Microsoft Project enables the definition of lag in elapsed days. This works great to model the 24-7 nature of material cure time. However, there are situations when the elapsed time lag causes the critical path to become discontinuous. Let’s explore this further.
Lag is a helpful modifier that specifies waiting time between predecessor and successor tasks. In Microsoft Project lag is expressed either in business days or elapsed days. Elapsed days are calendar days and are scheduled 24-7; weekends are not skipped, but are included in the lag daily count. This makes the elapsed days option helpful for modeling the 24-7 curing of material like concrete.
A problem, however, occurs with elapsed time when the cure process ends at the close of business Friday. This situation generates 2-days total float for all upstream tasks. The default Microsoft Project setting is to define critical tasks as having 0-days or less total slack. So these upstream tasks become non-critical, and generate a critical path discontinuity.
This article demonstrates a way to model elapsed time lag and maintain a continuous critical path in Microsoft Project.
Our demonstration Microsoft Project schedule is in Figure 1.
Note that the task ‘E – Strike Forms’ predecessor has a 6-days lag modifier. This lag modifier does not include the weekends, so there are actually 8-days total between task ‘D – Pour Concrete’ and task ‘E – Strike Forms’. We want weekends included in our lag count, so we need to change the lag modifier to elapsed days. Select the task tab, properties ribbon group, and information. This displays the Task Information dialog, Figure 2.
Note the 6d lag modifier on predecessor task ‘D – Pour Concrete’. In Figure 3 we change the lag modifier from 6d to 6ed.
With this simple adjustment our lag modifier now includes weekends, and there are 6 calendar days total between task ‘D – Pour Concrete’ and task ‘E – Strike Forms’, Figure 4.
Our elapsed time modifier helps us fine tune the schedule by more accurately modeling the 24-7 material cure time.
Let’s now change the elapsed day lag modifier to 4ed, Figure 5.
Now our disconnected critical path problem occurs, Figure 6.
The situation is our concrete cure process concludes Friday night. This generates 2-days total float on all activities upstream of task ‘E – Strike Forms’. These activities can delay 2-days without delaying task ‘E – Strike Forms’.
Because of this allowable 2-day delay, these activities are non-critical and are not along the critical path, which creates our critical path discontinuity. We want one continuous critical path through the network, so that we know our longest path.
To rectify this situation and maintain one continuous critical path we access schedule options advanced features, Figure 7.
In the advanced project option dialog. Look down below to the last setting. Adjust the critical task setting to “tasks are critical if slack is less than or equal to 2 days.” Now when we view our schedule, Figure 8, we see that we have one continuous critical path even though tasks upstream of task ‘E – Strike Forms’ have 2-days total slack.
Elapsed Days lag modifier is a nice feature in Microsoft Project that helps model the seven day a week nature of material cure. The definition of critical tasks, however, will require adjustment. Set the definition of critical tasks to 2-days or less on a five day workweek calendar and 3-days or less on a four day workweek calendar.
Another approach is to model the cure time as an actual task with a 7-day workweek calendar, but with no resource assignments. With this method you will still have to adjust the definition of a critical task, accordingly.