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## Estimating Activity Duration

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**Estimating Activity Duration**• Time Interval • Time Interval is selected according to the nature of the activity (seconds - minutes …) • It is common practice in construction industry to use calendar day. • Use one and only one time unit for any schedule • Estimating Activity Duration 1.From Company’s Record 2.From Standard Estimating Guide 3.Interviewing Field Personnel**Weather and Contingency Allowance**• Two approaches for assignment of weather allowance: 1.Add the Weather Allowance at the end of the project as a separate activity 2.Add Weather Allowance to those affected by the weather. • Can add weather allowance at the end of each construction segment (site preparation, foundation …etc. • Other contingency items • Procurement Allowance • Strikes • Time Zone • The close of the work period immediately preceding the start of the project.**Precedence Diagram**• Advantages of Using Precedence Diagram 1.No dummy activity are required. 2.A single number can be assigned to identify each activity. 3.Analytical solution is simpler. • Numbering System Same concepts used in Arrow diagram • Even or odd or in fives or tens • Smaller numbers to the left • Each Activity has its unique number.**Redundant Linkages**20 A 30 B 40 C (a) Incorrect Representation 20 A 30 B 40 C (b) Correct Representation**Closing the network to givesingle beginning and ending nodes**A E B C F (a) Incorrect Representation A E Start Dummy Finish Dummy B C F (b) Correct Representation**Retaining Wall Precedence Diagram**Wall 2 20 Footing 2 Wall 1 10 Footing 1 40 Wall 2 Footing 2 Footing 1 30 Wall 1**Precedence Diagram -Sequence Stepped and Numbered**10 A 25 E 5 Start 35 Finish 15 B 20 C 30 F 1 2 3 4**Advantage of Using Precedence Diagram**1.No dummy activities are required. 2.A single number can be assigned for identify each activity. 3.Analytical solution is simpler.**Constructing the Diagram**1.Constructing the Diagram • Activities with no dependencies arranged at the left • Create a dummy start called “Contract Award” • Ball-of-string provides a mean to determine sequence steps for each activity. 2.Arrange Activities in a Sequential Order • Start by activity “Contract Award” • Possible sequence step are these activities located at the arrow head.**Create Activity List**• List all the possible activities beginning with an active verb. (strip, repair...) • Consider all procurement activities • Obtain vinyl floor covering • Obtain cabinet • Painter availability. “Labor Procurement” • Consider the relationship among the activities • What does each activity depend upon? • The planner must ask the field personnel how they plan for the job, and why things should be done the way proposed.**Constructing the Precedence Diagram**1.Create Activity list and eliminate redundancies. 2.Construct Rough Diagram “Ball of string” • Activities with no dependency arranged at the left • Create dummy start if you have more than one start “Contract Award” • Ball-of-string provides a mean to determine sequence steps for each activity. 3.Arrange Activities in a sequential order • Start by first activity “Contract Award” • Possible sequence steps are those activities located at the arrowhead**Additional Rules/Practices**1.Event numbering is usually entered after completing the diagram. 2.Common practice to assign i, j in such a way that the number at the tail is small than that at the head. 3.Choose some numbering pattern such as • Even number • Odd number • Numbers in fives or tens**Tabular Format (Precedence Diagram)**• Can be carried out on the tabular form without preference to the diagram. • Advantage: The time required to construct the diagram is eliminated. • Determination of early start and finish date • Start by time zero and add duration to get EF for A, B, C, D • For E choose the maximum EFD of 8 or C**Determining Lag**LAG5-10 = ESD10 - EFD5 = 4 - 4 = 0 LAG15-25 = ESD25 - EFD15 = 12 - 7 = 5 LAG20-25 = ESD25 - EFD20 = 12 - 6 = 6**Determining The Free-Float**• Free Float is the minimum of the lag of the link that follow an activity. • Look at the “Pre. Column” to find the same number value of EF for terminal activity “not following act.” FF35 = LFD35 - EFD35 = 20 - 20 = 0**Determining the Total Float “Backward”**• Take TF of the terminal activity = 0 or FF35 = LFD35 - EFD35 = 20 - 20 = 0 TFI = Min (LAGIJ + TFJ) TF30 = Min (LAG30-35 + TF35) = Min (7 + 0) = 7 No other link TF20 = Min LAG20-25 + TF25 = Min 6 + 0 = 6 LAG20-30 + TF30 1 + 7 { } { }**INDF**• If the free float is zero - no INDF INDFJ = FFJ - MaxI (TFI - LAGIJ) INDF20 = FF20 - Max(TF5 - LAG5-20) = 1 - (0 - 0) = 1 INDF30 = FF30 - Max TF15 - LAG15-30 TF20 - LAG20-30 =7 - 5 - 0 = 2 6 - 1 { } { }