Construction Costs Estimate for Civil Works. A Model for the Analysis During the Preliminary Stage of the Project

摘要

In order to estimate the cost of construction it is necessary to identify all the elements having expense and to provide the corresponding economic values accordingly to the level of detail of the project. Given the high number of variables characterizing the engineering project, it is required to have simplified schemes able to facilitate the study and management of the project. In particular, a civil work needs to have a concise representation through a suitable classification system, which allows to identify sets of homogeneous elements such that the complexity of the analysis is reduced. The classification systems commonly treated in literature are indeed based on the assumption that the building process can be broken down into simple elements able to give an efficient representation of the whole project. In the present paper, we first analyze the main classification systems of civil works, highlighting features, advantages and problems. Then, starting from the classification system proposed in Italy by the UNI 8290 regulation, which has been implemented and extended to multiple levels of detail, it is defined a Work Breakdown Structure (WBS) with the aim to be the reference for the description, the economic analysis and the management of the project already in the preliminary design stage and, later, also in its final planning stages and execution. Operationally, the decomposition of the project, aimed at identifying the processes needed to ensure the production of the work site, is the first step of the procedure, which is then followed by the quantification and the subsequent allocation of unit prices resulting from price lists. In these additional steps, we resort to semi-analytical estimation procedures, which allow us to draw up the Metric Computation (MC) and the Estimate Metric Computation (EMC) also in the preliminary design phase. The use of a simplified base model for the decomposition of the project at the stage of preliminary analysis, can improve the accuracy of cost estimates, otherwise based on rough and often significantly approximate evaluations which follow from baseless estimates when compared to the macro-processing items. Increasing the accuracy of the cost estimates is of primarily interest as it can ensure higher margins of investments in the technical and economic feasibility of the project on the territory, which may cover the infrastructures, the urban planning, the implementation of new technologies for the environment and the rational use of the land. In this way, it is possible to reduce the risk associated to the project initiative, also allowing a unique decomposition scheme of the project. Such scheme, adopted since the preliminary study, can be then integrated through the following phases of the final and executive project.