chap 5 schmtds

7/29/2019 Chap 5 Schmtds

1/13Construction Project Management Page 1

Chapter - V Scheduling Methods

Work Breakdown Structure

Bar Chart Basic Networks

Critical Path Method (CPM)

Program Evaluation and Review Technique (PERT)


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Construction Project Management Page 2

Work Breakdown Structure

The construction process starts with the construction planning which is the prime activity

in Construction Project Management (CPM). Planning is the identification of the tasks which need to be

done for the completion of the work (project). It is achieved by the technique known as Work Breakdown

Structure (WBS).

The various steps involved in WBS are as follows :-

a) Breaking the project into recognizable systems or units.

b) Dividing each system or unit into functional sub systems.

c) Detailing the various tasks involved in completion of each sub system.

d) Listing the activities in a task in hierarchical order.

e) Each activity is so chosen that it can be planned, scheduled and executed.

Scheduling

It is the process of fitting the work plan to a time frame. The activities are sometimes

interdependent or independent. For example, Roof slab casting can't take place unless the reinforcement has

been placed in position and reinforcement can't be placed in position until the shuttering has been completed

Like this the various inter-related activities of the project need to be studied and scheduled accordingly for the

efficient functioning.

The independent activities takes place irrespective of the completion or delay of the other

activities. For example, reinforcement and erection of shuttering can be done independently and simultaneously

Some of the conventional methods for Scheduling explained below.

Bar Chart (Gantt Chart)

Henry Gantt developed this specialized chart around the years 1910 - 1915 and

popularized in West. The early application of this was tracking the progress of ship building.

A Bar chart is a linear calendar of project activities for planning of a project. The chart is two - dimensional

with Time along the X-axis and Activities along the Y-axis.

The Start of the horizontal bar indicates the start of an Activity. The activities which can

run concurrently i.e. which are independent of other activities are depicted by parallel bars. The sequential

activities which can happen only after the completion of the previous, are depicted by bars starting from the

time when the first activity ends. ( shown in fig. )

Let us take the following example of construction of column footings in a building :-

Activity Duration

A Layout and excavation of column footings. 2 days

B Laying of lean concrete and curing. 3 days

C Fabrication of foundation reinforcement. 2 days

D Erection of shuttering and placing of reinforcement. 2 days

E Casting of column foundations. 2 days


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Bar Chart

Monitoring

Coloring and Hatching bars technique was associated with the Bar charts to check out the

progress of different activities. As shown in the bar chart, the darkened (black) part of bars indicate the level of

completion of the particular activity against time (in days). Let us take a particular day, say day 8 and try to find

the progress of the activities. If we see, at the end of day 7 only 50 % of work is completed against the planned70%.

This review of progress would indicate an Urgency to expedite the laying of foundation concrete by including

more resources to avoid delay of subsequent activities. But without this additional impovement on Bar Chart

site engineers would not know the extent of delay in planned schedule and may fail to take timely corrective

measures.

Limitations of Bar Charts

a) Interdependency of activities - The bar chart fails to clarify the interdependency of an activity on one

or more of other activities explicitly.

b) Monitoring of progress - The bar chart has no provision to indicate the progress of work at any given

time and hence cannot be used as a monitoring tool.

c) Critical activities - The are no such indications in the bar charts, which can help us in finding the

critical activity of the project cycle, as the delay of this critical activity could affect the completion of

project.


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d) Quantum of work- The quantity of work to be carried out in each activity is not indicated, thus it is not

easy to work out the requirement of resources for execution.

Advantages of Bar Chart

a) In case of small projects having limited number of activities, the application of bar chart is very much

successful.

b) ven in the case of a large project, Bar chart cn be used as an excellent tool at the foreman level, where it

becomes easy in understanding the split sub-activities of a main activity.

Basic Networks

Networks are logical and chronological graphic representation of the activities that

constitutes a project (events composing a project). basically the networks are of two types, Arrow network and

the Node network. In an arrow network the activity is mentioned over the arrow and so it is also called as

Activity On Arrow (AOA) networks. In case of the node network, the activity is mentioned inside the node and

so it is called AON i.e. Activity On Node.The representation of a simple AOA and AON network below gives the idea of basic

networks in construction management.

The circles in the AOA and the squares in the AON network represents the nodes, the activities are represented

by the alphabets and the arrow heads indicate the hierarchy of activities.

The nodes represent events ; as an event is the point of time when an activity starts or ends. Each arrow connect

two nodes, the fr om nodeand the to node(depicted by circles and squares in AOA and AON respectively). The

basic difference between AOA and AON networks is that, in AON or node networks the dummy activities are

not depicted separately.


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Logic : In the above network we have, the activities B and C following after A. This means B and C can't

happen when A is not completed. But in reality B and C may occur consecutively or concurrently depending

upon the resources availability, site conditions etc. And once both B and C are done, activity D can start.

Dummy activities : These are fictitious activities which are inserted into arrow (AOA) networks to make it

logically correct and to distinguish between identities of activities. The AOA network below will explain about

the dummy activities or dummies.

Redundancies: An important thing to note that while building a arrow network you can have not only a

logically improper network but also a proper one with redundant dummy activities. This fact is especially true

for complicated networks. Redundant dummy activities are not logically improper: they are just redundant ( ).

Critical Path Method

The Critical Path Method is the extension of the basic networks in which the activities are

grouped into two groups i.e. Non critical and Critical activities. This identification of criticality of the activities

involves the understanding of the following terms :-

Earliest event time

Forward pass

Latest event time

Backward pass Float and its calculation.


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Earliest event time

It is the earliest time by which all activities leading to an event are completed. In practice

it is represented by TE and is written in a triangle above the event node. The iniitial node in a network is alloted

TE=0.

The Rule for evaluating TE- Add duration of activity to earliest event time of the immediately preceding event

if there are more than one activities for which this event is the end event, then repeat the same process for each

activity and the highest of the time so calculated would be the earliest event time for that event.

The network below explains the TE calculation,Forward pass

In forward pass we move from left to right in forward direction and calculate the TE of

all events starting from initial event till the end event. Earliest event time of initial event is taken equal to Zero.

T of event 10 = 0 and earliest event 20 can occur only after the completion of activtiy A i.e T for event

20=0+5= 5 days

T of 30 = 5+10 = 15 days and T of 40 = 5+8 = 13 days

T of 50 will be largest of earliest completion of activtiy 30-50 and T of 40

earliest completion of 30-50 = 15+7 = 22 days.

T of 40 = 13 days therefore T of 50 = 22 days.

Similarly T of 60 will be largest of earliest completion of activity 40-60 and 50-60.

Earliest completion of activity 40-60 = 13+12 = 25 days.

Earliest completion of activity 50-60 = 22+6 = 28 days.

Therefore T of 60 = 28 days and T of 70 = 28+4 = 32 days.

Fig. AOA network (Activity duration in days)


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Latest event time for an event

The latest time by which an event should occur to keep the project on schedule is called

the Latest event time of that event. It is denoted by TL and we consider the latest event time of last event equal

to its earliest event time and then move backward. It is written below the node in square.

The Rule for evaluating TL - Deduct duration of activity from latest event time of immediately succeeding

event, if there are more than one activities (paths) to reach immediate successor event, then repeat the process of

deducting activity duration from latest event time of immediately succeeding activity for all paths and choose

the lowest of the times so calculated and that would be the latest event time of event under consideration

Backward pass

In case of backward pass we assume the latest event time of the end event equal to its

earliest event time and then move from right to left reaching the first event and on the way we work out the

latest time for each event and note the same below the event node. If calculation is error free the Initial event

will have the latest event time equal to Zero. Let us consider the above network, in that for event 70 to occur on

32 days event 60 must occur on 28 days as activity 60-70 needs 4 days for completion. And for event 60 to

occur in 28 days the event 50 should occur in 28-6 = 22 days. Activity 40-60 duration is 12 days so event 40

should occur not later than 28-12 = 16 days but considering backwards path via dummy activity 40-50 event 40

should occur not later than 22-0 = 22 days, choosing the least value of 16 days, we can say that events 50 and

60 can on their latest time when event 40 occurs in 16 days.

TL of 30 = 22-7 = 15 days

TL of 20 = 15-10 = 5 days via activity 20-30

= 16-8 = 8 days via activity 20-40

Therefore TL of 20 = 5 days (choosing least of 8 and 5 days)

Float

Float is the extra time available for an activity over and above the activity duration. There

are three types of floats namely Total Float (TF), Free Float (FF) and Independent Float (IF).

(a) Total Float (TF) :- It is the maximum time by which an activity can be delayed without delaying the

completion time of Project. Total Float of an activity is related to all other activities and use of Total float will

affect the floats of both preceding and succeeding activities.

Total Float (TF) = Total time available to execute activity - duration of activity.

TF = (TLh - TEt) - d

(b)Free Float (FF) :- If we don't wish to interfere with the floats of succeeding activities the activity must

finish at the earliest event time of head event so that succeeding activity can start at its EST (Earliest Start Time

of an activity), therefore

FF = (TEh - TEt) - d


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(c) Independent Float (IF) :- This is the real reserve of the activity because by using IF we will not be

interfering either with the preeding or succeeding activity and for this to happen activity should start at latest

start time of tail event and finish at the earliest event time of head event.

IF = (TEh - TLt - d)

Critical Activities

The activities of a project can be grouped into two groups i.e. Non Critical activities and

Critical activities. Non Critical activities are those activities for which Total float is not zero and Critical

activities are those for which the Total float is zero. As there is zero Total float in Critical activities, any delay

in their completion will cause corresponding delay in the project completion time.

Critical Path

The path joining the critical activities in a network is known as Critical Path. It is the

longest path in the network and the time taken on this path determines the completion time of project.

To locate critical path total float in respect of all activities is worked out and critical

activities (TF = 0) are identified and joined together to identify the critical path. Double or Bold lines are used

to mark the critical path in network so that it is prominently visible. A network may have more than one critical

path. The network below will explain the Critical path method of Scheduling.

Consider the activity table and the corresponding AON network.

Activity Immediately Preceding

Activity (IPA)

Duration (in

days)

A - 5

B A 8

C A 6

D B 9

E B,C 6

F C 3

G D,E,F 1

The Forward Pass

The project starts with activity A, which starts at the beginning of day 1 (end of day 0). It

takes 5 days to finish activity A: it finishes on day 5 (end of the day). At this point, activities B and C can start.

Activity B takes 8 days: it can start on day 5 (directly after activity A finishes), so it can finish as early as day

13. Similarly, activity C can finish on day 11 (5 + 6). Activity D follows activity B. It can start on day 13 (end

of B) and end on day 22. Activity E must wait till both activities B and C are finished. Activity C finishes on

day 11, but activity B does not finish till day 13. Thus, activity E cannot start till day 13. with 6 days' duration,

activity E can then finish on day 19. Activity F depends on activity C only. Thus, it can start on day 11 and

finish on day 14. The last activity, G, cannot start till activities D, E, and F are finished. Through simple


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observation, we can see that activity G cannot start till day 22 (when the last activity of D, E, and F finishes).

Activity G takes 1 day, so it can finish on day 23. The Completed network is shown below

For this example, we have calculated two types of dates: 1. The expected completion date of the project: day 23

2. The earliest date when each activity can start and finishThese dates are called the early start (ES) and theearly finish (EF) dates for each activity. As you will soon learn, an activity cannot start earlier than its ES date

and cannot finish earlier than its EF date, but it may start or finish later than these dates. In mathematical terms

the ES time for activityj (ESj)is as follows:

ESj=max(EFj)

where (EF1)represents the EF times for all immediately preceding activities. Likewise, the EF time for activity

j (E15)is as follows:

EF=ESj+Durj

where Dur is the duration of activity j. The forward pass is defined as the process of navigating through a

network from start to finish and calculating the early dates for each activity and the completion date of the

project. See Figure \

The Backward Pass

Now let us start from the end of the project and work our way back to the start. we

already know the end-of-project date5: day 23. Activity G must finish by day 23. Its duration is only 1 day, so it

must start no later than day 22(23 - 1) so that it does not delay the project. Similarly, activities D, E, and F must

finish no later than day 22 so that they will not delay activity G. Through simple computations, we can find

their late start dates: activity F: 22-3 =19; activity E: 22 - 6 = 16: and activity D: 22- 9 =13. Activity C must


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finish before activities E and F can start. Their late start dates are 16 and 19, respectively. Clearly, activity C

must finish by the earlier of the two dates, day 16, so that it will not delay the start of activity E. Thus, its late

start date is day 10 (16 - 6). Similarly, activity B must finish by the earlier of its successors' late start dates: day

13 for D and day 16 for E. Therefore, the late finish date for activity B is day 13 and its late start date is day

5(13 - 8). The last activity (from the start) is A: It must finish by the earlier of the late start dates for activities B

and C, which are day 5 for B and day 10 for C. Consequently, the late finish date for activity A is day 5, and its

late start date is day 0 (5 - 5). In mathematical terms, the late finish (LF)time foractivity j(LSj)is as follows

LF=min(LSk)

where (LSk) represents the late start times for all succeeding activities. Likewise, the late start (LS) time for

activityj (LSj) is as follows:

LSj=LFjDurj

The backward pass is defined as the process of navigating through a network from finish

to start and calculating the late dates for all activities. This pass, along with the forward-pass calculations, helps

identify the critical path and the float for all activities. If you refer to Figure , you can see that for some

activities (light lines), the late dates (shown under the boxes) are larger (i.e., later) than their early dates (shown

above the boxes). For other activities (thick lines) late and early dates are the same. For the second group, we

can tell that these activities have strict start and finish dates. Any delay in them will result in a delay in theentire project. We call these activities critical activities. We call the continuous chain of critical activities from

the start to the end of the project the critical path. Other activities have some leeway. For example, activity C

can start on day 5, 6, 7, 8, 9, or 10 without delaying the entire project. As mentioned previously, we call this

leeway float

There are several types of float. The simplest and most important type of float is total float (TF):TF = LS - ES (or) TF = LF - EF (or) TF = LF - Dur - ES

With the completion of the backward pass, we have calculated the late dates for all activities. With both passes

completed, the critical path is now defined and the amount of float for each activity is calculated.

Activity Duration ES EF LS LF TF

A 5 0 5 0 5 0

B 8 5 13 5 13 0C 6 5 11 10 16 5

D 9 13 22 13 22 0

E 6 13 19 16 22 3

F 3 11 14 19 22 8

G 1 22 23 22 23 0


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PERT (Program Evaluation and Review Technique)

The program evaluation and review technique (PERT) is an event-oriented network

analysis technique used to estimate project duration when individual activity duration estimates are highly

uncertain. PERT applies the CPM to a weighted-average duration estimate. PERT is considered a probabilistic

method.

Concept of PERT

Like the regular (deterministic) CPM, PERT uses logic networks to calculate the

completion date of a project or the date of any other event in the schedule. In PERT, a probability (likelihood) is

associated with any event date. This probability depends on uncertainty in the durations of the activities that

lead to the desired event (e.g., project completion). PERT realizes that actual durations vary from those

assigned, so it attempts to compensate for this variation with a time range during which activity durationsmay realistically occur. This topic is discussed in more detail after the details of PERT are covered.

Working Of PERT

PERT uses a probabilistic approach, which requires a duration frequency distribution for

each activity. In most cases, such distributions are unknown or unavailable. Because of this, PERT requires the

user to set three durations that constitute the practical range of the duration for each activity. These three


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durations give a distribution for the activity, and we can produce the statistical mean and variance for the

duration of the activity. When we need to compute the probability that a certain event, such as Substantial

Completion of a building, will occur by a certain date, we need to take into account all preceding events,

starting at the beginning of the project and including the continuous chain of activities till Substantial

Completion. Using the central limit theorem, PERT treats the means of the durations of these activities as a

normal distribution, no matter what distributions these durations followed. PERT then uses simple statistics to

calculate the mean and variance (or standard deviation) of the time required to complete the chain of events

leading to Substantial Completion. It calculates the probability that Substantial Completion will occur by a

particular date, or, conversely, it calculates the date that the Substantial Completion event will happen with a

certain level of confidence (probability).

PERT Calculations

As mentioned previously, the path (chain of activities or events) leading to the examined

event (e.g., Substantial Completion) must be chosen. For each activity on that path, three durations must be

estimated:

To: Optimistic Duration

Tm: Most Likely Duration

Tp: Pessimistic Duration

The preceding values are estimated by the scheduler or project manager, who uses his or

her experience and good judgment to do so. The optimistic duration is the amount of time the activity will take

if everything goes smoothly and efficiently. The pessimistic duration is the duration under the worst-case

scenario. Both values must be within the realistic, although perhaps unlikely, realm of expectations. The mean

weighted value for these three durations is called the expected duration (Te). It is calculated as follows:

Te = To + 4Tm + Tp

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The weights assigned to these times (coefficients of To, Tm, and Tp) may be adjusted, but the denominator

must equal the sum of all weights. The weights in equation above represent a population of durations made up

of 16.7% (one-sixth) optimistic (To); 66.7% (four-sixths) most likely (Tm); and 16.7% (one-sixth) pessimistic

(Tp). Most likely Duration (Tp) is the time for completion of an activity under normal conditions i.e.

conditions are not ideal, minor mishaps may happen.

Applications of PERT

PERT is used for works which are first of its kind, where time estimates for various

activities are neither known nor can be estimated with any certainty. The emphasis is given on reducing project

completion time without cost constraints.


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