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

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)

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

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.

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

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

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?

WBS linked to OBS(Organization breakdown structure)

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

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

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

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

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

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

https://youtu.be/mnrp1cWUDY0

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 https://www.smartsheet.com/critical-path-method

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

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?

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?

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

Step 3: Forward Pass

Forward Pass

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

Backward Pass

Backward Pass

Activity times in perspective

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

Critical Path Analysis Example

Example 1

Example- solution Critical Path

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

CPM Example

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”

Total and Free Float

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)

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.

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

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

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

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

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.

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.

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.

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.

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.

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

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.

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

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 300cy/day

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.

Activity Durations Means data can be used for computing activity durations

CSI Format- 50 Divisions

RS Means- 30 days free online access https://www.rsmeansonline.com/

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.

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

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

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

Questions Please send me your questions at aaraheem@utep.edu