1. Lecture 2. CPM Schedule Network Logic Diagrams Activity Durations
Presented by: Dr. Adeeba Abdul Raheem

2. What we learned last week?
Can you explain what is the difference between planning and scheduling? (Hint: Lecture 1-slides 10-13)
Which method of scheduling is preferred when the construction project is repetitive in nature? (Hint: Lecture 1-slide 51)

3. Lesson Objectives Plan a schedule How to draw precedence diagram?
Difference between total and free float?
How to determine activity durations?

4. CPM Network Scheduling Development & Background
CPM was developed in 1950’s by the US Navy for the Polaris Missile Program
Starting in 1956, a joint venture between the Dupont and computer firm Remington Rand devised the Critical Path Method to tackle the interrelationships of separate activities within a project schedule.

5. Planning A Schedule- DGD, DED
Define scope, method statement and sequence
Generate the work breakdown structure (WBS)
Develop the organization breakdown structure (OBS)
Determine the relationship between activities
Estimate activities time duration, cost expenditure, and resource requirement
Develop the project network
Plan
Define the scope of work, method statement, and sequence of work.
2. Generate the work breakdown structure (WBS) to produce a complete list of
activities.
3. Develop the organization breakdown structure (OBS) and link it with work
breakdown structure o identify responsibilities.
Schedule

6. The Work Break Down Structure (WBS)
An organizational tool
Useful for defining the scope of work
Consists of:
Goal statement for project
Subdividing goal into
smaller portions

7. Break the Project into Activities
Could use a Work Breakdown Structure (WBS)
Keep trades and contractual divisions separate
May even want to separate crews
Keep field work separate from shop or office work
Determine the level of Detail
Include submittal process?
Include procurement of material?
Include curing time for concrete?

8. WBS linked to OBS(Organization breakdown structure)

9. Start and Finish times of an activity
Early Activity Start (ES): Earliest time an activity can start—as determined by the latest of the early finish times
Early Activity Finish (EF): Earliest time an activity can finish— determined by adding the duration of the activity to the early start time
Late Activity Start (LS): Latest time an activity can start without delaying the project completion
Late Activity Finish (LF): Latest time an activity can be finished without delaying project completion

10. Linkages/ Activity relationship
Excavation- Foundation
Pour foundation- Level concrete
Wiring- Inspect electrical
Less frequently used- milestone completion

12. Activity Relationships
Finish-to-Start with a delay
Finish-to-Start Relationship with a 28-Day Delay

14. Activity Relationships
Activities with Start-to-Start with a Delay Relationships

16. Activity Relationships
Activities with Finish-to-Finish with a Delay Relationships

18. Activity Relationships
Activities with Start-to-Start and Finish-to-Finish Relationships

20. https://youtu.be/mnrp1cWUDY0

21. Steps to Create A CPM Schedule
Construct the Logic Diagram
Add the Durations
Calculate the Early Dates (forward pass)
Calculate the Late Dates (backward pass)
Calculate the Total Float
Mark the Critical Path
Please go through the following website to have more details on various steps to create CPM Schedule

22. Step 1:Construct the Logic Diagram
Best Scheduling Practices Suggest only One Beginning Activity and only One Ending Activity for the Project

23. Make Sure the Logic is Correct
Review Logic Diagram with ALL Team Members
Ask question #1 of every activity - Does this activity really have to be done before each of the following activities can start?
Ask question #2 – What other activities need to be done before this activity can start?

24. How Do You Determine the Durations of the Scheduled Activities?
Step 2: Add the Durations to the Logic Diagram
How Do You Determine the Durations of the Scheduled Activities?
From Subs
Estimating or Productivity Manuals
Company Records of Productivity Rates
Flat out Guess
What was it we were trying to manage?

25. Node formats for CPM diagram There are various formats that are being used to draw the nodes in precedence diagrams. Some of the examples are as follows (We’ll be using format A for this course): A C D B

26. Step 3: Forward Pass

27. Forward Pass

28. Step 4: Latest Start & Finish Steps
Begin at ending event & work backward
LF = Maximum EF for ending activities
LF is latest finish; EF is earliest finish
LS = LF - Activity time
LS is latest start
LF = Minimum LS of all successors for non-ending activities

29. Backward Pass

30. Backward Pass

31. Activity times in perspective

32. Step 5 and 6: Critical Path Analysis
Provides activity information
Earliest (ES) & latest (LS) start
Earliest (EF) & latest (LF) finish
Slack (S): Allowable delay
Identifies critical path
Longest path in network
Shortest time project can be completed
Any delay on activities delays project
Activities have “ZERO” float

34. Critical Path Analysis Example

35. Example 1

36. Example- solution
Critical Path

37. Calculating Durations & Float You can download the excel file for this problem online to practice

38. CPM Example

39. Types of Float 1. Total Float
“The amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint.”
2. Free Float/ Activity Delay
“The amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint”

40. Total and Free Float

41. Types of Float
3. Shared (Path, string, interference) float: Shared float will interfere with the float of later activities 4. Independent float: Belongs to one activity only. It is not shared by the other activities
Independent float (IF)
Shared float (SS)

42. Types of Float
Negative float: By having fixed the start and finish date, where the finished date is set earlier than the date by forward pass
The amount of time the project schedule is beyond the client imposed milestone. It is called 'negative' float since in this case the project completion need to be advanced not delayed.

43. Activity Relationships
Start-to-Finish with a Delay — Relationships

44. ACTIVITY DURATIONS TYPES OF ESTIMATES
The type of estimate will determine the detail and accuracy of the schedule
Estimates are either preliminary or detailed
Preliminary estimates render ± 30% accuracy
Preliminary schedules are only as accurate as the preliminary estimate

45. ACTIVITY DURATIONS
What will we need to know in order to accurately estimate the project?
Quantities
Crew & Equipment requirements
Productivity of Crew & Equipment

46. Bridge Schedule- Example
Task #
Task Description
Duration (workdays) 1
Survey and layout 2
Rough-grade access road on east bank 3
Drive timber piles for east abutment 4
Construct east abutment 5
Drive timber piles for midstream pier 6
Complete midstream peer 7
Place steel girders east span 8
Place timber decking east span 9
Drive timber piles west abutment

47. ACTIVITY DURATIONS The breakdown of our work elements for the bridge
The first element is “Survey and Layout”
What information do we need?
We need to know what Crew & Equipment is necessary to compete this element.
We need to know how much surveying and layout is required.
Then we need to know the amount of surveying and layout our crew can perform in a day.

48. ACTIVITY DURATIONS What are the units for survey and layout?
They might be acres/day
Or points/day
Or structures/day
Or some other logical measurement
Let’s say that our preliminary layout will be for the road, the approaches, and general control points.
For our project we assume that it will take one day for control and one day to provide construction stakes for the other elements.
Therefore, this is a 2-day duration.

49. ACTIVITY DURATIONS What is our Crew & Equipment?
We can establish our crew & Eqpt. as
Lead surveyor $30/hr
Instrument man $20/hr
Rodman $15/hr
Assistant $10/hr
Instruments $25/hr
Truck $35/hr
We usually establish a “Crew cost” and “Equipment Cost”
$75/hr.
$60/hr.

50. ACTIVITY DURATIONS
Let’s look at our second element, “Rough Grade access road East bank”
What are the quantities?
We can determine the quantities by calculating quantities based on the drawings.
What are the work elements for this item?
We first must clear the area where we are to construct our road.

51. ACTIVITY DURATIONS In what quantities do we measure clearing? Acres
Are all acres the same?
No, we have clearing defined as:
Light
Normal
Heavy
Or perhaps some other terms, but this is a generally acceptable definition.

52. ACTIVITY DURATIONS Let’s say we have light clearing.
What do we need to accomplish light clearing?
We will need:
Foreman $30/hr
Equipment operator $20/hr
Laborer (2 each) $20/hr
Bulldozer $75/hr

53. ACTIVITY DURATIONS What’s the next task of work element #2?
Using the stakes installed by the surveyors, the bulldozer will “rough grade” the area for the road.
This means it will excavate required “cut” material and spread required “Fill” material.
What does “rough grade” mean anyway?
It means bringing the surface to within a few tenths of a foot of its final required elevation.
Is this all we need for “rough grade?”
We will need a compactor if fill is to be brought in.

54. ACTIVITY DURATIONS
So, let’s see what we will need in the way of Crew & Equipment for this element.
Foreman $30/hr
Equipment operator $20/hr
Laborer (2 each) $20/hr
Bulldozer $75/hr
Compaction attachment $20/hr

55. ACTIVITY DURATIONS What are we assuming?
We are assuming that this same bulldozer and the crew will do both clearing and grading.
What are we missing?
We need some productivity
How about:
Clearing (light) 5ac/day
Grading cy/day

56. Rough Grade access road East bank
ACTIVITY DURATIONS
Work Element
Units
Quantity
Crew cost/unit
Eqpt cost/unit
C&E Productivity
Total Cost
Duration in Days
Clear for access road
Acres 2
$70/hr
$75/hr
5 ac/day
0.4
Rough Grade access road East bank cy
1200
$95/hr
300 cy/day 4
Etc.

57. Activity Durations
Means data can be used for computing activity durations

58. CSI Format- 50 Divisions

59. RS Means- 30 days free online access

60. Delays
WEATHER – the effect of weather on a project duration must be considered.
When weather delay a project that delay must be considered in the schedule.
Weather can mean a number of things:
Rain delay
Cold weather delay
Hot weather delay
Storm (hurricane) delay
Etc.

61. DELAYS
Other Delay – other factors can cause delays that extend project duration.
Inspection
Material delivery
Rework
Permitting
Unforeseen (archeological artifacts)
Etc.

62. DELAYS
Accounting for Delays – if it is anticipated that a genuine delay due to factors such as weather are real, the amount of delay must be added to the schedule.
This additional time can be added in a variety of ways:
Increase the durations of activities that would actually be delayed by rain
Add specific delay activities
Add a % increase to critical activities

63. Uncertainty in Duration Estimates
Uncertainty – durations are estimates and therefore contain some uncertainty
Scheduling is the basis for the management of time on a construction project
It is essential for the prudent scheduler to have a clear understanding of the uncertainty actually associated with duration estimates
It should be recognized that duration is not an absolute value

65. Questions
Please send me your questions at