The second step in creating a scheduling network model is to order the activities listed in the WBS. Jim explains this process and illustrates the process using the concept of immediately preceding activities or IPAs, which are also called predecessors.
- Once I've defined and listed all of my activities which includes construction activities as well as any procurement and management activities I need on the schedule, it's time to order the activities, basically sequence them. It sounds easy, but ordering these activities is not necessarily as straightforward as it might seem. For each activity you'll need to determine which activities must precede it, which activities must follow it, and which activities can be done simultaneously with it. Remember at the beginning of the course when I talked about history and the fact that some of the first computers ever built were used to figure out scheduling? Maybe now you can start to see why.
This does get complicated, especially if you have hundreds of activities, which is really common on a construction project. Our example here of a tennis court construction has a fairly small scope of work, and we already have dozens of activities. But don't panic, I'll keep this as straightforward as possible. We need to run through these steps by hand so you can see how they work and so you really understand the terms and the concepts, but we're only going to use a handful of the activities that we created and then at the end we'll let the computer do the heavy lifting on the whole model.
Let's start ordering the activities. All we really need to do to answer all of the questions I said we needed to answer to put these in order is to take each activity and identify each one's immediately preceding activities, or IPAs. Some people will also just refer to this as identifying each item's predecessor. If we identify each activity's immediate predecessor then the network will address all of those other questions. Let's look at how this is done. I'm going to go back to my WBS and list those activities and put them in rough order in a table.
I'm going to label each activity on the left side of the table with a letter, A, B, C, D, and so on. And then a column on the right side of the activity, I'm going to list each activity's IPA. Those activities that immediately precede it and that have to be done before I can start. Anything in this table that doesn't have an IPA means that I can theoretically start it at the beginning of the job. In our example here, you can see that I am saying that the surveying and the site fence can both be started simultaneously at the beginning of the job.
Neither activity has an IPA listed in the table. Remember, in the right column I only need to enter the activity's immediate predecessor. So for example, for my Concrete activity I only need to list Install Post-Tension Reinforcing as the IPA. I don't list that and Grading because Grading is the IPA for Install Post-Tension Reinforcing. The only time that I'll list multiple immediate predecessors is when I have an activity that relies on multiple unrelated activities, so let's look at that.
For example, the Clearing activity. It lists both the Site Fencing and the Surveying as an IPA. I want both of those activities done before I start the Clearing. Just think through the logic for each activity and ask yourself, what activity on my list has to be done before I start this one, and then use that to create your table. From this table I'm going to use this information to create a network diagram, or more specifically, a precedents diagram. This diagram is our network model and it will help us convert this table over into a schedule.
Let's see what this looks like.
This course identifies the steps needed to develop a proper plan, and demonstrates how that plan is transformed into a construction schedule. Throughout the course, instructor Jim Rogers shares examples of his own successes and failures in the areas of construction planning and scheduling, so as to lend real-world context to the concepts he covers.
- Types of schedules
- Planning versus scheduling
- Work breakdown structure
- Developing a schedule
- Creating a network model
- Assigning durations, costs, and resources
- Identifying the critical path
- Letting the software do the calculations
- Checking and updating the schedule
- Scheduling's impact on productivity