HAMMOCK ACTIVITIES

A hammock activity—an activity that spans other activities—is a scheduling technique used in CPM. It can be best understood by looking at Figure 18.2.

The Dewatering activity shown in Figure 18.2 is the hammock activity—the duration is the result of the activities it spans. If there is a change in the duration of those five activities,

FIGURE 18.2 Dewatering is an example of a hammock activity

the duration of the hammock activity automatically changes to equal the total duration of the activities spanned. The hammock activity in this case shows when the dewatering is operative. In the schedule in Figure 18.2, Dewatering must start after the building is excavated and continue until the permanent drainage system will be in effect as the slab is placed. Hammock activities are also used when haul roads are maintained or during the procurement process. Hammock activities are unique in that their duration automatically changes with the activities they span. Their duration is not input into the computer. However, they do need to be identified in the software as a hammock activity in order for this to happen. Each software program is unique in how this is done.

CONSTRAINTS

A constraint is a restriction or boundary on the start or finish dates of an activity. Constraining an activity’s start or finish dates is another technique used in CPM schedul- ing. Constraints are used to ensure that an activity meets a deadline or when an activity must happen before or after a certain date. The most commonly used constraints are deadlines (no-later-than constraints) and potential delays (no-earlier-than constraints). They can be attached to either the start or finish of an activity. For example, a deadline can be put on the start or finish of an activity (the activity must start or finish no later than the date specified) or a potential delay (the activity can start or finish no earlier than the date specified).

Typically, an experienced scheduler would enter the project schedule into the com- puter without constraints first and examine the project purely on the CPM logic. Constraints are then added, one at a time—watching to see how each one affects the entire schedule. Remember, constraints override the CPM logic. If a mandatory start constraint is included on an activity, that activity will start on that date regardless of the CPM logic and whether or not the predecessors are finished. Or, if the project is ahead of schedule, the constrained activity will not automatically adjust to the possible new early date. Again, constraints override the logic, therefore, they must be used with care.

Date constraints are most often used for deadlines and delivery dates. They affect not only the constrained activity but also its predecessors and successors (and by extension the entire project). Therefore, they should be used sparingly. Too many constraints dimin- ish the value of using CPM because activities that are constrained cannot be as freely rescheduled when other parts of the project are rescheduled. The next several sections describe the various types of constraints.

Chapter 18 • Other Scheduling Techniques 175 Start Constraints

Start constraintsspecify that an activity can start no earlier than, or no later than, a speci- fied date. For example, if an activity cannot begin until the delivery of a piece of equip- ment on a particular date, make that date an early start constraint. The activity can start on or after that date, as calculated according to the logic of the schedule, but not before. If an activity must start by a certain date, attach a late start constraint.

Finish Constraints

Finish constraintsare used to specify that an activity must finish no earlier than, or no later than, a specified date. For example, if an activity must finish by a certain date, attach a late finish constraint. If an activity can start at any time but cannot finish until a particular date, perhaps after a scheduled customer inspection, attach an early finish constraint.

Mandatory Constraints

Mandatory constraintsforce an activity to establish a definite date for its start or finish. No matter what happens, that activity will take place on the mandatory constrained date. Use these constraints with caution; they control activity dates whether or not they are con- sistent with schedule logic. Therefore, an activity with a mandatory constraint could be scheduled before its predecessor.

Start-on Constraints

Start-on constraintsare equivalent to applying both a “start-no-earlier-than” and “start- no-later-than” constraint to an activity. It sets the early and late start dates equal to the specified date, but protects schedule logic (unlike the mandatory start constraint), so the activity can have positive or negative float.

Expected Finish Constraints

Expected finish constraintsforce the duration of an activity to depend on its scheduled finish date. Attach an expected finish constraint and the computer will calculate the dura- tion for the activity from its early start date to the expected finish date you specify. If the activity is under way, the computer will calculate its remaining duration as the difference between the data date and the expected finish date. Because it is calculated, the software typically will display the duration with an asterisk.

Float Constraints

Float constraintsaffect the scheduling of an activity, but, unlike date constraints do not override schedule logic. There are two types of float constraints: zero total float constraint and as late as possible float constraint.

ZERO TOTAL FLOAT CONSTRAINTS. A zero total float constraint causes an activity to be scheduled as soon as possible. It then has zero total float. The activity will be sched- uled to occur on the first dates it can occur and it will appear critical. The float will disappear.

AS LATE AS POSSIBLE FLOAT CONSTRAINTS. An as late as possible float constraint is used to set the dates of an activity so that the activity is scheduled as late as possible without delaying the early start date of its earliest successor. This eliminates any free float on the constrained activity. This constraint is commonly used for payments and deliveries.