Calendars Improve Modeling
Ensuring CPM scheduling calculations provide the correct result across calendars is one of the most important reasons to use computerized scheduling tools. These tools are able to automatically adjust the start and end dates of schedule activities but as the scheduler you must clearly understand what is being done by the underlying calculation method.
The introductory discussion of schedule calculations was very straightforward, but as with most topics introductions rarely cover the complexity of reality. This is certainly the case when considering how to perform schedule calculations using actual crew work times and calendars encountered on real job site. In the standard schedule calculation an activity may begin no earlier than its early start time and end no later than on its late finish time. A one day activity, for example, starts on day 1 and ends on day 2. If you were to explain such a schedule to a construction crew, how would you explain it to them so that they will know what to do?
Take a moment and consider what you would tell the crew about when to start and stop the activity and how that is the same or different from the dates identified by the introductory CPM scheduling calculation’s early and late start times..
The answer of how best to explain the starting and ending of activities to construction crews, and other project stakeholders, is one of the keys to understanding why the introductory CPM scheduling algorithm is deficient for effective communication. When explaining to the construction crew you might explain that Monday the crew is to start the job and have it finished by Monday close of business.
As noted at the very start of this tutorial, CPM schedules are just a model of the construction process. As with any model it has some benefits and also some weaknesses. Luckily the weakness of the basic model may be improved, in this case, by adding the impact of calendars and actual work times on the basic construction scheduling algorithm. By adding calendars we introduce the idea of early and late start and finish dates, as opposed to early and late event times.
8 hour work day
Typically, construction crews work in shifts of 8-hours, Monday through Friday. The number and timing of “weekend days” varys across the world, however a five-day work week is typical. Let's consider an activity that is a one day activity and the impact on schedule calculation algorithms for this activity.
Let's assume that an activity is scheduled to begin on Tuesday, 15 October, takes one day to complete, and is on the critical path. The early start date of the activity is, therefore, 15 October. The early finish date of a crew working an 8 hour shift would also be 15 October. Therefore the adjustment to the standard CPM calculation rule, early finish even time = early start even time + duration, is that the early finish date = early start date + duration - 1.
Since the activity is on the critical path we know that it's late finish date must also be 15 October. As with the forward pass, the backward pass calculation must also be updated to take into account the effect of calendars. Since the late finish and late start date of the example activity must be 15 October when considering a one day duration of a crew working an 8 hour shift the rule for calculating the backward pass must be: late start date = late finish date - duration + 1.
Five or Seven day work weeks
If on a given project site crews will only be working Monday through Friday, it does not make much sense to tell a crew to show up for work on Sunday and complete by the following Sunday. To accurately reflect the crew work week calendar commercial scheduling tools provide significant flexibility to represent working weeks. In some scheduling tools it may even be possible to consider multiple shifts per day. For most projects, however, there are only two calendars that should be used.
A five-day work week schedule should be used for all production activities since this calendar is the typical 40 hour work week. If a crew works more than 40 hours, or in a compressed schedule, such as 4 ten hour days per week, then such complexity is likely to be available. Increasing the complexity of the schedule will however require significant double checking of all starting and ending dates where activities of different calendars share prior or successor activities.
Envision a set of activities that span the entire project. These activities do not have to be on the critical path, they just need to go from the start of the project (or your current progress data) to the end of the project. If all activities on the path have the same calendar then these activities are likely to have the same amount of total float. If that total float is the minimum total float, then of course your path is the “critical path.” Now consider what happens when activities on a single path through the schedule have different calendars. It is possible, in this case, for the activities on that path to not have the same total float amount.
The lesson is that models should be just as complex as needed, but no more. This is because more complexity could reduce the use of the schedule as a project communication tool.
In addition to the five day work week for production work, most schedules would be expected to have one additional calendar – a seven day work week. Non-production activities should be represented by seven-day work week schedules. Examples of non- production activities include concrete curing and submittal review times. It should be obvious that tasks that require evaporation occur on continuous schedule. If activities such as concrete curing, paint drying, etc… need to be included in your schedules to model important constraints, they should be included with calendars of seven day per week calendars.
Five-day work week example
To show the early start and early finish dates as they relate to calendars let’s look at the fictional calendar for a given month of interest shown in the table below. To begin, the blue, bold dates are the dates that crews are allowed to work based on a typical western 40-hour work week. Sundays and Saturdays are non-work days, and shown as undecorated text in the table below.
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | |
| 7 | 8 | 9 | 10 | 11 | 12 | 13 |
| 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| 21 | 22 | 23 | 24 | 25 | 26 | 26 |
| 28 | 29 | 30 |
Now, let’s take a fictional activity that will take five working days. The table below shows the working days needed if the activity starts on the first day of the month. Scheduling this five-day activity produces an early start date of Monday, the first, and an early finish date of Friday, the fifth.
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | |
| 7 | 8 | 9 | 10 | 11 | 12 | 13 |
| 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| 21 | 22 | 23 | 24 | 25 | 26 | 26 |
| 28 | 29 | 30 |
Now, let’s consider that the fictional five working day activity cannot begin until Wednesday the 10th. The table below shows the working days needed if the activity starts on the first day of the month. Scheduling this five-day activity produces an early start date of Monday, the first, and an early finish date of Friday, the fifth.
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | |
| 7 | 8 | 9 | 10 | 11 | 12 | 13 |
| 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| 21 | 22 | 23 | 24 | 25 | 26 | 26 |
| 28 | 29 | 30 |
In the five day activity that starts on the 10th, shown above, note that the CPM schedule calculation algorithm must take into account that there is no work on Saturday or Sunday, the 13th and 14th. If we were to simply look at the early start date, the 10th, and the late start date, the 16th, then we would have thought that the activity would actually have been seven days in duration.
Holidays
From the point of view of a CPM scheduling algorithm holidays are non-work days that may occur during the normally scheduled working week. In the example below consider the impact on the schedule if the calendar shown above is for the month of November. In the US there are two bank holidays in November. November 11th is Veterans Day. November 25th is Thanksgiving. Scheduling software systems typically allow you to provide the holidays that will impact your project. Rescheduling the activity shown above to take into account the Veterans Day holiday produces in the schedule shown in the figure below.
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | |
| 7 | 8 | 9 | 10 | 11 | 12 | 13 |
| 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| 21 | 22 | 23 | 24 | 25 | 26 | 26 |
| 28 | 29 | 30 |
Typically lists of holidays are entered in scheduling systems against specific calendars. This is appropriate since seven-day non-production activities will continue through the holiday dates, just as they would through any weekend. In general bank holidays will be added to five-day work weeks and no holidays will be added to seven-day work week calendars.
Modeling Point Delays with Holidays
A use of holidays that may not be obvious at first is that holidays may be used to model any non-work data associated with a given calendar. This includes delays such as site access restrictions or weather delays that impact all activities using the specific schedule. For example, if you wanted to determine the impact to the schedule of a week long period where work cannot continue add those holidays to the appropriate calendar and you will be able to recalculate the schedule with the new non-work days taken into account.
Questions for your consideration.
How does the formula for calculating total float from event times differ when calculating float using scheduled dates based on calendars and holidays.
What factors would you consider when applying milestones to schedules that use five-day and seven-day work week calendars?