Piotr Jaśkowski

The method for improving stability of construction project schedules through buffer allocation: Statybos vykdymo grafiko stabilumo užtikrinimas paskirstant laiko rezervus

Abstract. The actual completion time of construction projects is rarely in accordance with initial plans. This is due to unforeseen events that affect the project execution. In the paper, the authors propose a method of improving the construction schedule reliability. The method is based on the idea of buffers allocation, whose proper sizing helps to reduce negative effects of random conditions on the progress and efficiency of project activities. The authors adopt a proactive approach to the construction of robust schedules to cope with multiple disruptions during project execution, applicable to two processes starting policies (as soon as possible or no sooner than on a predefined date). The proposed method, based on simulation technique and mathematical programming, was illustrated by an example. The results obtained by means of the proposed method were compared, in terms of schedule stability, to those of the float factor heuristic and starting time criticality heuristic procedures. The method allowed...

Simulation modelling construction project with repetitive tasks using Petri nets theory

Resources selection and allocation at the project planning stage is an important issue for reducing project cost, duration and risk. Existing planning and scheduling methods overlook aspects of limited production capacity of construction companies (contractors) due to the fact that they are simultaneously engaged in realization of some projects. This paper presents a new methodology for project scheduling with repetitive processes using Petri nets based approach. The paper starts with an overview of current developments in the Petri nets theory. We then propose an efficient computational method based on simulation of Petri net model for construction project planning and subcontractor agreement analysis. An example of construction project simulation research is presented to illustrate the method of project planning and resources allocation.

Scheduling construction projects with resources accessibility limited and changeable in time

Abstract This paper aims at solving the problem of minimising the construction project duration in deterministic conditions when the accessibility of renewable resources is limited and changeable in time (workforce, machines and equipment). Particular construction processes (with various levels of complexity) must be conducted in the established technological order and can be executed in different technological and organisational variants (different contractors, technologies, and ways of using resources). To solve this problem the authors are using evolutionary algorithm. For the assessment of solutions generated by evolutionary algorithm, the authors have worked out a heuristic algorithm (of resources allocation and project duration calculation). This methodology seems to produce similar outcomes when juxtaposed with other solutions obtained by research works carried out using comparable methodologies. The paper contains an example of practical application of evolutionary algorithm for construction proje...

Modelling contractor’s bidding decision

Abstract The authors aim to provide a set of tools to facilitate the main stages of the competitive bidding process for construction contractors. These involve 1) deciding whether to bid, 2) calculating the total price, and 3) breaking down the total price into the items of the bill of quantities or the schedule of payments to optimise contractor cash flows. To define factors that affect the decision to bid, the authors rely upon literature on the subject and put forward that multi-criteria methods are applied to calculate a single measure of contract attractiveness (utility value). An attractive contract implies that the contractor is likely to offer a lower price to increase chances of winning the competition. The total bid price is thus to be interpolated between the lowest acceptable and the highest justifiable price based on the contract attractiveness. With the total bid price established, the next step is to split it between the items of the schedule of payments. A linear programming model is proposed for this purpose. The application of the models is illustrated with a numerical example. The model produces an economically justified bid price together with its breakdown, maintaining the logical proportion between unit prices of particular items of the schedule of payment. Contrary to most methods presented in the literature, the method does not focus on the trade-off between probability of winning and the price but is solely devoted to defining the most reasonable price under project-specific circumstances. The approach proposed in the paper promotes a systematic approach to real-life bidding problems. It integrates practices observed in operation of construction enterprises and uses directly available input. It may facilitate establishing the contractor’s in-house procedures and managerial decision support systems for the pricing process.

Minimizing Project Cost by Integrating Subcontractor Selection Decisions with Scheduling

Subcontracting has been a worldwide practice in the construction industry. It enables the construction enterprises to focus on their core competences and, at the same time, it makes complex project possible to be delivered. Since general contractors bear full responsibility for the works carried out by their subcontractors, it is their task and their risk to select a right subcontractor for a particular work. Although subcontractor management has been admitted to significantly affect the construction projects performance, current practices and past research deal with subcontractor management and scheduling separately. The proposed model aims to support subcontracting decisions by integrating subcontractor selection with scheduling to enable the general contractor to select the optimal combination of subcontractors and own crews for all work packages of the project. The model allows for the interactions between the subcontractors and their impacts on the overall project performance in terms of cost and, indirectly, time and quality. The model is intended to be used at the general contractors bid preparation stage. The authors claim that the subcontracting decisions should be taken in a two-stage process. The first stage is a prequalification – provision of a short list of capable and reliable subcontractors; this stage is not the focus of the paper. The resulting pool of available resources is divided into two subsets: subcontractors, and general contractors in-house crews. Once it has been defined, the next stage is to assign them to the work packages that, bound by fixed precedence constraints, form the projects network diagram. Each package is possible to be delivered by the general contractors crew or some of the potential subcontractors, at a specific time and cost. Particular crews and subcontractors can be contracted more than one package, but not at the same time. Other constraints include the predefined project completion date (the project is not allowed to take longer) and maximum total value of subcontracted work. The problem is modelled as a mixed binary linear program that minimizes project cost. It can be solved using universal solvers (e.g. LINGO, AIMMS, CPLEX, MATLAB and Optimization Toolbox, etc.). However, developing a dedicated decision-support tool would facilitate practical applications. To illustrate the idea of the model, the authors present a numerical example to find the optimal set of resources allocated to a project.

Updating Linear Schedules with Lowest Cost: a Linear Programming Model

Many civil engineering projects involve sets of tasks repeated in a predefined sequence in a number of work areas along a particular route. A useful graphical representation of schedules of such projects is time-distance diagrams that clearly show what process is conducted at a particular point of time and in particular location. With repetitive tasks, the quality of project performance is conditioned by the ability of the planner to optimize workflow by synchronizing the works and resources, which usually means that resources are planned to be continuously utilized. However, construction processes are prone to risks, and a fully synchronized schedule may expire if a disturbance (bad weather, machine failure etc.) affects even one task. In such cases, works need to be rescheduled, and another optimal schedule should be built for the changed circumstances. This typically means that, to meet the fixed completion date, durations of operations have to be reduced. A number of measures are possible to achieve such reduction: working overtime, employing more resources or relocating resources from less to more critical tasks, but they all come at a considerable cost and affect the whole project. The paper investigates the problem of selecting the measures that reduce durations of tasks of a linear project so that the cost of these measures is kept to the minimum and proposes an algorithm that could be applied to find optimal solutions as the need to reschedule arises. Considering that civil engineering projects, such as road building, usually involve less process types than construction projects, the complexity of scheduling problems is lower, and precise optimization algorithms can be applied. Therefore, the authors put forward a linear programming model of the problem and illustrate its principle of operation with an example.

Assignment problem and its extensions for construction project scheduling

The assignment problem consists of allocating renewable resources (construction equipment, crews, or contractors) of limited availability to a set of activities. The classical model for this problem minimizes the total time or cost of completing all activities with the assumption that each activity is assigned to one particular resource. This paper systematizes and describes extensions of these assumptions, considering the effects of task sequence: parallel, serial and hybrid (modeled by means of network methods). This study proposes algorithms for the solution of presented models, which can be used in construction project scheduling.