BIM METHODOLOGY AS A SUPPORT TO THE QUANTITY TAKE-OFF Bernardo Ferreira e Silva Extended Abstract Master s Degree in Civil Engineering Supervisors Profª. Alcínia Zita de Almeida Sampaio Jury President: Supervisor: Member: Profª. Alcínia Zita de Almeida Sampaio October 2016
1. Introduction The Quantity Take-Off (QTO) is an essential process in project, because it allows to manage the costs involved in construction, supporting the investment analysis, as well as the decision-making and resources planning [1]. QTO can be applied through the construction process, where valid results can be obtained in the early stages that will assist the estimation of a preliminary cost. Before construction, it can be used as a planning tool with the construction activities. The BIM model guarantees the automatic update of results obtained in QTO after any change in the model [2]. During the works, it can be used to control the economic part of the construction. The traditional quantity take-off, through manual measurement of the different project elements, is based on 2D drawings, that can present inconsistencies and, therefore, might produce errors on the results obtained [3]. The level of automation in the industry has been a major concern over the last years with the academic community working hard to raise it [3]. Nowadays, BIM technology presents a valid alternative to the traditional process, allowing better results to the quantity takeoff process and raising the automation present in the project, while facilitate the information management in construction [4]. BIM is an automated tool based on a virtual model where information is automatically generated from it. BIM tools allow various uses, like visualization and clash detection, based on the automation and transfer of information using the virtual model. Using BIM to create bills of quantities will reduce the time spent on obtaining them and, therefore, will reduce costs on the project [5]. Despite these considerations, this feature is overlooked and BIM is mainly used to visualization. The utilization of BIM in QTO process is still target of reservations in the industry, due to some lack of information about the advantages provided in BIM utilization as a QTO tool. The strengths of using BIM in a QTO process are: Faster execution; QTO in design phase; Costs analysis through the project; Reliable Results; Competitive Advantage. But there are some problems identified, such as: Lack of technical standards; Insufficient interoperability between systems; Changes in the company, due to the necessity of collaborative work and new work methods; Additional work in obtaining quantities that can t be obtained automatically. This work aims to contribute to the knowledge of the BIM utilization in the QTO process, while demonstrating that it has more advantages and provide some solutions to the problems. Particularly, it compares both methods, traditional and BIM, used in the process, applied to a real structural project. 1
2. Case Study 2.1. Guidelines to BIM modelling The 3D model is the base of the QTO process with BIM. The modeling should follow certain guidelines, suitable for the objective of the model. In order to guarantee that the BIM model will provide reliable results, the following standards should be applied to the modeling process: It is important that all the objects and components of the model are modeled under the same circumstances and standards. Consistency of the model is necessary to ensure homogeneity on the results; Each object should be created using the appropriate tool, depending on which software is used. All the information relevant to the QTO should be entered on the tool, such as dimensions and materials. Some objects can have two function types: structural and non-structural. Is important to differentiate the layers by type to obtain bills of quantities classified in function types; Interoperability is a main concern when exchanging information between different software. The person responsible for the QTO need to know the limitations of the file format used and be able to overcome them, in order to maintain the results quality; Clash Detection is important to ensure the quality of the model, and the model should be adapted accordingly to that analysis. This tool is available in most of the software available. 2.2. Case Study Modelling In this research, a real case study was used: a project of a school in Faro, Portugal. The documents provided consists in 2D drawings of the structural foundations (Footings, Columns and Lintels) with rebar detailing. The modeling process was divided in phases, listed on the Table 1 Table 1 - Modeling phases Activity 1º Creation of levels and grids 2º Column insertion 3º Footing placement 4º Lintel definition 5º Footing rebar detailing 6º Model Checking 2
Before modeling, it s necessary to import, through Revit, the 2D drawing from AutoCAD with the elements that will be modeled, to be used as a basis for the BIM model. The first step is to create levels and grids that will support the 3D modeling. The levels are useful to limit the height of columns and footings, as it is possible to pin the top of the columns to the level and change the height of those columns automatically. The grids provide a place to attach columns and assist their placement. The Figure 1 shows a representation of the grids defined. Figure 1 - Grids Defined As it can be observed, the intersection between grids normally coincide in the center of the columns, to ease the placement. Next step is to create the different columns as a parametric object, duplicating a standard concrete column and adapting the dimensions. With the help of grids and the layout imported from AutoCAD, inserting the columns is an easy process, where the level that limits the column height is chosen in the software. For this work, all the columns have the same height (3 meters). The Figure 2 shows a representation of the columns on Revit. The process to create footings is analogous to the column process. Standard footing is duplicated and all the footings with different dimensions are created. To facilitate the placement of footings, the columns were created first so it is possible to use them as a reference. The software identifies the base of the column and place it correctly. Revit has the capability to facilitate the modeling and to place adjacent elements in a simple way. 3
Figure 2 - Columns representation The way to create and place lintels is, equally, based on the standard element duplication. Taking into account that all the lintels have a square section and do not exist, in Revit, a tool to create lintels, the beam tool was used due to the similarity between those elements. After creating the different types of section, the lintels were placed 1.25 meters above the top of the footings, to ensure that no lintel conflicted with the footings. For quantity take-off results, the columns pass through the lintel. The Figure 3 shows a 3D model of the foundations. Even using the columns as a reference to place the lintels, each individual placement was checked to guarantee that there weren t errors in the alignment. Figure 3 - Foundations 3D model To illustrate and study the rebar detailing in structural elements only the footings were considered. An add-on from Revit was used in this process, Reinforcement. This allows an automatic detailing, after providing measurements and information like steel grade. The add-on calculates the number of bars and the spacing between them, placing them inside the element. 4
This process needs to be executed on each case and a manual verification is needed to ensure the correct rebar bending and placement. The Figure 4 illustrates a footing rebar. Figure 4 - Footing rebar detailing To finish, is necessary to check the model to verify that all the elements are modeled with the correct dimensions and in the correct location, based on the drawings provided. This way, the results from the automatic quantity take-off are rigorous and represent the reality. This process guarantees the reliability of the model, and is the last step to finish the modelling process. The Figure 5 represents the final model of the foundations, with all the elements modelled. Figure 5 Foundations BIM Model The creation of the BIM model is simplified by the advanced modelling capabilities of Revit, however, some limitations were identified, particularly on the rebar detailing. Despite the 5
limitations, modelling in BIM brings unique advantages that compensate the initial difficulty of working with BIM. 3. Bill of quantities 3.1. Traditional method The traditional quantity take-off consists in measuring all the elements of the building manually, using the technical drawings. In this paper, the measurement regulations used were from Laboratório Nacional de Engenharia Civil, Medições em Construção de Edifícios [6]. The elements of the case study are simple, with common geometric shapes, so the volume formula is multiplying the three dimensions (length, width and height). By analyzing the foundation plan, the quantity of concrete needed to build the footings, columns and lintels was obtained. To obtain the steel quantities from the footing rebar, it is necessary to measure the length of the bar, taking into account the overlaps and hooks. This process takes a lot of time due to the necessity of measuring both rebar (superior and inferior) and guarantee the correct quantity take-off. The time taken and the quantities measured can be observed on the Table 2. Table 2 - Quantities and time taken Element Time spent (min) Quantities measured (m 3 ) Material Footings 25 119,52 m 3 Columns 35 33,31 m 3 Concrete Lintels 60 102,53 m 3 Footing Rebar 90 4239 kg Steel Total 210 Related to rebar modelling, due to the difficulties found in the BIM modelling process of the footing rebar, only this rebar was quantified. The process of modelling the columns and lintels would require a great amount of time and the results would be different between the traditional method and BIM. 3.2. BIM method Using a Revit functionality, called Schedules, a list of quantities is created based on the element type chosen and the properties needed, such as area, volume or material. It is possible to require a sum of quantities of the same property, represented on the list, that will facilitate the delivery of results like quantity of concrete needed to build the footings. The Table 3 represents a list of footings classified by type, showing the number of footings of each type, the material and the volume. This information is useful to support the planning of formwork. The process to obtain this table took 5 minutes. 6
Table 3 - Footings Schedule To obtain the other elements, the process is analogous. After the modelling phase, the quantity take-off is a simple process and the results obtain are trustworthy if the modelling was correctly executed. 4. Results Analysis The Table 4 presents the values obtained through both methods. The footings and columns returns the same result due to simplicity of the take-off in the traditional method. Table 4 - Quantity take-off (Traditional and BIM) Element Traditional method (m 3 ) BIM method (m 3 ) Difference (m 3 ) Footings 119,52 m3 119,52 m3 0,00 m3 Lintels 102,53 m3 103,11 m3 0,58 m3 Columns 33,31 m3 33,31 m3 0,00 m3 Rebar (Footings) 4239 kg 4199,03 kg 39,97 kg In the case of the Lintels, the difference is not significant. The justification to this result is that the project has some uncommon intersections between columns and lintels, where the lintel wraps the column and, per measurement rules, it is not accounted in the column, as is seen in the Figure 6. 7
Figure 6 - Intersection zone between column and lintel Regarding the footing rebar, the difference verified is due to the length of the bending, confirmed with the manual comparison between both methods. It is possible to correct this situation on Revit, but the increase in time spent doesn t compensate the difference of less than 1%, considering the steel wastes in rebar. Constraints and Limitations The production of a BIM model requires training to the modeler to acquire knowledge and experience to conceive proposals, manipulate information and obtain results. This training requires a great amount of time to adapt to a new methodology and the results aren t visible initially, a situation that can move away potential interested parties. The initial investment, that requires software licenses, new hardware and training is a constraint that will limit the successful implementation of BIM. The modelling process requires good practice and experience to return reliable results, and a model created to QTO may not be an accurate representation to 3D visualization. The rebar modelling in Revit is a difficult process and not user-friendly, although all the applications developed and incorporated in BIM software. The results obtained aren t reliable and the rebar modelling is a limitation to BIM and a point to improve to the software companies. 8
5. Conclusions This document presents an experimental study in order to evaluate the advantages of using BIM on the QTO process, and make a comparison between the traditional method and BIM. The results obtained in this research allows drawing the following general conclusions: 1) The results obtained from the BIM model are, in most cases, reliable and easily obtained, reducing the time spent on the QTO process. 2) The results obtained with BIM are as accurate as traditional method and aren t affected by the human error in the QTO process, if the modelling process was well executed. 3) The study of alternatives to the project is almost instant and automatic, allowing to evaluate a wide variety of options and choose the less expensive, reducing costs. 4) The rebar modelling needs further development by the software companies and the results returned should only be representative of the quantities required. 5) Even with the initial constraints, such as a high up-front investment and training required to the professionals, the utilization of BIM in the QTO process is beneficial, in place of the traditional method. References [1] Doloi, H.K., Understanding stakeholders perspective of cost estimation in project management, International Journal of Project Management, 29, pp 622-636, 2011. [2] Forgues, D. et al, Rethinking the Cost Estimating Process through 5D BIM: a Case Study, Construction Research Congress, ASCE, 2012. [3] Monteiro, A., Martins, J.P., A survey on modeling guidelines for quantity takeoff-oriented BIM-based design, Automation in Construction, 35, pp 238-253, 2013. [4] Parreira, J.P., Implementação BIM nos processos organizacionais em empresas de construção um caso de estudo, Dissertação Mestrado. Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2013. [5] Shen, Z., Issa, R.R.A., Quantitative evaluation of the BIM-assisted construction detailed cost estimates, Journal of Information Technology in Construction (ITcon), Vol.15, pp 234-257, 2010. [6] Laboratório Nacional de Engenharia Civil, Medições em Construção de Edifícios, LNEC, Lisboa, 1983. 9