Delay and Disruption - Types of Programme of Works

© Daniel Atkinson 2001 22 November 2001

SUMMARY

The term "programme" is not a legal term, but takes its meaning from the contract.  There are several types of programme, each with its uses and limitations.

1. The Meaning of Programme

The term “programme” is not a legal term. It takes its meaning from the contract.

It is suggested that the reference to "programme" in a construction contract will require at least the basic information of a list of verbal descriptions of activities or operations together with a list of dates for commencement and completion of each activity.  The extent of the list of activities will depend upon the contract description of the programme.

Standard Forms of contract do not adopt a common approach to the description of programmes, the role of the programme in the administration of the contract and the obligations and rights as to programme.  It is suggested that in the absence of a detailed description of the form and content of the programme, it is necessary to identify the intended role of the programme in the contract, in order to decide the type of programme required by the contract.  This aspect is dealt with in the Article The Role of the Programme.  The obligations of the parties as to programme is dealt with in the Article Obligations as to Programme.

The different types of programme used in projects is described below.

2. Types of Programme

The construction industry uses a number of different types of programme to manage construction.  Different terms are commonly used for the same types of programme, much of which is historic and of little practical relevance.  There are two types of programme in most common use - the Bar Chart and the Linked Bar Chart.

The Bar Chart

The most frequently used type of programme is a Bar Chart, which is a list of those activities required for the project.  The planned start and planned finish of each activity are shown in a time grid and are connected as a bar.  The bar therefore represents the duration of the activity.  The assumption usually made is that the bar represents a continuing uninterrupted activity, but this may not be intended.

An example of a simple Bar Chart is shown to the left.  The "Equipment Project" shown is the design and manufacture of an item of equipment, the construction of a piled foundation and installation of the equipment at the site.

In order to simplify the presentation and increase the utility of the Bar Chart each listed activity represents a number of discrete tasks, which together make up that activity.  For example, in the "Equipment Project", the activity "Procure Supplier" will include the tasks of preparation of tender documents, the review of tenders received, possible negotiations and the order for the equipment.  This aspect, common to all programmes, contributes to the inherent ambiguity of the terms "start" and "finish" of an activity.  Unless the activity description clearly states discrete verifiable tasks to mark the start and finish, the programme will lack precision.

For example, the finish of the activity "Procure Supplier" will be defined by the order for the equipment, which in many cases will be identified by the issue of the written order, or the written acceptance of the tender, or the instruction to commence work.  The definition of the "finish" of the activity could be said to be inherent in the description and in practice verifiable by a document.

In contrast, the activity "Erect Equipment" is less well defined.  It will include placing the equipment in its final position and probably will include connection of holding down bolts and any grouting necessary, and possibly connecting to the required power supplies, although this is not clear.  The activity description does not make clear whether the activity includes testing of the equipment.  The activity description "Erect Equipment" does not therefore define the task which will mark the finish for the activity and reference will need to be made to other contract documentation.

The Bar Chart is also commonly referred to as a Gantt Chart.  It is said that there is a difference between a Bar Chart and a Gantt Chart, but this is a matter of semantics and of little practical significance.

The Bar Chart is easily prepared and can be used to show estimated timing and duration of activities, or to record actual progress.  It does not require computers or special software and can be drawn easily by hand.  Experience shows that it is the most natural representation of activities used by engineers and project managers.  The types of activities are not limited in any way, since the Bar Chart is simply a diagramatic representation of the time characteristics of an activity.  So, for example, the activity could be the operation of design or construction, but equally could be the timing and duration of the use of different resources.  There is no logical restriction to the number of activities that are shown against an activity.  "Multiple line-bars" are useful to show the duration of the same type of activity in different areas of the project for instance, usually differentiated by colour or shading.

The Bar Chart does not model the inter-relationship between activities, and does not model the consequences on expected completion, if the actual timing or duration of an activity is not met.  So, for example, if an activity is started later than shown on the Bar Chart, the Bar Chart does not allow the effect on completion to be analysed, without additional information.  The Bar Chart simply shows that an activity started later than planned.  Similarly if an activity requires a longer duration than shown on the Bar Chart, this only shows that an activity took longer than estimated.  The Bar Chart therefore simply provides a model of the time characteristics of the activity, and does not model the relationship of the activity with the time characteristics of the project.

The absence of logic links between activities means that the use of the Bar Charts is limited to monitoring progress rather than forward planning of the project.  It is used in the initial stages of delay analysis to compare planned and actual progress so as to identify problem activities.  Care is required, since the implicit assumption that the planned durations were an accurate and still valid estimate, may not be correct.

Network Programmes are a model not only of the activities and their durations, but of their interdependence.  They represent the time characteristics of the Project, not just each individual activity.  The additional project dimension means that it is necessary to consider the introduction of additional activities which do not represent tasks, but contingencies in time for risk events or "time risk allowances".

The most used Network Programme is the Critical Path Method which models the construction logic links between the activities.  The construction logic represents those factors which define the construction sequence of the project and include:

• the method of working, showing how the project is to be carried out and the sequence of activities;
• the construction constraints, which may be access dates for parts of the site or release dates for information or delivery dates for work by others;
• the resource restraints, which recognise the limited availability of plant, equipment, labour and supply of materials.  The resource restraint is usually modeled by the sequential linking of activities which use the same resource.  Experience shows that the resource restraint is often not appreciated when delay analysis is carried out. 

There are two types of Critical Path Programmes; Activity-on-Arrow Programmes and Activity-on-Node Programmes.

Activity-on-Arrow

The Activity-on-Arrow Programme produces an Arrow Network in which each node represents either the beginning or end of a discrete activity and the arrow linking the nodes is the activity.  The nodes are numbered and the activity is identified by the numbers of the nodes at the start and the finish.  In the example to the right, the activity "Design & Fabrication" is identified as Activity 21-22.  Node 21 represents the beginning of Activity 21-22 and node 22 the end of Activity 21-22.  Node 22 is also the start of Activity 22-23.  This demonstrates the logic inherent in the Activity-on-Arrow which is finish-to-start.   The arrow activity is not drawn to a time-scale but the duration is annotated as shown in the example.  The Activity-on-Arrow Network is useful in representing the flow of work, but its use has declined in construction.

Activity-on-Node

In the Arrow-on-Node Programme each node is an activity with a duration and the arrows represent the logic link between the activities.  The Programme uses finish to start relationships or links which are the same as used in Activity-on-Arrow Programme.  In the example to the right, Activity 30 is "Design Fabrication" and is linked to Activity 29 and 31.  Activity 30 can start once Activity 29 is completed. The Precedence Network Method is now the most common form of Activity-on-Node Programme and uses the possibility of defining the links between activities by relationships other than finish-to-start.  This method permits not only start-finish links, but start-start and finish-finish, as well as allowing a time dimension to be added to the link in the form of a lag or lead. The choice of logic link depends on which link accurately models the particular restraint.

Logic Links

Logic links other than finish-to-start are used because the assumption in Activity-on-Node programmes that activities are indivisible is not usually correct.  In order to reduce the complexity of programmes activities are defined which are made up of a considerable number of separate tasks.  In many situations it is not necessary for all the tasks to be completed before the follow-on activity commences.

This is demonstrated in the extract from the Equipment Project above.  The activity "Design Equipment" will logically only start when the activity of "Procure Supplier" has been completed.  The finish-to-start activity therefore accurately represents the relationship between these two activities.  If the fabrication of the equipment is not to start until all the design is completed, then the finish-to-start link will correctly describe the relationship between activity "Design Equipment" and "Fabrication".  It is more likely that a phased design/construct approach will be adopted, if the pace of design is faster than that of fabrication.  Once part of the design is completed, this will allow fabrication to commence. This is shown in the expanded programme to the right.  As successive parts of the design are completed then other parts of the fabrication are released as shown.  The representation of this logic requires the activities to be broken down into the constituent tasks if finish-to-start links are used.

If start-to-start links and finish-to-finish links are used then the simplicity of the presentation can be maintained.  This is shown to the right. The activities of "Design Equipment" and "Fabrication" are shown as continuous single activities, but with start-to-start and finish-to-finish logic links and each with a lag.  The start-to-start link has a lag which is calculated as the time required to complete the first part of the design so as to allow the start of fabrication.  The finish-to-finish link has a lag calculated as the time required to complete fabrication following the finish of the last part of the design.

The facility to define the relationship of activities both in terms of the type of logic as well as with a time dimension, makes the Precedence Network Method a most powerful and flexible method of programming.  The assumptions made must be carefully examined when carrying out any delay analysis or management through programming analysis.  If for instance the initial design of the equipment in the above example is delayed, then the lag in the above start-to-start link will need to be adjusted to take account of the delay.  In any analysis, the time dimension of links which are not based on real time factors need to be examined carefully to establish that they still accurately model the relationship between activities.

Linked Bar Chart

The introduction of logic links to create the Linked Bar Chart allows the time dimension of the project to be defined by the relationship of activities one to the other.  The "Equipment Project" shown above as a Bar Chart is shown to the left, as a Linked Bar Chart.  The introduction of logic links and the presentation in a cascade format, means that the operations involved are easily understood and particularly the restraints on each activity.  This structure and presentation has meant that the Chart has become the main management tool for deciding for example which activities have priority to achieve the project completion date and for example how to allocate resources for optimum efficiency.     

Projects are not static.  When progress does not follow the programme, the Linked Bar Chart allows the analysis of the consequences of events that have occurred.  More dynamically the Chart allows prediction of the effect of events yet to occur.  This is possible with modern software which allows precedence networks to be constructed by linking directly on screen the bars representing activities.  The Linked Bar Chart produced by such software is a full precedence network creating a mathematical model of the project.  As with any model, the assumptions made of the duration of activities and the links between activities, need to be checked to ensure they remain valid for the analysis. 

Critical Path Network

In order to make management decisions and to establish the priority of actions, it is necessary to interrogate the precedence network.  

One important attribute which is relevant to the obligation to complete by a specified date, is the critical path.  Those activities which can be least delayed without affecting the Date for Completion are said to be on the critical path.  The line through those activities is the critical path to completion and is usually generated by modern software

This is shown to the left for the "Equipment Project".  The site construction involving the installation of piles, construction of formation and Floor Slab and the erection of the Equipment at site are shown as being critical.  If the logic is actually followed, and the start or finish dates for any of these activities are different to programmed, then the completion date will be affected (unless the changes cancel out).  If the planned date is to be maintained then priority will need to be given to the critical activities.

The activities which are not on the critical path will have "float".  This is generated by the software and sometimes shown as the difference between the earliest and latest start dates. There are various types of float, all of which are an expression of the relationship of an activity to other activities and milestones. The term float used here is the period by which an activity on a programme may be delayed before the programme shows an effect on the date for completion. The activities with the least float are on the critical path to completion.

The emphasis on the programme is important because float is a function of the model represented by the programme, but may not accurately represent the consequences of starting an activity later than it could have been started. The construction logic on many programmes is kept simple in order to produce a workable programme so that management decisions can be taken. A sophisticated approach is to use a "Float Trend Analysis" in which the tendency of activities over time towards becoming critical is monitored and used as a means to decide priority of action.

Other types of programme which are used to manage projects include the Linear Scheduling Method or Line-of Balance Programme. This type of programme allows the different rates of productivity of parts of an activity to be modeled, depending on its location on site or the period of year when the activity is carried out.  It also allows the inter-relation between different trades in an area to be modelled.

An example is shown here based on the "Equipment Project" above, but in this instance with accelerated construction at site.  The construction on site requires piling, formation and floor slab in four areas.  The work will progress from Area 1 to Area 4 using the same resources for each part of the work.  The progress of the piling will be less in areas 3 and 4 than in areas 1 and 2 because of the ground conditions.  This difference in productivity is not easily shown on a Bar Chart unless the activities in the areas are separately identified.  The progress is easily shown on a Line of Balance.  The pace of work in Formation is much greater than either piling or floor slab, so the timing for this work is in fact dictated by the Floor Slab.  This is shown on the Line of Balance, which is used to demonstrate the effect of events on productivity and the movement of resources in the site.