Semi-automatic and specification-compliant cost estimation for tendering of building projects based on IFC data of design model

Abstract

Cost estimation for tendering of building projects (TBP cost estimation, hereafter) is one of the most critical tasks in the architecture, engineering, construction, and facilities management (AEC/FM) industry. However, it is currently both time-consuming and prone to error because of the working complexity and comprehending deviation caused by the traditional two-dimensional representation of design. The advent of Building Information Modeling (BIM) technology enables a potential solution to these problems through the support of open data exchange standards such as the Industry Foundation Classes (IFC) standard. This paper discusses the key issues for semi-automatic and specification-compliant TBP cost estimation based on the IFC data of the design model, taking TBP cost estimation for the architectural and structural engineering of the cast-in-place concrete structure projects as an example. A Chinese national mandatory specification for TBP cost estimation is introduced as a representative specification and is interpreted into a computer-processable format. An IFC-based construction product information model of TBP cost estimation is established. A general process map and the involved key algorithms for semi-automatic and specification-compliant TBP cost estimation based on the IFC data of the design model are formulated. The application of the process map and algorithms in developing a prototype BIM-based software application for TBP cost estimation and the results of applying the prototype in an actual building project in China are presented. This paper lays a sound foundation for developing BIM-based software applications for TBP cost estimation in China, and demonstrates a feasible approach to satisfying the similar requirements in different countries.

Introduction

Cost estimation is one of the most critical tasks concerned by all participants in the architecture, engineering, construction, and facilities management (AEC/FM) industry throughout the lifecycle of a building project, especially in the tendering phase after design is completed. In practice, cost estimation for tendering of building projects (TBP cost estimation, hereafter) generally consists of three major processes, i.e., classifying all construction products that constitute a building project into assemblies or items, taking off the quantities for these assemblies or items, and computing the project cost [1]. Specifications for standardizing these processes, such as UniFormat and MasterFormat, need to be strictly complied with in order to develop an effective and accurate estimate for tendering.

The development of information technology in the AEC/FM industry has resulted in the emergence of numerous software applications for TBP cost estimation. These achievements have greatly improved estimators' working efficiency. However, in practice, when using the traditional two-dimensional representation of design, estimators still have to manually extract useful information from printed drawing sets or CAD drawings, or manually rebuild a specific three-dimensional model for TBP cost estimation [2]. Due to the working complexity and comprehending deviation in these processes, TBP cost estimation is still time-consuming and prone to error [3].

The advent of Building Information Modeling (BIM) technology enables a potential solution to these problems through a unique and continuously updated model throughout the project lifecycle [4]. Theoretically, all the information can be shared directly among different phases with the support of open data exchange standards for BIM, such as the Industry Foundation Classes (IFC) standard [5]. Thus, information contained in the BIM-based design model can be shared directly in TBP cost estimation, which can reduce the heavy human workload and manual errors in traditional work.

Several studies have revealed that cost estimation based on the BIM-based design model is more efficient and accurate than that based on the traditional design alone [3], [6], [7], [8]. Meanwhile, some BIM-based software applications for cost estimation have emerged, such as Innovaya [9] and Vico Estimator [10], in which the BIM-based design model can be imported to conduct effective TBP cost estimation. However, few of these software applications support the IFC standard, which currently is the most comprehensive data exchange standard for BIM, and thus they are rather vendor-dependent. Moreover, the following deficiencies in specification-compliance prevent them from being efficient and accurate enough for TBP cost estimation. First, although construction products can be classified into assemblies or items according to certain conditions [9], it is still a human-intensive work because estimators have to manually set such conditions to comply with a certain specification strictly. Second, their quantity takeoff completely depends on the imported design model [11] without considering the quantity takeoff rules as discussed in the following section, and thus may result in an inaccurate estimate. In a word, specification-compliant TBP cost estimation based on the BIM-based design model, especially the IFC data of the design model, is still far from being mature, especially in reducing the human-intensive work and developing an accurate estimate.

This paper discusses the key issues for semi-automatic and specification-compliant TBP cost estimation based on the BIM-based design model, where the IFC data of the design model is used to widen the applicability of the findings. Considering that the cast-in-place concrete (CIPC, hereafter) structure projects are widely constructed in China, TBP cost estimation for the architectural and structural engineering of this kind of projects is taken as an example. First, a Chinese national mandatory specification for TBP cost estimation is introduced as a representative specification and is interpreted into a computer-processable format. Then, an IFC-based construction product information model of TBP cost estimation is established, and a general process map and the involved key algorithms for semi-automatic and specification-compliant TBP cost estimation based on the IFC data of the design model are formulated. Finally, the application of the process map and algorithms in developing a prototype BIM-based software application for TBP cost estimation and the results of applying the prototype in an actual building project in China are presented.