Estimating Activity Duration by NyhZaG

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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
     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
Redundant Linkages
20                30
A                 B




                                    40
                                    C

     (a) Incorrect Representation




20                30                40
A                 B                 C


     (b) Correct Representation
   Closing the network to give
single beginning and ending nodes
             A                  E

             B

             C                  F
         (a) Incorrect Representation

             A                  E

 Start                                   Finish
Dummy        B                          Dummy

             C                  F
         (b) Correct Representation
     Retaining Wall Precedence
              Diagram

            Wall 2                       20
                                      Footing 2


Wall 1                       10                    40
                          Footing 1               Wall 2
              Footing 2

          Footing 1                     30
                                       Wall 1
    Precedence Diagram -
Sequence Stepped and Numbered
          10     25
          A      E



   5      15            35
  Start   B           Finish



          20     30
          C      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”
       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
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
 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
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
       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


  =6       {
  TF20 = Min
                            } { }
                LAG20-25 + TF25 = Min 6 + 0

                LAG20-30 + TF30     1+7
                INDF
• If the free float is zero - no INDF
  INDFJ = FFJ - MaxI (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

								
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