, use the latest or largest value of EF of the preceding activity as the ES of the current activity being considered
Project Scheduling: Precedence Diagram Method 12 | P a g e STEP 1:
ESA = 0 (starting activity)
EFA = ESA + durationA = 0 + 5 = 5 STEP 2:
ESB = EFA + (Lag or Lead Time) ESB = 5 + 0 = 5
Project Scheduling: Precedence Diagram Method 13 | P a g e STEP 3:
EFB = ESB + durationB EFB = 5 + 3 = 8
Figure 8a. Forward Pass Calculation for F-S relationship
b) General Rule in calculating Early Start and Early Finish dates based on Forward pass calculation for Finish to Start relationship with Lag and Lead Times
For Lag Time (+); ES of the Current Activity = EF of the Preceding Activity + (Lag Time) and EF of current activity will be equal to ES of current activity + Duration (See Figure 8b) If there is a Lead Time (-) between two activities; ES of the
Project Scheduling: Precedence Diagram Method 14 | P a g e STEP 1:
ESA = 0 (starting activity)
EFA = ESA + durationA = 0 + 5 = 5 STEP 2:
ESB = EFA + (Lag or Lead Time) ESB = 5 + 2 = 7
STEP 3:
EFB = ESB + durationB EFB = 7 + 3 = 10
Figure 8b. Forward Pass Calculation for F-S relationship with Lag Time
Project Scheduling: Precedence Diagram Method 15 | P a g e
For Other Relationships aside from F-S
For F-F relationship, to get the EF of the preceding activity, EF CURRENT = EFPREVIOUS + (lag or lead). (See Figure 9.)
If in case that there are other relationships aside from F-F with the current activity, EFCURRENT can be also determined by adding ESCURRENT with its duration. Then assess which value for EFCURRENT is higher or latest.
Project Scheduling: Precedence Diagram Method 16 | P a g e STEP 1: ESA = 0 (starting activity) EFA = ESA + durationA EFA = 0 + 5 = 5 STEP 2: ESB = ESA + (Lag or Lead Time); (S-S relationship) ESB = 0 + 1 = 1
Project Scheduling: Precedence Diagram Method 17 | P a g e STEP 3:
To determine EFB consider the following: 1. EFB = ESB + durationB EFB = 1 + 3 = 4 or 2. EFB = EFA + (Lag or Lead Time); (F-F relationship) EFB = 5 + 0 = 5
*Given there are two (2) possible values for EFB, choose the bigger or later value for EFB
Therefore; EFB = 5
Figure 9. Forward Pass Calculation for S-S and F-F relationship
Backward Pass Calculation
For Finish-To-Start Relationship
a) General Rule in Calculating Late Start & Late Finish dates based on Finish to Start relationship
Late Finish of the Current Activity = Late Start of Previous Activity minus lag or lead time; LF of final activity = EF for the final activity of the project
Late Start of the current activity = Late Finish of the current activity – its duration
Project Scheduling: Precedence Diagram Method 18 | P a g e For Multiple Predecessors, use the earliest or smallest value of
LS of the preceding activity as LF of the current activity
STEP 1:
LFB = EFB (final activity) LFB = 8
LSB = LFB – durationB LSB = 8 – 3 = 5
Project Scheduling: Precedence Diagram Method 19 | P a g e STEP 2:
LFA = LSB - (Lag or Lead Time) LFA = 5 - 0 = 5
LSA = LFA - durationA LSA = 5 – 5 = 0
Figure 10a. Backward Pass Calculation for F-S relationship
b) General Rule in Calculating Late Start & Late Finish dates based on Backward Pass calculation for Finish to Start relationship with Lag and Lead Times
For Lag Time (+); LF of the Current Activity = LS of the Preceding Activity - (Lag Time) and LS of the current activity will still be equal to LFCURRENT – duration. (See Figure 10b) If there is a Lead Time (-) between two activities; LF of the
Project Scheduling: Precedence Diagram Method 20 | P a g e STEP 1: LFB = EFB (final activity) LFB = 10 LSB = LFB – durationB LSB = 10 – 3 = 7 STEP 2:
LFA = LSB - (Lag or Lead Time) LFA = 7 - 2 = 5
LSA = LFA - durationA LSA = 5 – 5 = 0
Figure 10b. Backward Pass Calculation for F-S relationship with Lag Time
Project Scheduling: Precedence Diagram Method 21 | P a g e
For Other Relationships aside from F-S
For S-S relationship, to get the LS of the preceding activity, LSCURRENT = LSPREVIOUS – (lag or lead). (See Figure 11)
If in case that there are other relationships aside from S-S with the current activity, LSCURRENT can also be determined by subtracting LFCURRENT with its duration. Then assess which value for LSCURRENT is lower or earliest
Project Scheduling: Precedence Diagram Method 22 | P a g e STEP 1: LFB = EFB (final activity) LFB = 5 LSB = LFB – durationB LSB = 5 - 3 = 2 STEP 2: LFA = LFB – (Lag or Lead time); (F-F relationship) LFA = 5– 0 = 5
Project Scheduling: Precedence Diagram Method 23 | P a g e STEP 3: To determine LSA consider the following: 1. LSA = LSB – (Lag or Lead Time); (S-S relationship) LSA = 1 – 0 = 1 or 2. LSA = LFA - durationA LSA = 5 – 5 = 0
*Given there are two (2) possible values for LSA, choose the lower or earlier value for LSA
Therefore; LSA = 0
Figure 11. Backward Pass Calculation for S-S relationship with Lag Time
Project Scheduling: Precedence Diagram Method 24 | P a g e
THINGS TO REMEMBER
In PDM, the relationship between the current and
previous activities can be better calculated by using two or more of the four possible logical relationships than by using only one relationship.
o In a PERT/CPM diagram, which is only limited with F-S relationship, additional nodes and dummy can be used to represent activities that are concurrent. As shown in Figure 12, the activities in the bar chart are presented in PERT/CPM showing the correct relationships by breaking each activities into repetitive activities and with the use of dummy.
o Using PDM, however, can be simplified the calculation of the same by using two of the four possible relationships together with lag and lead times, the same correct relationship can be shown in Figure 13 with only three activities.
Calculation of the critical path requires consideration of
the logical relationship of each activity.
o The critical path in PDM is continuous from the beginning to the end of the diagram. Just like in PERT/CPM, the total float of an activity must be equal to zero to be considered critical. Usually activities using F-S relationship without lag are considered to be critical.
o In PDM, a scheduler should observe completeness and show all logical relationships among activities, avoiding any unnecessary or redundant relationships. Too many arrows will complicate calculation of task dates during the forward and backward passes. However, while avoiding redundancies, one must be careful not to oversimplify the schedule.
Project Scheduling: Precedence Diagram Method 25 | P a g e Fi gu re 1 2. Bar C hart a nd P ER T/ C P M of S u b b as e C ou rse, Ba se C o urse, a n d P C C P
Project Scheduling: Precedence Diagram Method 26 | P a g e Fi gu re 1 3. Ba r C hart a n d P DM of S u b b as e C ou rs e, Ba se C ou rs e, an d P C C P
Project Scheduling: Precedence Diagram Method 27 | P a g e Usually activities with a start-to-start relationship will
also have a finish-to-finish relationship.
o Failure to show finish-to-finish relationships among activities that have start-to-start relationships indicated on the logic diagram may change the logic and the critical path resulting in inaccurate duration among the activities.
In a comprehensive schedule all activities except the first
and last nodes should have at least one preceding activity and at least one succeeding activity.
o Dangling activity refers to an activity that is open ended where there are no activities connected either on the left or right side. This activity also has no activity relationship originating either left or right side and should be avoided as this is considered as poor scheduling practice.
o In a PDM diagram, more dangling activities may prevent a realistic critical path preventing the schedule from accurately measuring the delays and any changes in the time frame.
Project Scheduling: Precedence Diagram Method 28 | P a g e
Project Scheduling: Precedence Diagram Method 29 | P a g e Fi gu re 1 4. S a mp le P E R T /C P M of R oad P roj ec t
Project Scheduling: Precedence Diagram Method 30 | P a g e Fi gu re 1 5. P DM of Sa mp le P E R T/ C P M fro m Fi g ur e 14
Project Scheduling: Precedence Diagram Method 31 | P a g e
