EVALUATING SURROGATE MEASURES OF CONSTRUCTION PROJECT SCHEDULE ROBUSTNESS

Year 2011, Volume: 3 Issue: 2, 263 - 272, 01.12.2011

Piotr Jaskowski Slawomir Biruk Agata Czarnigowska

Abstract

Projects are subject to various uncertainties that have negative effect on activity
durations. This is most apparent in the case of construction projects. The actual
completion time of construction projects is reported to be rarely in accordance
with initial plans. A schedule considered optimal with respect to project duration
may become infeasible due to disruptions caused by uncontrollable factors.
Deficiencies of the existing methods of project scheduling gave rise to the
worldwide search for predictive (or proactive) scheduling that is expected to
provide robust schedules (immune to disturbances), thus counteracting instability
and “nervousness” of a project plan.
The baseline schedule (execution plan prepared prior to the project execution) is
to be reliable in terms of not only the total project makespan, but also timing of
particular tasks and activities – related with resource management. The main
reason for the planner’s insisting on stable baseline schedules is the necessity of
“advance booking” of key staff or equipment (to guarantee their availability) and
keeping fixed delivery dates (as required by suppliers or subcontractors).
A stable schedule with acceptable makespan performance should minimize the
instability cost function, defined as the weighted sum of the expected absolute
deviations between the predicted start times and the value that the random
variable of start time will assume during schedule execution. Computational
burden of optimizing this direct measure of schedule robustness in a real-life
project environment is quite high. Developing surrogate quantitative measures to
provide a good estimate of schedule robustness is essential for building efficient
robust scheduling algorithms. For this reasons, the aim of this paper is to
evaluate the quality of free-slack-based measures for a benchmark project. The
new approach to increasing schedule robustness, based on buffer sizing and
allocation, is proposed and tested against the existing free-slack times relocation
approaches.

Keywords

Project management, Robust scheduling, Stable baseline schedule

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