CHAPTER 9 Float Types INTRODUCTIONFLOAT TYPES The different

CHAPTER 9 Float Types

INTRODUCTION-FLOAT TYPES ▪ The different types of floats not well understood ▪ Results in mismanagement of the project ▪ Not easy to understand when first introduced to them ▪ Review this chapter several times until they are understood

INTRODUCTION ▪ Most managers feel CPM’s main benefit is the critical path ▪ Important to timely job completion ▪ Point of focus for completion date ▪ Float activities allow more precise management of the job ▪ Correct use of float takes pressure off many aspects of the job ▪ Better control of resources and cost ▪ Float is the key ▪ Understanding different types of float is important

TOTAL FLOAT ▪ The only float we have discussed at this point ▪ The difference of early start and late finish days or ▪ The difference of late start and late finish days ▪ Inexperienced manager may think that float belongs to that activity alone ▪ When that float is used, it takes float from other activities as well

SHARED FLOAT ▪ A difficult concept to understand Subs W, I and N see the 11 days of float and each plans on using the extra days. B uses 2 days W uses 15 days N uses 14 days S uses 2 days The project now takes 46 days instead of 24. Look closer • If W uses float of 11, finishes on day 17 • Activity I can no longer start on day 6 • I’s early start day is now 17 • The same as I’s late start day • Activity I is now critical with 0 float • N also becomes critical • N cannot start until day 19 when I finishes • The 11 days of float are shared with W, I & N

SHARED FLOAT Float is like money in the bank: you would like to save it for a later date when it may be of greater need. • If Y and H use their float day • it would not affect later activities • nor can earlier activities rob them of their float • Y and H do not share float

SHARED FLOAT ▪ Another example: • If activity J uses 2 available float days • finish on day 7 • Activity B • robbed of 2 float days • becomes critical • finishes on day 13 • Activity N • also becomes critical • J, B and N have 2 days of SHARED float • Activity L • uses 1 day of float • will this rob V? • Yes, 1 day

FREE FLOAT ▪ The amount of time an activity can be delayed without delaying the early start of activities following it. ▪ The amount of time an activity can be delayed without taking away float from later activities ▪ Looks only on later activities, not earlier activities ▪ Determined by ▪ Subtracting earliest finish time ▪ From the earliest of the early start times that directly follow

FREE FLOAT If Activity Y uses the 1 day of float • It does not affect Activity U • Because U can’t start until day 7 • Activity Y has 1 day of FREE float The key to determining FREE float: Look at the early end and the early start of the next activity. If they are the same, there is no FREE float 22 X If Activity H uses 4 days of float it will not affect Activity S • Activity S cannot start until day 22 anyway • Activity H has 4 days of FREE float If Activity W uses any of the 11 days of float • It will delay Activity I • Activity W has no FREE float If Activity I uses any of the 11 days of float • It will delay Activity N • Activity I has no FREE float If Activity N uses any of the 11 days of float • Does it affect Activity S? • No, Activity S’s early start is day 22 • Activity N has 11 days of FREE float • Activity N is at the end of a string of SHARED float The last SHARED float activity always becomes FREE float • But the FREE float may be used by W or I leaving N with no float • N’s FREE float does not mean it can be given away because it is also SHARED float

FREE FLOAT Another example: If J uses any float will it rob float from B? • J’s early finish is day 5 • B’s early start is day 5 • Yes, it will rob float. • There is no FREE float Does Activity B have FREE float? • No, early start and early finish are same day Does Activity N have FREE float? • Yes, 2 days • Activity J and Activity B may have already used it • Activity J, B & N have SHARED float • Activity N’s chance of using the FREE float may be small

FREE FLOAT Look at Activity L and Activity V • They have 1 day of SHARED float • If Activity L uses it, Activity V is delayed Activity V could use both float days without affecting Activity I • Therefore, Activity V has 2 days of FREE float Why is FREE float helpful to management? • Free float can be shared with previous activities • If prior activities use it, it cannot be used for later activities • It is crucial to not give away FREE float too early in the job • The extra days may be needed later • FREE float used too early increases the chance of a later project finish date • Only INDEPENDENT float can be given away • Will discuss this next class period

INDEPENDENT FLOAT ▪ Independent float is float that belongs to one activity and that activity alone. ▪ It is not shared with any activity earlier ▪ It is not shared with any activity later ▪ Independent float can be given away by management because it has no affect on other activities

INDEPENDENT FLOAT Both Activity Y and H are on the critical path with no other activities interfering Activity Y If Y uses float will it affect Activity U? • Activity U does not start until day 7 • use of 1 day of float past EF day 6 = 7 • This means Activity Y has FREE float If Y uses float will it affect Activity B? • Activity B and Y do not share float • no earlier activity can take Y’s float Activity Y has INDEPENDENT Float Activity H If H uses 4 days of float will it affect Activity S? • 18 + 4 = day 22=ES of Activity S. • So no effect • Activity H has FREE float If H uses 4 days of float will it affect Activity U? • Activity U and H do not share float • Activity U cannot take Activity H’s float Activity H has INDEPENDENT Float

INDEPENDENT FLOAT Activities W, I and N share 11 days of float They do not have INDEPENDENT float However, what happens if Activity I finishes with the 11 days of float unused? Activity N will then have INDEPENDENT Float • No earlier activity will take the 11 days of float • Activity N has FREE float because • Activity S does not start until day 22 • EF of 11 plus 11 days of float = day 22 We see that FREE float has a chance to become INDEPENDENT float if predecessors are finished and their float is available.

INDEPENDENT FLOAT To recap: INDEPENDENT float is not shared in either direction ▪ left or right ▪ before or after All INDEPENDENT float is also FREE float

INDEPENDENT FLOAT Activity J, B & N share 2 days of float Activity N has FREE float Can Activity N also be INDEPENDENT float? • If Activity J and B do not use the 2 days of float Activity V Shares 1 day of float with Activity L 1 day of float is FREE float • (does not affect Activity I) The 1 day of float is also INDEPENDENT float • It is FREE float • And it cannot be taken by Activity L Note: If Activity L does not use the shared 1 day of float, then Activity V will have 2 days of INDEPENDENT float

INDEPENDENT FLOAT ▪ Most texts and software do not discuss or calculate SHARED float or INDEPENDENT float. ▪ The ability to analyze these two floats prevents the manager from making the mistake of giving away float that may end up damaging the project. It is essential to realize that most of the time float is shared, and it is not independent.

AUTHOR’S CHART WITH FLOAT Total Float Shared Float Free Float Independent Float TF SF FF IF Notice the critical path has no float in any category

NEGATIVE FLOAT Negative float is obtained by having a fixed project start date and a fixed project finish date. With the finish date set at a time earlier than would be calculated by the forward pass. If the backward pass is done with a number smaller than 24 • The result would be NEGATIVE float • This means the project is already that many days behind schedule

NEGATIVE FLOAT A manager typically does not want to see negative float on a project There are times when it is beneficial If there is a forced completion date and negative float appears it shows management ▪ the path or paths of activities where acceleration is needed ▪ exactly how many days need to be eliminated from each path NEGATIVE float is typically shared ▪ The team does not need to reduce each activity by the number of negative days ▪ Just eliminate that many days from each path or string of activities

NEGATIVE FLOAT Many managers ▪ Only input the start date in the computer ▪ Keep an eye on the finish date to make sure it meets requirements ▪ Prefer to have finish date that is earlier than project requirements ▪ To allow for likely project delays

WHEN TO GIVE AWAY FLOAT OR KEEP IT Temporary Power If electrician is type of person whose word is his bond tell him he could start on day 4 (end of day 3) and end on day 9 If electrician is a little slow, or undependable tell him to start on day 4 and end on day 6 Save 3 extra days of float in case he needs longer

WHEN TO GIVE AWAY FLOAT OR KEEP IT Shingle Roof 5 days of INDEPENDENT Float Just needs to be finished sometime this week No need to push people beyond what is reasonable

WHEN TO GIVE AWAY FLOAT OR KEEP IT 6 20 20 20 5 6 26 26 25 0 5 3 29 Durations can be adjusted with float Windows & Doors can be given 3 days of shared float with Brick Front Side – change duration to 6 • Brick Front Side would still have plenty of float left This affects Install Siding and Paint Exterior leaving 5 days of shared float between them instead of 8 • Management can increase duration of Install Siding by 2 more days • And still have 3 days of float for Paint Exterior

TO WHOM DOES FLOAT BELONG? Many general contractors have the attitude that float belongs to them They simply give Early Start and Early Finish dates to the subs They would not increase durations as we just did above If window sub takes a couple of extra days ▪ It would impact start days of brick, siding and paint exterior ▪ Requires 3 phone calls to notify of the delay ▪ It is possible the subs are scheduled elsewhere on those days ▪ This could cause considerable delay

TO WHOM DOES FLOAT BELONG? If subcontractor thinks shared float belongs to them ▪ Creates confusion ▪ Can greatly delay the job Solution: ▪ Add shared float days to the duration ▪ Give subs only this adjusted Early Start and Early Finish days ▪ This way the float is hidden

TO WHOM DOES FLOAT BELONG? Sometimes project owner feels all the float should belong to them Contract may state that all activities must start and finish as early as possible, taking away all float This makes every activity a critical activity

TO WHOM DOES FLOAT BELONG? The Float belongs to the project and should be used by the activities that can benefit most by it