Gantt Charts
What is a Gantt (cascade) chart?
- A Gantt chart - also known as a cascade chart - is a graphical display of the activities making up a project
- a Gantt chart shows
- the critical activities
- the total float for non-critical activities
- the minimum project duration
- a Gantt chart shows
- Gantt charts can be used in resource levelling and scheduling problems when the number of workers for each activity is known
- in the first instance, a Gantt chart assumes one worker per activity
How do I draw a Gantt (cascade) chart?
- (Usually) a horizontal axis is drawn for time and this appears at the top of the Gantt chart
- Activities are then drawn as a series of bars (rectangles) underneath
- each activity is assumed to commence at its earliest event time
- i.e. at the early event time of its start node
- each activity is assumed to occur in a single block of time
- i.e. no breaks!
- e.g. an activity of duration 5 and early event time 4 would be drawn as a bar starting at 4 and ending at 9
- Critical activities are all drawn in the same horizontal line
- these have a total float of zero so are drawn back-to-back
- Non-critical activities
- are drawn one per line
- have their total float indicated by a bar drawn with a dotted line
- e.g. an activity of duration 4, early event time 7 and total float 3 would be drawn as a (solid) bar starting at 4 and ending at 11, with a dotted bar starting at 11 and ending at 14
- Bars are labelled with their activity name and duration
- floats are not labelled
- each activity is assumed to commence at its earliest event time
- For the activity network below
- the critical activities are highlighted and are A, C, G and I
- (the critical path is A-C-G-I)
- the minimum project duration is 23 (days)
- the critical activities are highlighted and are A, C, G and I
- The Gantt chart for the project would be constructed with
- a horizontal axis running from 0 to (at least) 23
- (0 to 25 keeps things nice!)
- critical activities A, C, G and I drawn back-to-back on a single line underneath
- activities B, D, E, F, H and J are each drawn on a separate line
- e.g. activity D will be drawn as a solid bar from 4 to 7 with a dotted bar from 7 to 12
- a horizontal axis running from 0 to (at least) 23
Exam Tip
- An exam question is likely to provide a grid and the axes for you to draw a Gantt chart on
- For a non-critical activity the dotted float bar can be seen as room that the activity bar can slide back and forth along to vary it's start and end time