Using the correct calendar in a programme might sound pedantic however there are some consequences if not managed properly.
A calendar within planning software will only allow the activity to be carried out within the working window on the calendar. Normally these would be 5,6 or 7 days per week calendars to set a working pattern however depending on the project there may be some more time sensitive working areas. These could be on railway possessions, road closures, tidal work or works in ecologically sensitive areas for example.
Calendars are assigned to the activities, the software then uses the logic to calculate when the work can be carried out within the working time. This may sound pretty straight forward however there is a very easy way to cheat all of this and its pretty common. All too often the works are just placed when the work can happen and constrained in place.
On the face of it this might seem fine as the activities are still showing when the work can happen however there are some issues with this short cut. Its easier to explain with an example:
The example programme below shows the same programme. The programme details the design and procurement before some initial enabling works to be carried out mid week. There are 3 possessions available and the plan is to work two of these, one for construction and one for commissioning with the third as a contingency.
The top programme utilises possession calendars, the one below utilises a 24/7 calendar to place the works when the possessions are and then constrains them in place.
Even though both programmes appear to be the same, the works are all happening at the same time, there are some differences. In the bottom programme the design and procurement is showing 33 days of float whereas the correct amount should be 10 days. This is because the possession calendars also impact the float amount based on how much available additional working time there is. In the short cut approach, as works are constrained on a 24/7 calendar, the float is calculated along this calendar and not strictly when the works can take place. This is highlighted more when we progress the programme and impact some change as shown in the example below:
In this progress update, both programmes show the same update however, now that change is impacting the programme, there are some issues.
The programme update shows an initial delay to contract award followed by an extended design duration. This has delayed the enabling works past the first possession date and as such in the version with calendars, the work has moved onto the next available calendar.
The cheated example below still shows the enabling works passing the available possession however because the works are on a 24/7 calendar, the works are just pushed to the next available day (which is not a true working day). Importantly here, the impacted works are not shown as critical and are still showing float on them. The construction works have moved to a date when there is no available working window and commissioning has not moved to the contingency possession.
This may still sound pedantic and in this case the planner may pick up on the shift and manually move the activities however the float is still showing an incorrect. On a complex project with multiple calendars, this becomes very complicated if not done properly.
As the cheat approach does not allow float to be shown correctly the critical path could be incorrect. This is where the cheat approach could start costing time and money.
Its very possible that an impact that would give rise to an EOT is missed using the cheat approach, it could also be the case that works are still planned for the dates in the impacted programme. As works are not possible on these dates then abortive costs and further impact may be realised.
So while it may seem pedantic to ensure works are placed on the correct working calendars, even with a simple example its clear that there can be significant consequences of using a short cut.
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