Managing PV Installations with a Gantt Chart: Page 2 of 3
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Step two: Create a network diagram. The network diagram depicts the relationships among the activities. Relationships, or dependencies, exist when certain activities cannot start or finish until you start or complete other activities. Those earlier activities are called predecessors. For example, “Install ac system” is a predecessor to “Commission and inspect.”
A network diagram graphically maps the order and relationships of a project’s activities (see Figure 4). The relationships and sequence of activities vary from project to project, based on the type of system you are installing and the construction means and methods. Usually you need to perform activities in sequential order. Four types of relationships define the order:
Finish to start: An activity starts after its predecessor activity has finished.
Start to start: An activity starts after its predecessor activity has started.
Finish to finish: An activity finishes after its predecessor activity has finished.
Start to finish: An activity finishes after its predecessor activity has started (rarely used).
You can add a lag time or a lead time to any of the above relationships. A lag time creates a delay between the predecessor and successor activities in a relationship. For instance, if you need to install an inverter on a concrete pad, you will need several days of lag time after finishing the concrete pad to allow it to set before you can start installing the inverter. A lead time allows sequential activities to overlap. For instance, you must perform the activity “Install roof attachments” before you can start the activity “Assemble racking.” If you cannot wait to start assembling the racking on the sections of roof with attachments already installed, you can get a 1-day head start (lead) on the “Assemble racking” activity by using a finish-to-start relationship with a 1-day lead time. This signals the installation team to start installing racking one day before finishing all the roof attachments.
Lag and lead times are opposites. A positive two-day lag time is the same as a negative two-day lead time. Many software programs accept only a lag time input. If you need a lead time, you must enter it as a negative lag time. For the sake of clarity in our example, all of the relationships in the network diagram in Figure 4 are finish to start, with no lead or lag time.
Finally, you determine the duration of each activity. The duration is the number of calendar days or hours it takes to complete the activity, not the actual number of person-hours or person-days worked. Each activity box in the Figure 4 network diagram shows a duration. You may base the duration of the activity on historical data, industry rules of thumb or an educated guess. With experience, your duration estimates will become more accurate. The crew size affects the duration, so adding more crewmembers to the installation team or working overtime can shorten it.
Step three: Develop the Gantt chart. The WBS and network diagram contain the information you need to create a Gantt chart. With these two steps completed, you are ready to bring them together and create the schedule. If you wish to use all the features of the schedule, doing it by hand is not a practical option. Many computer programs allow you to easily accomplish in minutes what takes hours by hand. Microsoft Project and Oracle’s Primavera are two programs commonly used in the construction industry, but many others are available, and you can find one to fit your budget and needs. Assuming you use a program to develop the Gantt chart, the process is similar with most scheduling software.
Start by entering the WBS into the program, maintaining the project, work packages and activities hierarchy. Next, enter activity information from the network diagram, including durations and relationships. Work packages inherit the start and finish dates of the activities they contain.
Add milestones to the schedule as necessary. A milestone is a significant event in a project, such as the completion of a major deliverable. It is common to include contracted milestones, such as substantial completion and final completion. Milestones are similar to activities, but have zero duration and no associated resources. A milestone is an informational item only that helps the project manager understand what activities and work packages the team must complete to reach that milestone.
Once you have completed the schedule, it shows the start and finish dates for each activity and work package, as well as for the overall project.