TO WHOM DOES FLOAT BELONG?

Many general contractors have the attitude that all the float belongs to them. They would simply give the early start and finish dates to the subcontractors and would not increase the durations as explained above. If this was the case with the warehouse schedule shown in Figure 9.9, and the windows sub took a couple of extra days, that would have an impact on brick, siding, and paint exterior. All their early start days would be delayed and the man- ager would have to make three phone calls in order to notify them of the delay. Then it could be possible that one or even all of those subcontractors have already scheduled their crews to be somewhere else on those later days so they won’t be able to get back to this warehouse job for days or even weeks, causing a delay to the project. The sharing of shared float could have eliminated this delay. If a subcontractor is a professional who really under- stands CPM and the types of float, give the computer disk to that subcontractor if it helps manage the project. Don’t try to hide information from team members. The fear is that someone who does not understand these concepts may use shared float, causing later activ- ities to be critical and potential delays to the project completion date.

If the whole management team is professional enough to recognize these different types of float and realizes when they are shared or independent, it will certainly make the job run more smoothly. But if they do not, and instead think that total float belongs solely to them when in fact it is shared (and they count on those extra days), the schedule will be in a state of confusion and the project will be delayed. That is why it is not a bad manage- ment idea to increase the durations a few days on the shared float activities and then give only the early start and early finish dates to the subs and suppliers. That way, the manager can hide some of the float in case it is needed later to better control the project and ensure the project will finish on time.

Chapter 9 • Calculating Total, Shared, Free, Independent, and Negative Float 87

Sometimes the project owner feels all the float should belong to the owner. The contract could even state that all activities must start as early as possible and finish as early as possible, taking away all the float. From a management point of view, this makes every activity critical. That would not be a very wise move, even from the owner’s point of view. In Chapter 5, the hotel project had an excess of 200 days of shared float on many activities. That type of owner requirement would force those activities to be completed around 200 days early and the owner would then have to pay interest on that money without being able to get a return until the project is finished. If the owner did not want to be a team player and, instead, wanted to use all the float to his or her advantage, the contract would state that all activities would start and finish as late as possible without delaying the project completion date. This would then make each activity start on the late start date and finish on the late finish date and keep the owner from paying more interest than necessary. It would also make each activity critical. A popular notion with experienced management teams is that the float belongs to the project and should be used by the activities that can benefit most by it.

Conclusion

It is important to be able to identify the types of float and then to develop a management style on how to use those floats to manage the project to help ensure the project’s success. As float is shared, it can take the pressure off as many of the activities as possible. It is float that gives the management team an opportunity to relax a lit- tle; every activity is not critical. With a tradi- tional bar chart schedule, which is not based on a network, there is no float. Therefore, the team’s understanding is that each activity must be done on the days shown. The correct identifi- cation and use of float can be a great asset to all working on the project.

If the attitude can be developed that the project really is a team project, all the float should belong to the project, and as a team member needs it, it should be available as long as it does not have an adverse effect on other team members. Float could be negoti- ated for the advantage of the team and the project based on prioritized needs. A later chapter shows additional applications of using float in order to balance resources, such as the number of workers or equipment and cash flow.

Application

Additional Examples of the Different Types of Float

Those who are having difficulty understanding the different types of float or who want to strengthen their ability to recognize the types of float may find the following example problems helpful. Most people find it easier to learn to dis- tinguish the different types of float by under- standing the logic rather than trying to memorize

a formula. A formula would be complex because it is necessary to look at all predecessors and all successors to each activity. The explanations in Figure 9.10 focus on the logical relationships.

Using the schedule in Figure 9.10, first we will look for independent float. Anytime an activ- ity is between critical activities, without other relationships, this is a case where the float will always be independent. There are other cases

R I H 3 3 0 3 5 8 A 4 8 12 C 7 8 15 H 2 17 15 P 6 23 17 0 3 0 Name of Activity Dur EF ES TF LF LS 0 8 3 10 2 14 17 0 23 4 7 3 D 3 6 3 0 8 15 E 6 7 13 1 8 14 0 17 15 2 8 5 J 3 16 13 1 8 4 14 1 17

FIGURE 9.10 A schedule to help analyze independent and shared float

where there may be independent float even if the activity is not between critical activities without other relationships. Notice activity D. It comes from a critical activity and goes to a critical activ- ity. If D uses the float and finishes on the end of day 8, it does not rob float from later activities. Therefore, D has 2 days of independent float. Activity D does not share float with anyone.

Next look for shared float in Figure 9.10. Whenever you see a string or path of activities with the same number of days of total float, it is a good guess the float may be shared with those activities in that string or path. Notice activities

H, E, and J; they all have 1 day of total float. If H

uses that day and finishes on the end of day 8, then E can’t start until day 8 and it now has zero float—it is critical. Activity J is in the same situ- ation—H or E can rob J of its float. Therefore, H,

E, and J all share that 1 day of float.

A closer examination of activity A in Figure 9.10 shows that if A uses both days of float, it will rob 1 day from J, forcing J to be critical. Therefore,

A and J share a day of float. Also notice that A can

finish on day 12 and J can’t start until day 13, so A has 1 day of independent float that can be used without affecting J. Because that day is not shared with earlier activities, it is independent. Therefore,

A has 1 day that is independent and 1 day that is

shared with J.

As another example of the different types of float, see Figure 9.11.

As Figure 9.11 is closely examined, notice that B and K both have 2 days of float and they are in a string together. It is probably shared. A closer examination reveals that if B uses those 2 days of float and finishes on day 35, that will rob K of all float, forcing K to become critical. Therefore, B and K share those 2 days of float. Now examine G. If G uses the 1 day of float and finishes on day 26, then B can’t start on day 25. Therefore, G can rob B and eventually K of 1 day of float each. Now we can see how G shares 1 day of float with B and K.

Next, look closely at activity F in Figure 9.11. Activity F looks somewhat independent, with a critical predecessor and a critical succes- sor. However, F also has B as a successor. Without the relationship with B, F would defi- nitely have those 6 days as independent float. Notice if F uses the 6 days and finishes on day 27, that will rob B and eventually K of 2 days of shared float. Hence, F shares 2 days with B and 2 days with K. Now notice that B can’t start until the end of day 25. Activity F could finish on the end of day 21, so F could go from day 21 to day 25 without robbing B of float. Those 4 days are independent to F.

If you are still having difficulty in recog- nizing the different types of float, study the schedule in Figure 9.12. If the difficulty per- sists, review some of the prior schedules in this chapter.

Chapter 9 • Calculating Total, Shared, Free, Independent, and Negative Float 89 J G A 10 10 0 10 15 25 B 8 25 33 I 10 48 38 A 8 56 48 0 10 0 Name of Activity Dur EF ES TF LF LS 1 26 11 27 2 35 K 13 33 46 2 35 48 48 0 56 16 26 10 F 11 21 10 R 12 26 38 0 26 38 0 48 38 6 27 16 C 10 18 38 0 26 10 38 0 48

FIGURE 9.11 Another schedule to help analyze independent and shared float

S B C 1 1 0 1 6 7 P 6 8 14 N 11 30 19 0 1 0 Name of Activity Dur EF ES TF LF LS 1 8 2 24 16 30 I 11 8 19 0 8 19 G 9 8 17 2 10 19 T 1 30 31 0 30 31 8 9 1 A 7 8 1 R 2 8 10 12 20 22 E 9 9 18 4 13 22 0 30 19 0 8 1 F 8 26 18 4 13 5 22 4 30

FIGURE 9.12 An additional example of a schedule to help analyze the different types of float

To evaluate the different floats within the schedule in Figure 9.12, we will first look for shared float. As you look for shared float, look for strings of activities with the same number of days of float. Notice C, E, and F; they are in a string and each has 4 days of total float. Upon close examination, it can be seen that if C uses the 4 days and finishes on day 13, E’s float is taken, forcing E and F to become critical. Therefore, C, E, and F share those 4 days of float. Does F also share with R? If R finished on the late finish day of 22, that would also rob F of all float. Therefore, F shares float with C, E, and R.

Taking a closer look at R in Figure 9.12, it is realized that R does not share float with the predecessor activity A. Activity R could finish at the end of day 10 and F can’t start until the end of day 18. Therefore, the 8 days are inde- pendent to R. Activity R continues to share the remaining 4 days with F.

Now examine activity B. If B used the 1 day of float and finished at the end of day 8, that would not affect any other activity because P and I cannot start until the end of day 8 anyway. Therefore, B’s 1 day of float is independent.

If activity P uses the 16 days of float and does not finish until the end of day 30, that does not affect later activities, nor does P share float with earlier activities. Therefore, those 16 days of total float are independent to P.

Continue looking at Figure 9.12 and exam- ine activity G, which has a critical predecessor and a critical successor. If G uses the 2 days of float it does not affect any other activity; there- fore, those 2 days are independent to G.

A final application of the principles in this chapter is to examine the warehouse schedule you developed in the prior chapters and identify the independent, shared, free, and total floats in that project. Remember, the current computer software will not figure the different types of float for you. To benefit from this information, you need to see the relationships for yourself, and that will also improve your ability to man- age using CPM. Review the prior information and examples and eventually it will click between your vision and your mind and these concepts will make sense. Don’t give up being able to recognize the different types of float. It will make you a better manager and leader. You will understand what many others do not.

91

INTRODUCTION

The use of lags can significantly reduce the number of activities in a schedule. To this point in the text, the logic has shown that one activity must finish before the next activity can begin. Sometimes in a project, the management team may want to overlap some of the activities or have a delay between activities. It may be logical for one activity to start and then a few days later the successor activity could start, without waiting for the predecessor to be completely finished. This overlapping of activities is done with the use of lags. It is also possible to use lags to show a period of time between activities, rather than an overlap. There are four types of logic relationships using lags: (1) finish-to-start (FS); (2) start- to-start (SS); (3) finish-to-finish (FF); and (4) start-to-finish (SF).

So far in the text, the only relationship that has been used is the finish-to-start relationship with zero lag. This is the standard or default relationship. It is graphically illustrated in Figure 10.1.