Hello, everyone. Welcome again. We will be talking in this module about activity precedence diagrams, or sometimes, we refer to as activity on nodes. Activity precedence diagrams is actually a node network diagram, uses four types of construction activities relationships. A finish to start relationship between two activities when an activity finish, another activity starts. The start to start relationship an activity needs to start, when another activity will start. A finish to finish relationship, when an activity finishes, another activity has to finish at the same time. And very, very rare to be used in our industry, start to finish relationship between construction activities. We will be talking in a different module about the types of these relationships, in much more details with much more examples. So, for the sake of the recording and our lecture for today, we are focusing on the activity on node or what we refers to as AON. Basically, activity on node is a graphical tool that shows a project's activities, or the list of the construction activities at your project, as nodes connected by arrows indicating their mutual dependency or indicating their logical relationships. So for example, if we have this as the activity on node in your project, we have how many construction activities? We have one, two, three, four, five, and six activities from A to F. So the construction activities, they are presented in the squares here. And the arrows, as I highlighted in the definition, they show the logical relationships between these activities. For example, activity B will not start until activity A finishes, and that we can see it from the logical relationship in the arrow between the two activities. The same thing will go to activity C. It will not start until activity A will finishes. However, let's go into another quick example. In activity F here, it will not start because it has two arrows coming from two construction activities, D and E. So once activity D and E finishes, the two arrows indicates activity F will start. So that being said, I want to highlight a very important definition in activity on node which called lag. So the lags help in presenting the situation in your projects when an activity will not start until certain time or a period of time, few days, sometimes few weeks, after the end of another activity. We'll take an example here. If you want to build a concrete wall, or you want to build a concrete column, let's say, there are around, maybe, for simplicity, five steps to build that. One, is to install the steel rebar of that column, and then you place the form of a concrete column around the steel rebar. Second, third, you start pouring the concrete at the form over the steel rebar. Fourth, you need to wait for the concrete to cure and to have, or to attain sufficient strength before you take the forms out. So activity five is you strip out, or take out the forms and then you have your concrete column. So if you noticed here, I gave you five steps. Steps number fourth is not actually a real activity, that it takes some like you need to put some time until you take out and shape the forms. So the activity, it doesn't require any resources. That what we call a lag. So for example, and what I explained here, the lag, it goes between the pouring a concrete, and taking out the form. And then we can have number 3 over the arrow indicating the lag of a waiting period until we take out the forms. And this number highlight that the minimum number of days that you need to wait, and below that minimum number, if you waited two days, will violate the rule of the lag. So if you can do three days, that's what required. Sometimes if you go to a fourth day and you take out the form, still okay. But don't go below the three days of the lag. So this is the minimum number. Another example could be building two identical walls in the same construction project using the same materials. So let's say building the first wall is task a and building the second wall is task b. The construction manager wants to delay task b for let's say, three or two days. This is let's say, due to the fact that the material used for both a and b for the two walls are a new type of materials. So, the engineer wants to learn from the first activity, the first wall. And then apply if there is anything learned to activity b, to the second wall. Therefore, the two tasks, a and b, have what we call start to start relationship with a lag time of two days. So activity a, the first wall will start. And before we start with activity b, we wait two days to have some lessons here about the new materials. And then we directly go to the starting of the activity b or the other wall. So again, if we want to sum it up, basically lag, it is the minimum waiting period between the finish or a start of an activity, and the start or a finish of that activity successor. I also, talking about lags in the activity on node diagrams. I want to highlight a specific thing, especially when the lag goes to a negative number. You might experience this example in couple of textbook, in couple of studies. It's very rare to be used in our industry, in the construction industry. But just in case, if you face something like that, we refer to a negative lag as the word lead instead of lag. When I present and talk in another module about the types of construction relationships, I will give you a quick example about the lead and the lag at that time.
