chapter 11 _ network models _ operations research

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Chapter 4 : Duality Theory,Sensitivity Analysis and GoalProgramming

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Chapter 11 : Network Models

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Chapter 11 : Network Models

Q 11.1 Define critical path and critical activities

Ans Critical path the path containing critical activities (with zero float) is known

as critical path.

Critical activity the activity, which can not be delayed without delaying the

project duration, is known as critical activity

Q 11.2 Differentiate between supercritical and subcritical activities

Ans Supercritical an activity having negative float is known as supercritical

activity.

Subcritical An activity having positive float is known as subcritical activity This

activity may be delayed without any delay m the project

Q 11.3 Differentiate between slack and float

Ans Slack It is the time by which occurrence of an event can be delayed

Q. 11.4. Enlist four types of floats used in network analysis.

Ans. (a) Total float.

(b) Free float

(c) Independent float

(d) Interfering float

Q 11.5 Define Free Float, Independent float, Interfering float as used inPERT chart.

Ans. Freefloat : Portion of the total float within which an activity can bemanipulated

without affecting the floats of subsequent activities.

Independent float: Portion of the total float within which an activity can bedelayed

without affecting the floats of proceeding activities.

Interfering float : It is equal to the difference between the total float and thefree float of the activity.

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Q. 11.6. What do you mean by dummy activity?

Ans. Dummy activity : An activity, which only determines the dependency of one

activity on the other, but does not consume any time, is called a dummy activity.

Q. 11.7. Define dummy arrow used in network.

Ans. Dummy arrow: It represent the dummy activity in the network. It onlyrepresents the dependency of one activity on the other. It is denoted bydash/dotted

line.

Q 11.8. Define dangling and looping in net-work models.

Ans. Dangling : The disconnection of an activity before the completion of all the

activities in a network diagram is known as dangling.

Looping (cycling) : Looping error is also known as cycling error in a networkdiagram. Drawing an endless loop in a network is known as error of looping.

Q. 11.9. Differentiate between event and activity.

Ans. Event: The beginning and end points of an activity are called events ornodes. Event is a point in time and does not consume any resources.

Activity : It is physically identifiable part of a project which require time andresources for its execution. An activity is represented by an arrow, the tail ofwhich represents the start and the head, finish of the activity.

Q. 11.10. Define (i) Network (ii) Path terms used in network.

Ans. (i) Network: It is the graphical representation of logically and sequentiallyconnected arrows and nodes representing activities and events of a project.

(ii) Path : An unbroken chain of activity ,arrows connecting the initial event to

some other event is called path.

Q. 11.11. Differentiate between CPM and PERT.

Ans. CPM.:

1. CPM is activity oriented i.e., CPM network is built on the basis of activities.

2. CPM is a deterministic model. It does not take into account in uncertaintiesinvolved in the estimation of time.

3.

CPM places dual emphasis on project time as well as cost and finds the trade off.between project time and project cost.

4. CPM is primarily used for projects which are repetitive in nature andcomparatively small in size.

PERT

1.PERT is event oriented.

2.PERT is a probabilitic model.

3.PERT is primarily concerned with time only.


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4. PERT is used for large one time reserach and development type of projects.

Q 11.12. Define crashing in network mode1s.

Ans. Crashing: The deliberatic reduction of activity normal time by puffing an

extra effort is called crashing. The crashing is being done by allocating moremanpower or by subcontracts.

Q. 11.13. Differentiate between:

(i) Crash project time and optimum project time

(ii) Normal cost and crash cost.

Ans.

(i) Crash project time: It is the minimum time by which the prOject may becompleted.

Optimum project time: The time corresponding to minimum project cost forcompletion of the project is known as optimum project time.

(ii) Normal cost : The cost associated when the project completed with normaltime

Crash cost: The cost associated when the project completed with crash time isknown as crash cost.

Q.11.14. Briefly explain resource allocations. Distinguish betweenResource smoothing and Resource levelling.

Ans. The resource allocation procedure mainly consists of two activities:

(z) Resource smoothing

(ii) Resource levelling.

Resource smoothing If the project duration could not be changed then the

resource allocation only smoothen the demand on resources in order that thedemand

for any resource is uniform as possible The periods of maximum demand forresource

are located and the activities are accordmg to their float values are shifted forbalancing

the availability and requirement of resources. The intelligent utilization of floatscan

smoothen the demand of resources to the maximum possible extent is calledresource

smoothing.

Resource Levelling : There are various activities in a project demanding varying

levels of resources.

The demand on certain specified resources should not go beyond the prescribed

level is know as resource levelling.

Problems


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Problem 11.15. Construct the network for the following activity data:

Solution. Network:


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Critical path 1—3—5—7—8—10 with project duration of 22 weeks.

Problem. 11.17. The time estimate for the activities of a PERT networkare given

below :

(a) Draw the project network and identify all the path through it.

(b) Determine the expected project length.

(c) Calculate the standard deviation and variance of the project length.

(d) What is the probability that the project will be completed

1. At least 4 weeks earlier than expected time.

2. No more than 4 weeks later than expected time.

(e) The probability that the project will be completed on schedule if theschedule completion time is 20 weeks.

(f) What should be the scheduled completion time for the probability ofcompletion to be 90%.

Solution. (a) Network


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Critical path—1 —3—5—6

Project duration = 17 weeks.

(c) Variance of the project length is the sum of the variance of the activities onthe critical.

(d) (i) Probability that the project will be completed at least 4 week earlier thanexpected time

2. Probability that the project will be completed at least 4 weeks later thanexpected

Time

Expected time = 17 weeks Scheduled time =17+ 4 =21 weeks

(e) Scheduled time = 20 weeks

(f) Value of Z for P = 0.9 is 1.28 (from probability table)

Problem 11.18. Consider the PERT network given in fig. Determine thefloat of each activity and identify the critical path if the scheduledcompletion time for the project is 20 weeks.


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Solution.

Critical path 10 - 20- 30 - 60 — 70.

Problem. 11.19. Table below show jobs, their normal thne and and costestimates for cost and crash time the project

Indirect cost for the project is Rs. 300 per day.

1. Draw the network of the project.

2 What is normal duration and cost of the project’

3 If all activities are crashed, what will be the minimum project durationand corresponding cost.


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4 Find the optimum duration and minimum project cost

Solution.

2. Critical path I — 2— 3 — 4 — 6

Normal duration of project = 20 days

Normal cost of project = Rs 9200.

3. Crashing

Crash following activities one day


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Minimum project duration 12 days

Cost corresponding to mm. project duration 16533.1

Minimum cost of project 15000

Optimum duration = 17 days.

Problem 11.20. The following table gives the activities in a constructionproject and other relevant information.

Draw activity network of the project.

(b)Find total float and free float of each activity.

(c) Using the above information “Crash” or shorten the activity step bystep untill the shortest duration is reached.

Solution.

Activity Network.1.


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(b)

Free float = Total float - Head event slack. Critical path 1-2-3—4-5.

(c) Crashing


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Problem 11.21. The activities, of a project are tabulated below withimmediate predecessors and normal and crash time cost.

1 Draw the network corresponding to normal time

2 Determine the critical path and normal duration and cost of project, ifthe indirect cost per day is Rs. 8.

3. Suitably reduce the activities so that the normal duration may bereduced by 2 days at minimum cost. Also find the. project cost for thisshortened duration.

Solution.


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Total project cost for reducing 3 days duration.

= Direct cost + Indirect cost + crash cost

= 3030 + 160 + 178 = Rs. 3368.


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chapter 11 _ network models _ operations research

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