Dynamic Minimum Lag Relationship - Construction Project Scheduling and Control - page 335

20%

A
8

B
10

Figure 12.4 SS Relationship with a percent lag

Can the Lag in the DML Relationship Be a Percentage?

The answer to this question depends on how we define percentage :IftheSS+ 20%
relationship shown in Figure 12.4 means a constant lag of 20% of the original duration
of activity A, that is, 20%∗8 ≈ 2 days, then it becomes just like a fixed time duration
(unless the original duration of activity A changes during updates, to be replaced by the
at- completion duration). In this case, the implementation in the DML relationship is
possible using the same methodology and equations explained earlier.
However, if the lag in Figure 12.4 means 20% of the remaining duration of activity
A, then this lag gets smaller and smaller as the work progresses until it becomes zero
toward the end, that is, simultaneous finish. This understanding of the percent-lag
negates the premise of DML (of maintaining a minimum lag) and is not compatible
with the DML equations.

Example 12.1

Assume the following two activities:

Activity Title Original Duration (days)

Roof Sheathing 20

Install Shingles 16

We need to maintain at least a 1-day lag between the two activities at all
times, as shown in Figure 12.5. Activities are contiguous. Assume activity B
(Install Shingles) to be linear.

1

0, 20 5, 21

Roof Sheathing
20

Install Shingles
16

5, 21

0, 20

1

Figure 12.5 Activity A (Roof Sheathing) as the predecessor an activity B (Install
Shingles) as the successor