Basic Construction Planning Techniques

Relevant Course:Construction Management

Relevant Department:Civil Engineering

Relevant Semester:6

Pre- requisite: None

This module will discuss the basic processes and techniques required to plan a construction project. Students will learn how to organize activities to develop Work Breakdown Structures, and to then create project networks. Basic network analysis methods such as the Critical Path technique as well as analysis of project floats will also be taught. The module will conclude with some practical insights on how to use these analytical techniques to improve project planning and performance.

Lecture - 1

Building Blocks for Planning

In order to understand construction planning, it is first important to have a preliminary understanding of projects.

1a. Introduction to Project Management

Projects are unique endeavors with a specific beginning and end. Differences will exist between any two projects. Consider the case of two similar small-scale residential buildings. Differences can arise in soil conditions, interior design, selection of contractors and so on. As a result, projects are 'learning impaired' - it is very difficult to translate knowledge gained or learnings from one project directly to another, due to the inherent differences across projects. This is in contrast with 'processes' that are followed in the manufacturing and other industries, that are repetitive and where a learning curve often results in improved performance over time.

While there are a number of project-based industries, the Architecture-Engineering-Construction (AEC) industry stands out both in terms of the size of projects undertaken (costs of construction projects are extremely high - even a small residential tower can easily cost Rs. 50 Crore), as well as the organizational complexity of executing these projects. The construction industry is highly fragmented and as a result, a number of different organizations are involved in executing a construction project ranging from design firms, consultants, main and subcontractors, vendors and so on. Given the challenges in coordinating amongst these agencies, the process of managing construction projects is highly complex.

Planning therefore plays a critical part on construction projects. Effective planning helps practitioners anticipate challenges and deal with them early on in a project, where the amount of money expended is low, and the flexibility of making an intervention is high. As the project progresses, changes are more expensive to make and budgets are often depleted, and as a result course corrections are not possible. It is for this reason, that considerable attention must be directed towards the planning process in construction projects.

1b. Work Breakdown Structures

Once a project has been awarded, the planning process can begin. The first task is to identify the various activities to be executed on the project. Once this is done, detailed construction methodologies for each of these activities should be identified. Once the methods are fixed, the resource requirements for each activity corresponding to these methods can be ascertained. Based on the productivity of the resources available (or chosen), durations for each of the activities can be calculated. Simultaneously, relationships between activities can also be fixed. These are the basic building blocks of creating a project plan.

The first and most important task, therefore, is to develop a detailed list of activities to be executed on the project. This is known as a Work Breakdown Structure (WBS). All other planning steps follow directly from the WBS, and therefore if the WBS is inconsistent or not comprehensive, then it is likely that a planning or execution failure will occur later on in the project. Normally, when developing a work breakdown structure, planners tend to think mainly of engineering activities. These are activities such as excavation, casting foundations, casting slabs and so on. While these are a vital part of the planning process, other kinds of activities or tasks must also be included in the WBS. These tasks can be grouped under the following categories

  • Procurement of Materials
  • Procurement of Equipment
  • Preconstruction activities (e.g. procuring permits, insurance etc)
  • Post-construction activities (e.g. commissioning, housekeeping etc)

Procurement of materials includes placing purchase orders, testing samples and so on. Procurement of equipment involves renting or purchasing equipment, transporting it to the project site and so on. Pre-construction activities range from obtaining the necessary permits to building approach roads and temporary structures, while post-construction activities relate to site clean-up, commissioning the structure and so on. These are all vital activities for which planning is needed. For instance, if a particular material has a long lead time, the purchase process must be carefully monitored. It therefore follows that this process must also be carefully planned, and therefore the set of activities relating to the procurement of this material must be made a part of the WBS for the project. A comprehensive and useful WBS that can guarantee good project performance will therefore feature activities within the above categories in addition to engineering activities.

1c. Bar Charts

Once a detailed WBS has been established, construction methods can be fixed, resources can be estimated and durations as well as activity relationships can be set. Once these preliminary planning activities are done, the most simplistic way of representing a project plan is through the creation of a Bar Chart, also known as a Gantt chart (named after Henry Gantt who is credited with developing this tool).

Most construction planners and engineers are very familiar with bar charts. As shown in the slides, a bar chart consists of a set of activities listed vertically one below the other. On the horizontal axis runs the projects timeline starting from the planned start date up until the planned finish date. Each activity is then represented as a 'bar' under this timeline. The bar starts from the planned start date of the activity and continues up until the planned finish date of the activity. Activities in the bar chart are listed in increasing order of their planned start dates and bars are drawn to scale, such that an activity whose bar is twice as long as another's can be said to take twice the time.

While representing activities in a bar chart, it is very important to note that a bar denotes an activity that is continuously performed. Therefore, when an 'excavation' activity is shown for a duration of, say, 15 days, the expectation is that the activity is performed continuously over 15 days. If on the other hand, the plan is to excavate two areas for 5 days each with a gap of 5 days in the middle, then this should be represented as two separate excavation activities in the bar chart.

Other kinds of information can also be represented in the bar chart. For instance, the person in charge of each activity can be show next to the activity. The status of the activity can also be shown using pre-determined symbols. The progress of an activity can also be shown by shading a portion of the bar in proportion to the actual percentage of the activity that has been completed. Running a vertical line through the chart on any given day can give a quick view of the activities to be performed on that date.

Bar charts possess several advantages and disadvantages. On the one hand, bar charts are simple tools, easy to create and offer a simple visual picture of how the project will be undertaken. This can be understood by a number of stakeholders including laborers on site. However, on the other hand, bar charts with large numbers of activities become very complex to read and interpret. Further, the relationship between activities is not explicitly represented in a bar chart. Therefore a person reading the bar chart will need to understand the logic used by the planner to understand the impacts to the plan if there is a delay in any given activity. These make bar charts highly difficult to use in large complex projects. However, they may be quite useful when projects are small and relatively routine in nature. Alternatively large projects can be broken into small sub-projects, within which bar charts can be used.

In the next chapter, we attempt to understand how these limitations of bar charts can be overcome through the use of other planning tools.

Videos for Lecture -1

Introduction

Work Breakdown Structure

Interaction and Work Break down Structure

Work Breakdown Structure (Continuation)

Interaction and Elements of Construction Network

Elements of Construction Network (Continuation)

4 Key dates

Interaction and Key dates (Continuation)

Session - 1 Basic Construction Planning Techniques - Lecture 1

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Lecture - 2

Networks

The deficiencies of bar charts can be overcome through the use of Network Diagrams as discussed below. Networks consist of nodes and links between nodes and are very effective in representing construction activities and projects.

2a. Network Diagrams

One of the main deficiencies of the bar chart is that relationships cannot be represented. As a result, if an activity is delayed, the bar chart cannot immediately provide information on which other activities may consequently have to be delayed. In order to solve this problem, it is useful to construct network diagrams. Network diagrams show activities as nodes, and relationships between activities as links between nodes. By following the links, one can easily determine which activities get affected, if a particular activity is delayed. By combining the relationship information in a network diagram with the activity duration information in a bar chart, a comprehensive Precedence Diagram can be prepared which can be used as a planning tool.

2b. Four Key Dates

Precendence Diagram can be subjected to Critical Path Analysis. However, before this is done, certain aspects of each activity are to be ascertained. First, there are four sets of dates that define an activity. These are:

  • Early Start Date (ES)
  • Early Finish Date (EF)
  • Late Start Date (LS)
  • Late Finish Date (LF)

The Early Start Date, as the name suggests, refers to the earliest date that an activity can be started. The early start date an activity is effectively the date right after the earliest finish date of the preceding activities. If there are multiple activities preceding an activity, then the earliest start date is the greatest of the earliest finishes of these preceding activities. For instance, in Figure 1 below, the Early Start date of activity C is 10. The Early Finish Date of an Activity can then be calculated by adding the expected duration of the activity to the Early Start. This denotes the earliest that an activity can be completed so as to release resources for subsequent activities.

The Late Finish Date of an Activity refers to the latest date that an activity can finish without causing delays to the plan. The Late Finish Date of an activity is effectively right before the latest date that a subsequent activity can start. If there is more than one activity that follows from a given activity, then the late finish date is the least of the late start dates of these activities. The Late Start date of an activity is therefore obtained by subtracting the expected duration of an activity from its Late Finish date. Figure 2 below describes the method of ascertaining the Late Finish of activities. The Late Finish of Activity C according to this logic is 6.

2c. Relationships Between Activities

In order to develop a Precedence Diagram, activities need to be related to each other. There are four main variants of these relationships. These are:

  • Finish to Start Relationships
  • Start to Start Relationships
  • Finish to Finish Relationships
  • Start to Finish Relationships

Furthermore, each of these categories can have three sub-categories:

  • Relationship with no Lag
  • Relationship with positive Lag
  • Relationship with Negative Lag

Therefore, overall, there are 12 possible kinds of relationships.

A Finish to Start relationship indicates that an activity can start once a preceding activity finishes. When there is no lag, the subsequent activity can start as soon as the preceding activity finishes. When there is a positive lag, then the lag amount must be specified as a unit of time, and the subsequent activity can start after the finish of the previous activity plus the lag time indicated. When there is a negative lag, the subsequent activity can start prior to the completion of the preceding activity, by a duration equivalent to the lag specified. As an example, Foundation works can start right after Excavation works. This represents a classic Finish To Start relationship. However, removal of shuttering can be done only after an appropriate period of waiting (for curing and other reasons) after the pouring of concrete. As a result, this is a Finish To Start relationship with a positive lag.

When the start of two activities are to be aligned, then Start to Start relationships can be used. For instance, prior to pouring concrete, both the form work as well as the reinforcement need to be prepared. Therefore, it is possible to represent these two activities as being connected by a Start-To-Start relationship such that the reinforcement activity starts when the form work preparation activity also starts. As another example, if a large area is to be excavated, it is possible that Foundation construction can start in an already excavated part of the site, prior to the completion of the full amount of excavation. These two activities can therefore be represented through a Start To Start relationship with a positive lag, where the start of the Foundation activity is linked to the Start of the Excavation activity, but after a lag period of a certain number of days to allow for enough excavation in an area of the site to have taken place.

When the finish of two activities are to be aligned, then Finish to Finish relationships can be used. For instance, it may be more useful to align the finish of the reinforcement and form work preparation activities so that concrete can be poured when both activities are completed. This can be represented by a Finish to Finish relationship with no lag.

Finally, while Start to Finish relationships are theoretically possible, they can almost always be represented as Finish to Start relationships by changing the order of the activities and therefore do not need to be dealt with separately.

Videos for Lecture - 2

Relationships

Representation for CPM Calculation

Critical Path

Float and Float Calculations

Float Calculations Continuation

Session - 2 Basic Construction Planning Techniques - Lecture 2

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Lecture 3

Project Managers must often answer a number of questions related to the project, such as the following:

  • When will the project be completed?
  • Which activities should I pay the most attention to?
  • Where do I have buffers on my project?

These and related questions can be answered by analyzing the project network or the precedence diagram that has been developed using the Critical Path Analysis Techniques.

3a. Forward Pass and Backward Pass

The network in Figure 3 below will be used to illustrate the critical path. Early Starts will be represented in the top left box, Early Finish on the top right, Late Start on the Bottom Left, Late Finish on the bottom right and Floats in the bottom center box. In the center the name of the activity and its duration will be represented.

The critical path method starts by conducting a Forward pass analysis on the network to obtain the Early Start and Early Finish times of each activity. The basic steps to be followed are:

  1. Start with the first activity on the network
  2. Traverse the network through the relationship arrows until you reach the end
  3. For each activity, the early start is the maximum of the early finishes of the previous activities
  4. The early finish is the early start activity duration
  5. Finish one level and then go to the next

Fig 4 below shows the results of the Forward Pass on this network.

The next step is to conduct a Backward pass analysis on the network to obtain the Late Start and Late Finish times of each activity. The basic steps to be followed are:

  1. Start with the last activity on the network
  2. Traverse the network backwards through the relationship arrows until you reach the start
  3. For each activity, the late finish is the minimum of the late starts of the previous activities
  4. The late start is the late finish - activity duration
  5. Finish one level and then go to the previous one

3b. Analysis of the Results

The difference between the Early Start and the Late Start or the Early Finish and the Late Finish gives the total float or buffer available. It is clear that for activities A and B, this value is zero (no buffer is available) whereas for the other activities some buffer is available. A and B are therefore considered to be critical activities.

Also, for such networks characterized by Finish-Start relationships, there will be at least one path where all the activities on that path are critical. This is called the Critical Path and the duration of this path will determine the duration of the project. There can be more than one critical path on a project. In this case, the path Start-A-B-Finish is the critical path. This means that any delay to A and/or B WILL result in a delay to the project, while delays to other activities may not result in delays to the project.

By using this algorithm, a planner can take a project, construct a precedence network diagram and analyze the diagram to find out the duration of the project (9 days in this case), the critical path and critical activities (where more attention needs to be paid) as well as the buffers on the project.

Session - 3 Construction Planning Techniques - Lecture 3

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Lecture - 4

Floats

It is important to understand the various floats that can be present on a project. There are 6 types of floats on a project:

  • Total Float
  • Free Float
  • Interfering Float
  • Independent Float
  • Start Float
  • End Float/Finish Float

4a. Total Float

Total Float is the amount of time an activity can delay but not delay the termination of the project. It can be calculated by subtracting the Early Start of a Project from the Late Start of the project or the Early Finish from the Late Finish. This is a property of the entire chain of activities from Start to Finish, passing through the activity where the completion is performed. This is therefore a shared quantity.

4b. Free Float

Free Float is the amount of time an activity can delay but not delay the termination of the project nor delay the start of any following activity. The free float of an activity is calculated by subtracting the Early Finish of the activity from the Early Start of the next activity. If more than one subsequent activity exists, then the activity with the earliest Early Start is taken for calculation. This is also a shared quantity and is shared by the sub-chain of activities starting from the beginning of the project and culminating at the activity under question. Free float is therefore a subset of Total Float.

4c. Interfering Float

Interfering float is the time span in which the completion of an activity may occur and not delay the termination of the project, but within which completion will delay the start of some following activity. In other words, this is also a shared float, and is shared by activities starting from the current activity up until the finish of the project, along the same chain. It is calculated by subtracting the Free Float from the Total Float. Free Float and Interfering floats are therefore subsets of Total Float.

4d. Independent Float

Independent Float is the time an activity might delay and not delay the termination of the project, not delay the start of any following activity and not be delayed by any preceding activity. It may be calculated through the following formula:

Where, 'i' refers to activities prior to the activity in question, 'j' is the current activity and 'k' refers to subsequent activities.

Unlike the floats discussed earlier, Independent Float is NOT a shared quantity. Rather, it is a sole property of the activity that is subjected to these calculations. This implies that when an activity delays by an amount less than or equal to its independent float, the floats of other activities (total, free or interfering) are not affected. On the other hand, if activities without an independent float are subject to delays, then the floats of other activities may be affected.

4e. Start and Finish Floats

Start Float is calculated by subtracting the Early Start of an Activity from the Early Finish of the activity. Finish or End Float is calculated by subtracting the Late Start of an Activity from the Late Finish of the activity. If a network consists only of Finish-Start relationships, then these floats will be the same as Total Float and there is no need to calculate them independently. On the other hand, if this is not the case, and other relationships such as Finish-Finish or Start-Start relationships exist within a network, then Start Floats may differ from Finish floats. Therefore, in such situations, Total Float cannot be calculated and Start and Finish Floats must be calculated separately.

4f. Float Analysis

Once a project network has been created and the critical path has been calculated, a comprehensive float analysis can be performed wherein each of the float types for each activity can be calculated. Figure 6 and Table 1 show a network and the associated float calculations.

It is clear from this table that A,B, E and G have no floats. D and F have some independent floats and therefore can be subject to delays without affecting other activities. On the other hand if C is delayed, the Float of F will be affected. On performing such analysis, the planner can use floats in a strategic manner. For instance, if there is a resource shortage on site, resources can be shifted from non-critical to critical activities at the cost of some float. Float analysis can also be used to understand how to apportion the blame for delays on a project. Activities cannot delay more than floats allocated to them. Finally, when preparing a schedule it is useful to note that some amount of float is desirable. Too little float indicates that the projects is subject to a high risk of delays while too much float potentially indicates an inefficient plan.

Session -4 Basic Construction Planning Techniques - Lecture 4

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Session - 5 Basic Construction Planning Techniques - Lecture 5

For questions 1-2 (a) draw the network diagram, and (b) find the Critical Path, ES, EF, LS, LF, Total Float (TF), Free Float, Independent Float and Interfering Float

Session - 6 Basic Construction Planning Techniques - Lecture 6

Videos for Lecture - 3

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