Ladder

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When drawing a network diagram it is common to come across a situation where activities are performed in parallel.

For example if a new cable were being laid in a trench, the three activities: Dig Trench, Lay Cable, Backfill, may not run sequentially. This is called a ladder.

The precedence diagram below shows that 1 day after the digging has started the cable laying can commence. For safety reasons, the cable laying cannot finish until 1 day after the digging. Similarly the backfilling of the trench can start 1 day after the cable laying and finish 1 day later. The lead times and lag times are shown on the start to start and finish to finish links respectively.

 

three activity ladder

 

When a critical path analysis is performed, this can give some results that need careful consideration.

 

Three activity ladder - calculated

 

The analysed network reveals that while the first activity is on the critical path, only the finishes of the second and third activities are critical, i.e. the activities have start float but no finish float.

When displayed in Gantt chart form, this means that the second and third activities need to be shown according to their latest dates with the float before the activity bar.

 

Three activity ladder - Gantt Chart

 

The use of start-to-start and finish-to-finish dependencies usually results in differences between an activity’s start float and finish float.

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11th August 2014Link to Italian translation added

Ladder

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