Critical path analysis

Identifying the critical path

As the project progresses, the critical path or paths are likely to change. For example, in a project to refurbish both the ground and first floors of an existing building, both floors are planned to commence at the same time, with the ground floor to be carried out in a period of 20 weeks and the first floor in 18 weeks. The project critical path would be shown through the ground floor activities, simply because the ground floor has the longer duration, and there would be 2 week's float for the first floor works.

The situation might change if the progress report at week 4 indicates that progress to the ground floor has gone according to the programme but the first floor has suffered delays and only 1 week's progress has been achieved, then the critical path will have changed. In this scenario there is still an estimated 17 weeks work to carry out on the first floor but only 16 weeks work left on the ground floor. Therefore, by week 4 of the project, the first floor has become critical and the ground floor now has 1 week of float.

The critical path might also change if:

• the contractor finds a better way of doing things and re-sequences the works;
• progress is better than anticipated in the programme;
• work is omitted, added or varied.

On a fast-track or complex project the critical path can change frequently and, at certain times, it may be in an almost constant state of change, which makes it exceptionally difficult if not impossible to define.

Construction project logic

Critical path analysis operates most accurately where the activity sequence is largely linear and where the options in respect of the logical relationships is limited. Within the construction process the logic can be best described as being either 'prescriptive' or 'preferential'.

Prescriptive logic applies to activity sequencing that must be adhered to; the placing of structural concrete must follow the erection of formwork and the fixing of reinforcement; the plastering operation must follow wall construction and so on.

Preferential logic gives a contractor some scope to choose and dictate a desired sequence in respect of certain elements of a project.

The extent of the prescriptive and preferential logic will depend on the nature of the works. A contract to construct a multi-storey concrete framed building will, by its very nature, dictate a prescriptive sequence and logic. Some degree of choice may be available to the contractor in respect of aspects such as the horizontal direction of progress, however the contractor obviously has no choice in respect of vertical sequencing other than to commence the structure at the lowest floor level and work up to the roof level.

The construction of the frame and envelope may mean that activity sequencing is largely linear and sequential and the logic prescriptive, once the building is watertight, and the internal walls and first fix trades are being executed, the logic may well become a mix of both prescriptive and preferential logic. When the second fix and architectural finishing trades are being undertaken the logic may become more preferential than prescriptive. For example, once the traditional wet trades of plastering and screeding are complete the works are often found to be 'opened up' for the commencement of many of the following finishing type trades. Such activities as suspended ceilings, doors, general joinery, second fix services, floor and wall tiling can all commence and although there are some prescriptive interfaces between these works in reality they, together with the other finishing elements such as decorations, soft floor coverings and the like, may all progress with an element of concurrency through to final snagging and handover.

Therefore, the suitability of critical path analysis for construction work may be dependent on the type of work; it is likely to be more suited to a project or series of activities where the sequencing and logic is largely prescriptive.