Critical path analysis

A critical activity is where the earliest and latest starts and earliest and latest finishes are the same. In figure 12, substructure network illustration, activities A,B,C.D and E are all critical and together they form the critical path. Taking activity B based on the duration of 5 days this activity must commence on day 10 to allow completion by day 15. If completion is delayed beyond day 15 then the start of the subsequent activity will be unable to commence on day 15. Likewise activities D and E and the completion of the works will also be equally delayed.

Figure 12 Substructre network

'Float' is the difference between the total time available for a particular activity and the activity duration - the 'spare' time available in which an activity can be delayed or prolonged without it causing any impact on the following activities and/or the completion date.

The substructure package network, illustrated by figure 12 above, indicates that Activities F, G, H and I have float. There is a difference between the earliest and latest start and finish times of these activities indicating that a limited delay will not affect the overall completion of the substructure works with the ability to complete activity E by day 30 still being maintained.

For example with activity I ('New service ducts'), the earliest that this activity can finish is day 18 and that the latest it can finish is day 25. Day 25 is the latest day that this activity can finish without delaying the completion of the substructure works on day 30. This being the case this activity can start later than planned or it can be prolonged, but, provided it does not finish beyond day 25, it will not cause the project completion to be delayed.

There are two types of float: 'total float' refers to the time by which an activity can be delayed or prolonged without affecting the project completion date; 'free float' refers to the time by which an activity can be delayed or prolonged without affecting the timing of any of the succeeding activities.

When an activity containing an element of 'total float' is progressively delayed, the amount of float will reduce. When all the 'float' has been utilised and the activity has zero 'total float' it will have become an activity that is critical to the original planned completion date. If delay to the activity continues, the float will become negative and as such is referred to as 'negative float'. Negative float indicates delay: 2 days negative float reflects a 2-day delay to the original planned completion.

Critical activities have zero float or if the project is in delay, negative float.

Resource driven logic

In some instances the activity relationships and timings may be driven by resource availability rather than by any necessary activity sequencing. For example, on a project to strip out and refurbish a 10-storey office block based on pure logical restraints it would be possible to commence the strip out on all floors on day 1. It would obviously require a very large labour resource of 10 separate gangs and associated plant and equipment to achieve this and in practice one would expect the work to commence at the top of the building and work down. If one single gang was employed starting at the tenth floor the start of the ninth floor strip out would be dependent on when the resources from the tenth floor strip out became available. In other words the ninth floor strip out would have a resource driven 'finish-to-start' relationship with the preceding tenth floor works.

Therefore, in this situation the logic is resource driven; the planned sequence and duration of strip out works is governed not by physical restraints but by resource utilisation.

Some of the more sophisticated software can carry out an operation called 'resource levelling' whereby the activity timings are set by the applied logic as well as allocated resource.