Time‐Cost Trade‐Off Among Related Activities

This paper describes the typical pragmatic approach that construction planners taken in performing time‐cost trade‐off (TCT). In general, projects have major dominant characteristics, operations, or resources whose planning affects or dominates planning of other operations and resources. Planning focuses first on the dominant characteristics and is then fine‐tuned in its details. Planners typically cycle between plan generation and cost estimating at ever finer levels of detail until they settle on a plan that has an acceptable cost and duration. Computerized TCT methods do not follow this cycle. Instead, they separate the plan into activities, each of which is assumed to have a single time‐cost curve in which all points are compatible and independent of all points in other activities' curves and that contains all direct cost differences among its methods. In general, these assumptions are not true for construction. Construction activities are related because they share methods and resources. Crashing activities usually require changes from normal, least‐cost methods and resources. Changes in one activity are not independent of changes in related activities. Therefore, normal computerized TCT techniques are conceptually wrong for construction and they are not useful in practice.