PDM Precedence Diagramming Method (PDM) Part 1 Con E 441 P05 A. Four (4) possible relationships 1. Finish to Start (FS). Start to Start (SS) 3. 4. Start to Finish (SF) B. Plus combined relationships SS with FF PDM PDM 3 PDM C. Can be time scaled with software D. Natural progression from the bar chart, easier to understand E. Disadvantage: unable to tell what part of an activity is related to another Finish to Start (FS) A. Most common for scheduling B. Succeeding activity cannot start until the preceding activity has been completed C. Three (3) possible forms using 1. Zero Lag most common. Positive Lag used for concrete to gain strength 3. Negative Lag - sometimes called LEAD PDM 4 PDM 5 Finish to Start (FS) (cont.) 3. Negative Lag succeeding activity can begin before preceding activities have been completed CAUTION Forward pass calculation could vary substantially between a F-S relationship with a (1) lag and a S-S relationship with a + lag Start to Start (SS) A. Draw activities one above the other B. Activities with SS with zero lag start concurrently C. Can use positive LAG D. Do not use negative LAG with SS very difficult to understand E. CAUTION Do NOT use alone, use in combo with FF, or will create errors in the critical path calculations!! PDM 6 PDM 7 1
Start to Start (SS) Start-to-start with positive lag A. Draw activities one above the other B. FF with zero lag most common C. Can use positive LAG D. Do not use negative LAG with FF very difficult to understand E. CAUTION Do NOT use FF alone. Use in combo with SS, or it may create errors in the critical path calculations!! Start-to-start with zero lag PDM 8 PDM 9 Can use all three forms of LAG 1. Zero. Positive 3. Negative (not commonly used, confusing) Zero Lag Finish-to finish with zero lag PDM 10 PDM 11 Positive Lag Start to Finish (SF) Not commonly used Not clear as to how to interpret May include LAG 1 PDM 1 PDM 13
Combo SS & FF SS used with FF to prevent errors in calculating the critical path Activities can be partially or completely concurrent Partially concurrent Start of one activity follows start of another by a few days (LAG = +) Completely concurrent Truly simultaneous activities PDM 14 Combo SS & FF Partially Concurrent EXCAVATION CONCRETE EXCAVATION 6 CONCRETE PDM 15 6 Combo SS & FF Completely Concurrent Logic Loops Avoid at ALL costs! Scheduling software will NOT accept Keep logic SIMPLE to avoid logic loops PDM 16 PDM 17 Logic Loops Dangling Activities All Dogs Have Tails EXCAV ALL activities MUST have an activity that follows WITH a logic relationship EXCEPTION Finish Milestone (No Tail) PDM Diagram with three logic loops PDM 18 PDM 19 3
Dangling Activities Scheduling software interprets ALL Dangling Activities as CRITICAL, and therefore on the CRITICAL PATH Primavera P3.1 and Sure Trak 3.0b both show Dangling Activities as CRITICAL (default setting in software-can be shut off at great peril to the scheduler & project manager) PDM 0 (white board examples) (more to follow in class on Monday) PDM 1 Precedence Diagramming Method (PDM) Part Con E 441 P06 PDM Activity Times ES = Early Start Time Earliest time activity can start after completion of predecessors EF = Early Finish Time Earliest time activity can be completed after completion of predecessors and activity in estimated duration LS = Late Start Time Latest time activity can be started without delaying project completion time LF = Late Finish Time Latest time activity can be completed without delaying project completion time PDM 3 Forward Pass ES for first activity is zero. EFACT = ESACT + DurationACT FS: ESACT = EFPRED + LAGARROW FF: EFACT = EFPRED + LAGARROW SS: ES ESACT =ES ESPRED +LAG LAGARROW SF: EFACT = ESPRED + LAGARROW Governing ESACT is the largest trial ES. Governing EFACT is the largest trial EF OR ESACT plus DurationACT, whichever is greater. Backward Pass LF for last activity is project duration. LSACT = LFACT -DurationACT FS: LFACT = LSSUC LAGARROW SS: LSACT = LSSUC LAGARROW FF: LFACT = LFSUC LAGARROW SF: LSACT = LFSUC LAGARROW Governing LFACT is the smallest trial LF. Governing LSACT is the smallest trial LS OR LFACT minus DurationACT, whichever is least. PDM 4 PDM 5 4
Calculating Float Total Float (if zero, activity is critical): TFACT = LFACT ESACT DURATIONACT Free Float FFACT = ESSUC -EFACT Start Float (if zero, start is critical): SFACT = LSACT ESACT Finish Float (if zero, finish is critical): FNFACT = LFACT EFACT If both Start Float and Finish Float are Zero, the entire activity is critical. PDM 6 HW #3 Due: P07 - Friday, Sept. 14 th @ 9:00 AM Chapter 3: problems: 1, 4, 6, 7 (See WebCT for corrections) Redraw on Engineering problems paper, straight lines, professional appearance (photocopies of problems in textbook NOT allowed). REMINDER: P07: Ch. 4-Activity Durations, Friday, 9/14 P08: Ch. 5-Time In Contracts (BRING TEXTBOOK): Monday, 9/17 P09: Scheduling Logic Skills, Assign HW #15, Friday, 9/1 P10: EXAM NO. 1-Chapters 1 4, Monday, Sept. 4 th See WebCT for split classroom assignments PDM 7 In-Class Examples (work example problem) PDM 8 5