Determination of float time for individual construction tasks using constraint-based discrete-event simulation

Abstract

In this paper we propose a methodology which combines Forward and Backward Simulation in order to extend discrete-event simulation by the ability to calculate float times. In the case of Backward Simulation, the simulation starts at the virtual completion date of the construction project and runs backwards in time until the start date. To determine float times it is important that the task execution sequence is in the same order for forward and backward simulation. Consequently we present an extension to the simulation concept that controls the execution order of the tasks within the backward simulation. By combining the results of the forward and backward simulation, it is possible to determine float times for each task while taking into account resources. A comprehensive case study illustrates the application of this new approach. One result of this is the determination of detailed float times for each task using discrete-event simulation. INTRODUCTIONThe creation of a detailed schedule for a construction project considering the available resources and precedence relationships between construction tasks is a challenging task for construction managers. The key measure of flexibility within the schedule is the float time (Raz et. al. 1996). It describes the time frame within which the execution of a task can be moved, or the duration leeway of a task without impacting on the total duration or the execution of subsequent tasks. The standard approach to determining float times in today's construction-process planning is the Precedence Diagram Method (PDM), including wellestablished methods such as the Critical Path Method (CPM) or Program Evaluation and Review Technique (PERT). Where material and resource restrictions do not need to be considered, i.e. only precedence relationships of tasks are crucial; the schedule can be analyzed by applying CPM or PERT methods and represented by an activity-on-node diagram. In this case the calculation of float times for tasks is a straightforward procedure. Starting with the first task, the execution time will be added to the start time.The result is the earliest end time of the first task as well as the earliest start time of the successor tasks. After finishing this so-called forward-pass, the earliest possible start time is calculated for each task. Subsequently, a backward-pass is conducted, which works in a similar manner. Starting with the last tasks, i.e. those that have no successors, the latest start and end times of all the tasks are calculated. The difference between the latest and the earliest start times defines the total float time of a task. The tasks without float times constitute the critical path. A delay in any of these tasks will result in an increase of the project's overall time span and therefore in a delay of the project itself. For this reason it is important to determine float times for each task, so that it is possible to identify tasks that are part of the critical path or have only short float times. If the availability of resources is limi...

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“…And enabler constraints are constraints regarding resources (requirements, availability, capacity, etc.) and information (Dori, 2016). These constraints have different degrees of flexibility.…”

“…In contrast, soft constraints (preference constraints) can be violated by certain extents (Wu et al , 2010). Technological dependencies, resource capacities and compatibilities and safety criteria are examples of hard constraints, and flexible execution strategies and the preferred allocated space to an activity for proper productivity are among soft constraints (Dori, 2016). Moreover, some of the constraints (i.e.…”

“…Konig et al (2012) also used this approach for construction planning automation. More recently, Dori (2016) used the CBDES for construction project schedule optimization and float time determination based on project-specific site conditions.…”

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“…Process modeling is a convenient technique to structure and document project behavior within construction projects (Cole 1991). Dori (2016) wrote that construction tasks have different dependencies upon each other that define the exact execution order. The End-Start relationship, also called the precedence relationship, is the most common one in use.…”