TR021: A Knowledge-Based Model of Time-Cost Tradeoff Analysis for Construction Schedules

"Buying time" is a common phrase in the English language. Historical approaches to applying this notion to formal critical path method (CPM) schedule analysis have suffered from two major problems. First, formal time-cost tradeoff analysis requires extensive numeric data which is difficult to acquire, is of suspect validity and requires complicated numeric manipulation to be of use. Second, the perceived value of time varies among the various project participants. One participant's analysis will disagree with another's because of their different perceptions of the value of shortening activity durations.

This thesis presents the conceptual architecture for JANUS, a knowledge-based time-cost tradeoff simulator that addresses these shortcomings of traditional approaches. The first problem is dealt with by utilizing a set of generic expediting tactics. These are standard means of shortening activity duration such as overtime, multiple shifts, additional crews or alternative equipment or methods. The second problem is dealt with by explicitly modeling the project participants, their goals and their relationships with one another. These extensions to traditional CPM analysis allow JANUS to evaluate the activity costs resulting from specific expediting methods more realistically and to simulate the effect of different contracting strategies on a project's schedule.

JANUS is an attempt to capture both the motivational and the technical components of buying time on a project JANUS uses activity sequence, time and cost relationships along with a set of generic expediting tactics for its time-cost tradeoff analysis. It also represents project participants and derives their goals from the owner's view of the project and the contractual provisions under which each project participant is employed. Each activity has a responsible participant who determines the value of time and therefore the maximum amount that may be spent expediting that activity. It can thus provide a more meaningful forecast of the likely results of alternative strategies - contractual as well as technical - to shorten project duration.