Jim lists discusses the different types of scheduling systems that have been applied to construction scheduling, including PERT and arrow diagramming, and explains that today the industry uses precedence network diagrams.
- I'm going to continue with the history lesson just a little bit and talk about the different types of schedules that we see in construction. And I'll take just a few minutes to explain why the different methods have been used over time, and what some of the limitations are on each one. First, I'm going to mention, and then we'll skip over, what I consider to be systems that are important to the history of planning and scheduling, but not really used much today. These are going to include the Program evaluation review technique, or PERT, and the arrow diagramming method, or ADM.
PERT was developed in the late 1950s by the US Navy, and it's a somewhat complex system that we just don't see used much in the construction industry. PERT takes into consideration the problem of uncertainty. That is, the fact that when it comes right down to it, when we make a schedule, we are just sort of picking a number, or a fixed value for each activity duration, when in real life there is probably an optimistic time duration, and a pessimistic time duration, and using historical data, we can arrive at the probability of hitting each one of those time frames.
Now that's a really complex discussion that takes a really good understanding of probability and statistics which I'm not a big fan of. Or at least, I find it really difficult and complex to teach that subject. I mention this here because I do think that it's important to understand that there is uncertainty in our schedules. We are picking a number. We say it's going to take 60 days to erect the structural steel frame, and it's probably not going to be exactly 60 days. In some really large, or very complex projects, it may be helpful to note just how much uncertainty there is in our schedule, and just what kind of effect it could have.
On most construction projects though, we just don't take it to this level; so enough about PERT. Let's move to the other method I mentioned, which was ADM, or the Arrow diagramming method. And I mention that here primarily for historical reasons because it was widely used for a long time in construction scheduling. And some people also called it activity on arrow. And this type of scheduling today has really been mostly replaced with the precedence diagramming method, or PDM.
This is really the basis for scheduling most construction projects today, using any of the commercially available scheduling programs, like Microsoft's Project, or Oracle's Primavera, or P6 software. Earlier, I mentioned bar charts, or gantt charts, and these are still widely used to display information regarding the construction schedule. In fact, if you're in charge of scheduling construction on a project, you'll probably be generating and submitting gantt charts all throughout the project, in order to give a simple picture of the planned progress of the job, and to show what order activities are to be done in, and when they start and stop on the calendar.
Determining this order of activities, and scheduling the project is also broadly referred to as the Critical Path Method, or CPM. And I'll talk more about that term later. Even though your output when you print or send the schedule to somebody will probably be a gantt chart, you'll use the system of precedence diagramming to help you schedule and arrive at that output. As we go through this course, you'll see how to schedule activities using this method. And as we do, you'll probably also begin to see how much work it is when you have to change a date or a duration inside of this diagram, or if you have to add a task or an activity.
That's where the limitations of precedence diagramming really come in. It's hard to update the schedule by hand. Not to worry though; that's what the computer software is for. Once we go through the actual process here in the course by hand, so you can see how activities and tasks are scheduled and linked using this process, we'll take a look at how computer software really removes any of the limitations of precedence diagramming, and makes the process quick and easy once you understand how it works.
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