# 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
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
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
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
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
• 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

=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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