Automated modification of compound elements for accurate BIM-based quantity takeoff

https://doi.org/10.1016/j.autcon.2020.103142Get rights and content

Highlights

  • Improper compound elements modeling causes issues in BIM-based quantity takeoff.

  • An automatic compound element modification (ACEM) method is proposed.

  • The ACEM method automatically modifies compound elements in BIM models.

  • Manual works can be reduced and accurate material quantities can be obtained.

  • The research contributes to the approach for automatic BIM model development.

Abstract

BIM-based quantity takeoff is faster, more accurate, and more reliable than traditional quantity takeoff. However, the quality of BIM models is a major issue. Quantity takeoff is inaccurate for compound elements such as walls and floors modeled as single model elements with defined material layers because the size and composition of each layer cannot be freely adjusted. Furthermore, overlapping regions of compound elements with other elements result in excess material quantities. Manual inspection and modification of each compound element are time-consuming, cost-intensive, and error-prone. This study proposes a method to automatically modify compound elements in BIM models by separating each layer into an individual model element and eliminating overlapping regions. Accurate material quantities for compound elements can be obtained from the modified BIM models. A prototype system was developed and the proposed method was validated by two case studies. Compared to manual modification, the proposed method successfully modified compound elements in BIM models in much less time, with accurate material quantities. The modified BIM models are also beneficial in the construction phase, which requires greater detail regarding BIM elements.

Introduction

Building Information Modeling (BIM) is a technology that represents a physical building as a digital model [1]. It is used to facilitate design, construction, and operation processes to form a trustworthy source for decisions [2]. In principle, a single centralized model is shared, exchanged, developed, and integrated among stakeholders from the building design phase to the facility management phase [2,3]. However, in practice, the development of different models for different purposes is a better approach to deal with model complexity and file size [4]. For example, designers make BIM models for the visualization and production of building drawings, while contractors make BIM models for cost estimation and construction planning. Fragmentation of BIM models causes gaps when models are transferred among parties in the design and construction phases owing to incomplete information and a lack of clarity regarding who is responsible for inputting the information [4]. Therefore, an understanding of information management with a precise BIM execution plan and specific level of development (LOD) milestones must be defined to address these issues [2,4]. The BIM execution plan is the plan that explains how and why the information of a project will be carried out by which delivery teams in which delivery phase of the assets [5]. LOD is the degree of geometry detail and attached information for each BIM model element in each design and construction phase [6]. All in all, when transferring BIM models among parties with different aims, the BIM models have to be modified not only with respect to the information in the model elements but also the geometry of the model elements.

Quantity takeoff is a task that is usually performed after the design development phase to prepare a bill of quantity for tendering the project [7]. Traditionally, quantity takeoff is a time-consuming and error-prone process because it is based on measurements of 2D construction drawings and human interpretations [7,8]. The recently developed BIM-based quantity takeoff is the fastest and most reliable method for measuring quantities from building designs [[9], [10], [11], [12]]. Nevertheless, each element in a BIM model must have appropriate details for accurate quantity takeoff [13,14]. If the details of a model are incomplete, the extracted quantity can be wrong. Quantities may absent from a model or deviate from actual values [14,15]. Therefore, the BIM models constructed for and used in the design phase must be inspected and modified for quantity takeoff.

A compound element is a building element with layered structures [16]. Examples of compound elements are walls, floors, roofs, and ceilings. Quantity takeoff for compound elements is complicated in both traditional and BIM-based methods because each material layer is found in a different location and may differ in size. In the traditional method, each material layer must be measured separately [17]. In the BIM-based method, each layer of compound elements must be modeled to represent the exact dimension of the actual construction in order to obtain accurate quantities [18]. However, the compound elements in a BIM model are generally treated as single model elements with defined material layers (see Fig. 1a) [19]. In this modeling method, the size and composition of each layer are not freely controlled or adjusted so deviations in material quantities are possible if each layer must have a different size and composition [8,[18], [19], [20], [21], [22]]. For example, the interior finish layer of a wall might reach the height of the ceiling, while the core structural layer of a wall reaches the soffit of the beam or the floor above. The solution is to separate each layer of the compound element into an individual model element (see Fig. 1b) [18,[20], [21], [22], [23]]. Furthermore, the compound elements in a BIM model could be modeled by overlapping with other elements causing excess material quantities (see Fig. 1c) [20,22]. For example, the walls may penetrate beams without subtraction. Modelers must eliminate overlapping regions in the BIM model to prevent excess material quantities (see Fig. 1d). Therefore, BIM models that are transferred from the previous design phases or BIM models that will be used for quantity takeoff must be inspected and modified or compound elements must be recreated to obtain accurate results. The modified BIM model, with its greater detail, is also essential in the construction phase. However, modifying each compound element layer as an individual model element is a time-consuming and cost-intensive process. Moreover, the complexity of 3D models increases the risk of human error.

In this paper, a BIM-based method is proposed that automatically modifies compound elements in BIM models by separating each layer into an individual model element and eliminating the overlapping regions of the compound elements. The objectives of this research were as follows: (1) to improve the accuracy of the quantities of compound elements extracted from BIM models, (2) to develop a method that automatically modifies BIM compound elements for accurate quantity takeoff, and (3) to reduce the working time and cost of editing or recreating BIM models. The proposed method provides the correct compound elements, which can also be used in the construction phase and are transferable in a collaborative work environment. The contribution of this research is the approach for automatic BIM model development, which reduces the working time, cost, and errors. Also, the study contributes to the understanding of BIM-based quantity takeoff issues from the incorrect BIM models.

The focal compound elements in this research are architectural walls and architectural floors or floor finishes because they are major architectural components in which each material layer usually has distinct dimensions. Each layer of architectural walls and floor finishes are measured as their surface areas [17,24,25]. Layers of compound elements that are reinforced concrete are excluded from the scope of the study because these layers are structural elements and they are measured as their volumes [17,24,25].

The research paper begins with a review of background information and related research. Subsequently, a detailed overview of the proposed method and the development of the prototype system is explained. Two case studies, a residential building and an office building, are used to verify the prototype system and to validate the proposed method. Material quantities for architectural walls and floor finishes are compared among original BIM models, BIM models modified by the proposed method, and manually modified BIM models (baseline). Moreover, the execution times of the prototype system are compared with the modification times of the manually modified BIM models. The results are discussed and the limitations of the proposed method are identified. Finally, the conclusions are summarized with a description of the scope for future research.

Section snippets

Traditional quantity takeoff

Quantity takeoff, a process in which quantities of building materials or work tasks in a construction project are measured, is a necessary task to obtain fundamental information for subsequent tasks in the design and construction process [7,8]. Therefore, the accuracy of quantity takeoff indicates the reliability of the following tasks, such as cost planning in the design phase, estimating costs in the pre-construction phase, schedule planning, purchasing materials, and tracking progress in the

Overview of the proposed method

An automatic compound element modification (ACEM) method is proposed as a framework to develop a prototype system. The ACEM method modifies wall and floor elements in a BIM model by fetching model elements as inputs to a system via an application programming interface (API). The calculation algorithms in the system automatically recreate wall and floor elements by separating each layer into an individual model element and eliminating the wall and floor regions that overlap with other elements,

Development of the prototype system

In this section, Autodesk Revit 2018.2 and Dynamo 1.3.3.4111 were used to develop the ACEM prototype system. Autodesk Revit is one of the most popular BIM software for architectural works that has modeling tools, a quantity takeoff tool, and an API for developing the prototype system. Dynamo is a user-friendly visual programming extension in Autodesk Revit requiring no programming background to develop and visualize results [52]. The explanation of the development of the prototype system is

Validation

To verify the ACEM prototype system and validate the ACEM method, material quantities of architectural walls and floor finishes from BIM models modified by the ACEM method were compared with the material quantities of architectural walls and floor finishes from original BIM models and manual modified BIM models (baseline). Furthermore, the execution times of the ACEM prototype system were also compared with the modification times of the manual modified BIM models. It should be noted that in

The ACEM method and prototype system

Based on two validation case studies, the proposed ACEM method can automatically modify compound elements in BIM models, which are walls and floors, by separating each layer into an individual model element and eliminating regions of overlapping compound elements. The ACEM prototype system was successfully applied to both case studies. The material quantities for compound elements extracted from the BIM models modified by the ACEM prototype system are as accurate as those for compound elements

Conclusion

Incorrect and incomplete compound elements result in inaccurate quantity takeoff for each material layer. Compound elements treated as single model elements with defined material layers are not flexible; the size and composition of each layer cannot be controlled and adjusted. Therefore, inaccurate material quantities are obtained when the size and composition of layers vary according to the actual requirements. Moreover, overlapping compound elements result in excess material quantities. For

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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