Precedence Networks - Construction Project Scheduling and Control - page 96

3. Finish-to-finish (FF) relationship : The FF relationship is also common and use-
ful. Examples of this type are as follows:
• Landscaping cannot finish until the driveway is finished.
• Backfilling a trench cannot finish until the pipe in the trench has been laid.
4. Start-to-finish (SF) relationship : The SF relationship is uncommon and
almost nonexistent in construction projects.
Important Comments about the Four Types of Relationships
1. Practically, the only two types of relationships that can be combined are SS and
FF, as shown in the previous example.
2. You can always use lags and leads with the four relationships. In fact, you will
have more need to use lags with SS and FF relationships than with the tradi-
tional FS relationship.
3. With regard to critical path method (CPM) calculations (discussed later in this
chapter), when you are using a combination relationship (SS and FF), usually
only one of the two relationships becomes binding (driving). This situation
may change if some criterion—such as duration, lag, or logic—changes.
THE PERCENT COMPLETE APPROACH
O'Brien and Plotnick (2009, p. 159-160) suggested a slightly different approach.
The relationship shown in Figure 5.8a means that activity B can start after A is at least
50% complete. In Figure 5.8b, the final 30% of activity B cannot progress until A is
completely finished. Although this approach makes sense, it did not gain popularity.
Perhaps the main reason is that in the CPM, early and late dates of an activity are
calculated using lags that have a definite number of days (constant) rather than a
percentage of completion of another activity (variable), although the conversion of
this variable to a constant is simple.
We must note here that MS Project accommodates such relationships. For
example, the relationship shown in Figure 5.8a can be entered as follows: B's
predecessor is A, relationship: SS + 50%. Oracle Primavera P6 does not accept a
relationship in this form: it must be in number of days.
In reality, the lags we use in the CPM are nothing but an estimate of the por-
tion (percent complete) of the preceding activity that must be completed before the
succeeding activity can start (or finish). For example, Figures 5.8a and 5.8b can be
redrawn as shown in Figures 5.8c and 5.8d. Note that the relationship shown in
Figure 5.8b is not equivalent to that shown in Figure 5.8d, but close enough. If you
try to do CPM calculations for both, assuming activity A starts the project, you will
find A's early dates are (0, 5). The last 30% of activity B is about 2 days. Thus, the
last 2 days of B must follow A (i.e., occur on days 6 and 7). However, the early start
of activity B is not restricted, so it can be zero (start of the project), in which case
the early dates of B are (0, 7), which is also true in Figure 5.8d, unless activity B is