Hammocks

When a number of activities are in series, they can be summarized into one activity encompassing them all. Such a summary activity is called a Hammock. It is assumed that only the first activity is dependent on another activity outside the hammock and only the last activity affects another activity outside the hammock.

On bar charts, hammocks are frequently shown as summary bars above the constituent activities and can therefore simplify the reporting document for a higher management who are generally not concerned with too much detail. For example, in Figure 20.22, activities A1 to A4

could be written as one hammock activity since only A1 and A4 are affected by work outside this activity string.

When a string of activities repeats itself, the set of strings can be represented by a configuration known as a ladder. For a string consisting of, say, four activities relating to two stages of excavation, the configuration is shown in Figure 20.23. This pattern indicates that, for example, hand trim of Stage II can only be done if

1 Hand trim of Stage I is complete

2 Machine excavation of Stage II is complete.

This, of course, is what it should be.

However, if the work were to be divided into three stages, the ladder could, on the face of it, be drawn as shown in Figure 20.24. Again, in Stage II all the operations are shown logically in the correct sequence, but closer examination of Stage III operations will throw up a number of logic errors which the inexperienced planner may miss.

What we are trying to show in the network is that Stage III hand trim cannot be performed until Stage III machine excavation is complete and Stage II hand trim is complete. However, what the diagram says is that, in addition to these restraints, Stage III hand trim cannot be performed until Stage I level bottom is also complete.

Clearly, this is an unnecessary restraint and cannot be tolerated. The correct way of drawing a ladder therefore when more than two stages are involved is as in Figure 20.25. We must, in fact, introduce a dummy activity in Stage II (and any intermediate stages) between the starting and completion node of every activity except the last. In this way, the Stage III activities will not be restrained by Stage I activities except by those of the same type.

An examination of Figure 20.26 shows a new dummy between the activities in Stage II, i.e.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

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