Weist’s Trigger Level Setting Heuristic Algorithm

Designed originally to smoothen manpower requirements in naval shipyards, since shop crew sizes generally must be sufficient to meet maximum manpower requirements, a schedule which reduces peak loads by increasing usage during slack periods would allow smaller shop sizes and hence reduce labour expense.

This model [1] tries to do this by scheduling all jobs at their earliest start times and then shifting some of them that occur at peak periods to later slack periods.

First, an early start schedule, along with the total slack values for all the jobs, is calculated by regular network procedures. A manpower loading chart is then generated. The program then sets the ‘trigger levels’, or resource limits, one unit below the peak requirement in each of the shops, and it attempts to reschedule the jobs so that peak requirements do not exceed trigger levels.

Jobs are then reloaded, or rescheduled, one at a time, until the trigger level is exceeded in some shop. All jobs active in that shop on the peak day are examined, and those which lack sufficient slack to be shifted beyond the peak period are discarded from consideration. Of those which remain, one is chosen at random and rescheduled to start at some point beyond the peak day.

Such a move may effect the early start dates and total slack of jobs following the one moved. These are recalculated and the loading is continued until all jobs are loaded or until the trigger level is again exceeded and another job is shifted. If the trigger levels of each shop are met, then all of them are reduced one unit again, and the process of loading and shifting repeated.

The trigger-level can be looked upon as a ceiling pressing down on the manpower requirements. Peaks are reduced by pushing them to the right. The ideal schedule would result in a rectangle as shown in the figure, but because of fixed sequencing of jobs, variations in crew sizes on different jobs, and the interactions of trigger levels in various shops, the ideal is improbable. It is possible that reducing a peak on one day by shifting a job to the right may result in a second, and perhaps worse, peak later, in the same shop or a different one.

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