Calculation of backward pass | Late start/Late finish time

Sunday, September 23, 2012 9:34
Posted in category Civil Engineering Practice
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Backward pass

Backward pass is a schedule calculation that determines the late start and late finish times of the activities under the condition that the project’s minimum duration be maintained.

A backward computational sequence through the logic network will produce

  • the last point in time that each network activity can start and finish, and still maintain the minimum overall project duration.

Calculation of backward pass

The backward pass calculation starts with the last activity in the network. This last activity is assigned a late finish time equal to its early finish time as calculated by the forward pass.

Late finish/Late start

 The late finish time of an activity is the last point in time that an activity may be finish.

To calculate late finish time and late start time of an activity, follow the precedence arrows backward through the logic diagram.

The late finish time of an activity is the latest point in time, that an activity may be finish without delaying the project.

Calculation of Late start/Late finish

To compute the late start time of an activity we have to subtract the activity’s duration from its late finish time.

The late start time is the latest time the activity may start without delaying the entire project.

Late start (LS) = Late Finish ( LF) – Duration

The preceding activity’s late finish time is the succeeding activity’s late start time. To determine the activity’s late finish time when more than one arrow tail leads away from its node, choose the smallest late start time of all the activities at the arrow’s heads. Logically an activity must be finish before all following activities may begin.

Using this backward systematic process, work through the entire logic diagram against the arrows. Compute all the late finish and late start times. This movement back through the logic diagram is known as backward pass. The late start time of the first activity must be zero.

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