Precedence Networks - Construction Project Scheduling and Control - page 95

1 1

0, 6 1, 8 4, 8 6, 9 7, 9

Subbase
7

Lay Pipe
4

Backfill
3

Compact
2

Excavate
6

1 2 1 1

Figure 5.7 Precedence diagram, for utility pipe installation project, using a combination
relationships and more lags

could finish at the same time. If this possibility is unacceptable, we can add lags, as
shown in Figure 5.7. In the network shown in Figure 5.7, Subbase cannot finish till
1 day after Excavate has finished, and Backfill cannot finish till 1 day after Lay Pipe
has finished.
One important note : These relationships are “driving in one way,” which means
that you can exceed the lag in one direction but not the other. For example, Subbase
cannot start till at least 1 day after Excavate has started. It is allowed to start more than
1 day after the start of Excavate. The same argument holds for the FF relationship:
Subbase cannot finish earlier than 1 day after the finish of Excavate, but it can finish
more than 1 day after the finish of Excavate.
The relationships shown in Figures 5.6 and 5.7 are called combination relation-
ships . In this type of relationship, two activities are related by two relationships. Some
computer software, such as Microsoft (MS) Project, does not allow combination
relationships.

THE FOUR TYPES OF RELATIONSHIPS
As mentioned previously, four types of relationships are possible in precedence
networks:

1. Finish-to-start (FS) relationship : The most common type of relationship is the
FS relationship. Many project managers still insist on using only this type.
Many examples of this type exist, such as the following:

• The concrete cannot be placed (poured) until the formwork has been built.
• The doors cannot be hung until door frames have been installed.

2. Start-to-start (SS) relationship : The SS relationship is common and extremely
useful. Examples of this type are as follows:

• Excavation for the foundation cannot start until clearing and grubbing
begins (usually with a certain lag; i.e., a certain percentage is completed).
• Laying felt on the roof cannot start until sheathing has started (also usually
with a lag).