Reliable planning improves project coordination and reduces disruptions and delays. It has been found that productivity increases of about 30% is achievable with improvements of Percent Plan Completed (PPC). This research proposes a constraint-based planning and scheduling system called Integrated Production Scheduler (IPS) to enhance plan reliability and improve project performance through modelling and resolving flow constraints in the construction work plan.

Constraint modelling and analysis provides an unambiguous means to identify the causes of uncertainties in workflow which has been neglected in conventional project management . Besides the process constraints representing the precedence relationships of construction tasks, the basic CPM (Critical Path Method) scheduling model is augmented with two additional types of constraints, i.e., Resource and Information (RI) constraints representing the availabilities of resource and information prerequisites, respectively. Incorporating these constraints will facilitate the management of underlying information and supply chain flow. For each RI constraint, the estimated available time is identified and incorporated in the augmented CPM equations to determine realistic task start and finish times so that the impact of RI prerequisite delays can be explicitly determined in project schedules. In this way, t he causes of task delays can be ascertained, namely RI availability from information and supply chain flow, or precedence, or both. Consequently, a better strategy for constraint handling can be adopted.

A methodology of key constraint analysis has been proposed to prioritise RI constraints and detect the most critical RI constraints (or key constraints). It is based on the theory of constraint principles of improvement, according to which the key constraints represent system bottleneck constraints that should be identified and resolved iteratively to improve system performance. If constraints are expedited without consideration of these key constraints, the improvement to project performance will only be localised without aiding overall project delivery.

The reliability of a work plan is further enhanced via managing a series of task buffers in the IPS. In this regard, four sequential task buffers (screening, pulling, shielding, and working) are deployed for strategically managing the RI constraints and making reliable work plan assignments. The deteriorating effects of variability in the task buffers have been demonstrated with a simulation model which benchmarks project performance and plan reliability. The results show that higher variability increase project duration, reduce throughput, reduce PPC, and increase Percent Plan Impact.

A prototype for the IPS (Figure 1) based on a collaborative scheduling framework has been developed with the employment of Internet technology such as Java, XML, and a 3-tier client/server computing model. It facilitates information and constraint management, which are vital for the task buffer management and ultimately, achieve reliable work plans and efficient production control at the project site. The IPS prototype comprises many functional objects playing important roles to automate certain simple and repetitive tasks in the system and greatly increase the speed of communication. With the involvement of all project participants in the constraint-based collaborative planning framework, it is feasible to attain new standards of project management leading to the lean construction processes featuring improved quality, timeliness, and transparency.

Figure 1 : The prototype of the Integrated Production Scheduler.

The current system is now being developed to achieve an industrial pilot capable of multi-project constraint management. The development is part of the Collaborative Engineering Research Program being funded by Infocomm Development Authority of Singapore with collaboration from industry and technology partners.

This work was performed by Assoc Prof David KH Chua in collaboration with Mr Shen Li Jun .