Dynamic Minimum Lag Relationship - Construction Project Scheduling and Control - page 330

INTRODUCTION
When the critical path method (CPM) was introduced in the late 1950s as a tool for
scheduling projects, activity on arrow (AOA) networks were used for depicting the
logic and performing the calculations. There was only one type of logic relationship:
finish-to-start (FS). Shortly later, activity on node (AON) networks were introduced
but also with FS relationship only; however, they were capable of incorporating lags.
In the early 1960s, Stanford Professor John Fondahl introduced precedence diagrams
with four types of relationships: finish-to-start (FS), start-to-start (SS), finish-to-finish
(FF), and start-to-finish (SF). Precedence diagrams revolutionized project scheduling
because they accommodated overlapping activities for the first time in CPM networks
without the need to split them.
Dynamic minimum lag (DML) is a new type of relationship in CPM networks.
Unlike the traditional relationships, the DML relationship is dynamic, not stationary
at the start or finish points of the predecessor or successor activities. It moves along the
work progress in the successor activity parallel to the work progress, not allowing the
successor to get ahead of (or undesirably close to) the predecessor (see Figure 12.1).
It maintains a minimum lag between the successor and the predecessor during the
entire course of work progress.

Why DML?

Traditionally in precedence networks, overlapping activities are connected with an
SS, an FF, or—in most cases—a combination SS + FF relationship. If productivity is
assumed to be linear in both the predecessor and the successor, and if actual work
progress conforms to the estimated productivities and durations, then the DML rela-
tionship will act the same as the SS and/or FF relationship.
However, we all know that things rarely, if ever, go as planned. There are many
situations in which the successor can theoretically get ahead of the predecessor or be
within less than the specified lag:

Definition: The dynamic minimum lag (DML) is a new type of logic relationship
in critical path method (CPM) scheduling. The DML relationship is dynamic,
moving along with the predecessor and successor activities in parallel with work
progress, to ensure that the minimum lag is maintained at all times, not just at
the start or finish of the predecessor or successor activity.
Assumption: The DML assumes that the predecessor activity can take any type
of linear or nonlinear distribution. The successor activity, however, is assumed to
be linear.

A
10

B
8

Figure 12.1 The dynamic minimum lag (DML) relationship