Other Scheduling Methods - Construction Project Scheduling and Control - page 311

to large projects that consist of large numbers of small activities. Some construction
projects consist of a few activities (usually with large quantities) that must be done in
the same order or sequence, such as heavy construction projects (e.g., roads, earth-
work, or utility piping). Let us consider a project to lay down 5,000 linear feet (LF)
of an underground utility pipe. The basic activities are Excavation, Prepare Subbase,
Lay Pipe, Backfill, and Compact. If we are to use CPM networks for this project, we
can take one of the following two approaches:
1. Create a project with only five large activities. Connect these activities with
start-to-start (with lags) and finish-to-finish relationships.
2. Divide each major activity into a number of subactivities (e.g., 50 subactivi-
ties), in which each represents a distance of 100 LF, and connect them with
finish-to-start relationships with no lags.
Steps Required to Build a Schedule by Using the LSM
Three simple steps (similar to the first three steps in the CPM, discussed in Chapter
4) are necessary to build a schedule by using the LSM.
1. Determine the work activities. As mentioned previously, we expect only a few
activities in LSM schedules.
2. Estimate activity production rates. Such an estimation is similar to determin-
ing durations. We still estimate durations, but we are more concerned with
production rates.
3. Develop an activity sequence, similar to determining logical relationships. All
relationships are start to start (with lags) with finish to finish (with lags too).
Before applying the LSM, we must make sure it is the most appropriate method.
How the LSM Works
In the LSM, since the x -axis represents time and the y -axis represents distance, the
slope represents the “speed,” or rate, of production (Figure 11.13). When we have two
or more activities, the production rate will differ from one to another. The horizontal
distance between two lines represents the float of the earlier activity. In the LSM, we
call it the time buffer . The vertical distance represents the distance separating the two
operations. We call it the distance buffer . See Figure 11.14.
Lines are not allowed to intersect, because an intersection literally means that the
successor has gotten ahead of the predecessor, which is impossible (see Figure 11.15).
Keep in mind that the “Location” label on the Y-axis works mostly with horizon-
tal construction projects, such as highways, utilities, and so forth. This label can be
changed to “Work Quantity” or another physical measure criterion for other types of
projects.