Estimating-Durations-Activities-EDGE.pdf
- 1. Determining Activities' Duration in the Construction Schedule Muhammad Zubair B.E., MS (UK), PMP
- 2. An Activity, Lag, or Event? An activity: Duration > 0, uses resource and has a budget A lag: Duration > 0, does not use resource, does not have budget An event (or milestone): Duration = 0, does not use resource , does not have budget An event likely has a responsibility, i.e. someone responsible Activities Duration Slide No. 2
- 3. Who Determines Durations? It is strongly recommended for the scheduler not to assign durations without consulting with the teams (crews / subcontractors) leaders. In the end, the project manager must take a careful look at the schedule and approve it; perhaps with some adjustments. This implies the approval of all team leaders and subs. Activities Duration Slide No. 3
- 4. How are Durations Determined? Several methods are used to determine durations, ranging from objective / calculated to subjective / guesstimated: A. Calculations based on average past performance or expected production for a worker or a crew. B. Calculations based on given equipment production rate. C. Estimation based on expert opinion. D. Estimation based on comparing to numbers in a reference. With assumptions in all cases. Activities Duration Slide No. 4
- 5. Source of Production Rates 1. Contractor’s own past records How close is the item considered to the one compared to? 2. Ask crew leaders / subcontractors 3. Commercial databases such as RS Means or Spon's price book 4. Internet How reliable? Activities Duration Slide No. 5
- 6. How are Durations Determined? Most importantly: Have all factors impacting the production been considered? A. In the “database”, and B. The considered project and activity? Activities Duration Slide No. 6
- 7. Productivity Numbers When keeping a production rate in the database (or importing it), some factors have to be considered: 1. Labor skill level. 2. Amount of resources allocated 3. Weather conditions 4. Design complexity 5. Learning curves 6. Fatigue 7. Multi-shift turnover 8. Soil condition (for excavation) 9. Height (for masonry, painting, etc.) 10. QA/QC and safety requirements 11. Work minutes / hour 12. Jobsite logistics and congestion 13. Any unexpected evets? 14. Does it represent “average production” to you? Activities Duration Slide No. 7
- 8. Productivity Numbers Now you have a “base” number as a reference, you need to make proper adjustments based on the current job situation. You can start with the same previous factors. For cost estimating purpose, there are other factors too. Activities Duration Slide No. 8
- 9. Adjusting Numbers Factors that have to do with: The design of the project The climate / weather: Temperature, humidity, wind, precipitation, visibility, lightning, Location / terrain / elevation Jobsite congestion and condition Workforce type and skill level Crew composition Type and condition of equipment and tools used Availability and readiness of materials Management style Safety regulations Activities Duration Slide No. 9
- 10. Importance of Durations Accuracy 1. Knowing how long the crews are needed 2. Calculating the cost of the activity 3. Determining the dates for dependent / succeeding activities 4. Calculating the expected finish date for the project • So in general, the accuracy of activities’ duration impacts both the time and cost aspects of the project. Activities Duration Slide No. 10
- 11. Inaccurate Durations Impact of inaccurate durations Delayed crews Subcontractors conflicts Wasted money Shortage of materials / crews (if actual date is earlier) Possible problems with cash flow Disrupted plans and chaos Impact on succeeding activities as well as the entire project Activities Duration Slide No. 11
- 12. Items in the RS Means Database Activities Duration Slide No. 12
- 13. Items in the RS Means Database Labor hours per CY = 𝐿𝑎𝑏𝑜𝑟 ℎ𝑜𝑢𝑟𝑠 𝑝𝑒𝑟 𝑑𝑎𝑦 𝐶𝑟𝑒𝑤 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑝𝑒𝑟 𝑑𝑎𝑦 = 12 800 = 0.015 Activities Duration Slide No. 13
- 14. 4,200 SF Elevated slab, 4 use, 18’ high Duration = 4200 / 495 = 8.5 ≈ 9 days Labor hours = 4200 x 0.097 = 407.4, or = 6 x 8 x 8.5 = 408 = 6 ∗ 8 495 # of workers Hours/ day Activities Duration Slide No. 14
- 15. Multi-Stage Activities Those activities performed by the same crew but with different nature or location Wood framing Flooring Those activities performed by different crews but result in completion of same finished product Reinforced concrete items Keep it as one activity or split it? Use the KISS principle! Activities Duration Slide No. 15
- 16. Pictures from https://pixabay.com/ Activities Duration Slide No. 16
- 17. Concrete Work Picture from https://pixabay.com/ Activities Duration Slide No. 17
- 18. Simplify… In general, it is better to break the activity whenever its portions are separated physically or by time. This simplifies all calculations: productivity, time, and money This also reduces the use of Start-to-Start (SS) and Finish-to- Finish (FF) relationships and lags, which simplifies even more. Activities Duration Slide No. 18
- 19. Intermittent Activities Electrical work: one activity, 23 days Electrical work: two activities, 4, and 3 days Electrical work 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Days Rough-in Finish Electrical Rough-in Electrical Finish 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Days Activities Duration Slide No. 19
- 20. Discontinuous Activities It will be better to separate intermittent activities when segments are separated by time periods This is better for control and many other scheduling functions If not separated, it may skew the percent complete and other dependent functions Activities Duration Slide No. 20
- 21. Discontinuous Activities Obtain Permit (90 days) Apply for permit, 1 day Permit issued Lag, 89 days Or Activities Duration Slide No. 21
- 22. Logic and Duration A B 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Days Clear and Grub Excavation A B 1 2 3 4 5 6 7 8 9 10 11 Days Clear and Grub Excavation 3 FS relationship: 17 days SS relationship with lag: 11 days A1 A2 B 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 C & G -1 C & G -2 Excavation FS relationship with splitting: 11 days Activities Duration Slide No. 22
- 23. Unplanned Stoppage Excavation was planned for 7 continuous days (Case A) But a technical problem forced a 6-day pause (Case B) Importance of activity notes Excavation 1 2 3 4 5 6 7 Excavation 1 2 3 4 5 6 7 8 9 10 11 12 13 Dewatering A B Activities Duration Slide No. 23
- 24. Multiple Crews Multiple crews (2+) Issues to be considered: Coordination Available space for work and safety Available equipment and maintenance Supervision, administration Crew #1 Crew #2 Activities Duration Slide No. 24
- 25. Multiple Shifts Multiple shifts (2+) Issues to be considered: Coordination, turn-over Supervision, administration Night lighting, security, facilities, and services Same equipment? Maintenance Productivity may not be the same (different weather conditions, visibility) 2nd (and 3rd) shift pay: Overtime? Crew #1 Crew #2 Activities Duration Slide No. 25
- 26. Multiple Crews and Shifts Multiple crews and shifts (2+) Issues to be considered: Coordination, turn-over Available space for work and safety Available equipment and maintenance Supervision, administration Night lighting, security, facilities, and services Productivity may not be the same for different shifts 2nd (and 3rd) shift pay: Overtime? Crew #1A Crew #2A Crew #1B Crew #2B Activities Duration Slide No. 26
- 27. Duration Adjustment For the excavation crew that has a recorded production rate of 800 CY / day, assume that adverse conditions results in 20% productivity reduction. Calculate adjusted: 1. Daily production 2. Labor hours / unit 3. Unit cost Activities Duration Slide No. 27
- 28. Duration Adjustment Adjusted daily production: 800 CY/day * (100% - 20%) = 640 CY / day Adjusted labor hours per unit: 0.015 / (100% - 20%) = 0.01875 ≈ 0.019 Or, 𝐿𝑎𝑏𝑜𝑟 ℎ𝑜𝑢𝑟𝑠 𝑝𝑒𝑟 𝑑𝑎𝑦 𝐶𝑟𝑒𝑤 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑝𝑒𝑟 𝑑𝑎𝑦 = 12 640 = 0.01875 ≈ 0.019 Note that as one parameter increases, the other one decreases, and vice versa Activities Duration Slide No. 28
- 29. Let’s Not Forget… The accurate estimation of crew production is important to both estimating and scheduling Unit Price = 𝐶𝑟𝑒𝑤 𝐶𝑜𝑠𝑡 𝑝𝑒𝑟 𝑑𝑎𝑦 𝐶𝑟𝑒𝑤 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑝𝑒𝑟 𝑑𝑎𝑦 Duration = 𝑇𝑜𝑡𝑎𝑙 𝑄𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝐶𝑟𝑒𝑤 𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑝𝑒𝑟 𝑑𝑎𝑦 Unit price and duration are inversely proportional to production rate Constant Constant Activities Duration Slide No. 29
- 30. Activities with Variable (Non- Linear) Production There are several ways to deal with activities with non-linear production: 1. If the productivity variation (non-linearity) is small, you can ignore it. For example, installing concrete blocks for a 5’ or 6’ wall. 2. Split the activity to homogeneous activities (each with assumed linearity) 3. Use the software tools to divide the activity into steps and distribute the resources per step Activities Duration Slide No. 30
- 31. Duration Risk Management How do you account for risk in the project? At the activity level, i.e. part of the duration? At the entire project level, i.e. independent of activities’ duration? or Both? Activities Duration Slide No. 31
- 32. Duration Uncertainty Excavation 10 days Most Likely Duration Optimistic Duration 8 Pessimistic Duration 18 Xo XP Expected Duration 11 Duration Risk Allowance Activities Duration Slide No. 32
- 33. Time Contingency It is not recommended to add extra time to activity duration due to risk calculations, but the risk impact is estimated for the entire path, and added within the project but outside the activities. If added within the activity, it will be used no matter what. It will become the “norm” for next time. So keep the activities durations lean! Activity duration may still have little “fluff” due to rounding. Activities Duration Slide No. 33
- 34. What If? What if the start of the project was postponed for a month or more? Slide No. 34 Activities Duration
- 35. Duration While Updating When updating the schedule, officially or unofficially, certain activities will be off the track; ahead or behind. If the schedule variance is small, it can be dismissed as normal and expected variation. If the schedule variance is major, you need to analyze and pinpoint the cause: A. Something went wrong with this activity, B. Certain factors were not considered when estimating the duration, or C. The number in the database was inaccurate Slide No. 35 Activities Duration
- 36. Duration While Updating Example: An excavation activity, 11,500 CY, using a crew with an estimated production of 700 CY/day. Estimated duration = 11,500 / 700 = 16.43 ≈ 17 days Five days later, 2,420 CY only were excavated: Baseline % complete = 30.4%, Actual % complete = 21.0% If everything was normal, i.e. no unexpected problems, then remaining duration must be corrected based on actual production, 484 CY/day: RD = 24 – 5 = 19 days, and NOT 17 – 5 = 12 days Slide No. 36 Activities Duration
- 37. Keep in Touch… Any comments? Any questions? [email protected] Activities Duration Slide No. 37