University of Twente Student Theses


Comparison of early-stage quantity estimation methods

Wee, P.J.A. van (2018) Comparison of early-stage quantity estimation methods.

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Abstract:Decisions made during the early-stage design phase account for 70-80% of the total costs. Therefore, early-stage cost estimation is decisive for the future success of a project. Efficiently and accurately estimating the quantities of the materials of a project is of great importance for the efficiency and accuracy of the cost estimation. Multiple quantity estimation methods can be distinguished; however, no Building Information Modelling (BIM) literature exists regarding the comparison of early-stage quantity estimation methods, because literature often focuses on near-fully developed designs. Therefore, this report focused on comparing early-stage quantity estimation methods. This study compared the overall performance of three early-stage quantity estimation methods: fully-automatic BIM-based QTO, semi-automatic BIM-viewer and manual quantity estimation using 2D drawings. BIMbased QTO concerns automatically extracting object quantity information of three-dimensional models and the BIM-viewer is a tool that displays a three-dimensional model including the dimensions of the objects in the project. The objective of this study was to compare the performance of automatic BIMbased QTO to using a BIM-viewer and manual quantity estimation for early-stage quantity estimation of cost determining objects, and to give a recommendation regarding implementing BIM to estimate product quantities in the work practice of Arcadis. The next paragraph describes the methodology this study used, the third paragraph describes the results of this study, followed by the conclusion. This study used interviews with cost engineers and literature regarding quantity- and cost estimation to determine the comparison criteria. This study used three criteria to compare the three quantity estimation methods, namely: efficiency, accuracy and traceability. The efficiency is defined as the ability to accomplish a job with a minimum expenditure of time. The accuracy is the degree to which the estimated quantity conforms to the actual quantity and the traceability is the ability to discover to what objects the quantities belong. The weights of the criteria have been determined by six cost engineers that assessed the importance rating of the criteria on a scale of zero to hundred. The six cost engineers assessed the importance ratings of the criteria for early-stage quantity estimation. Other criteria are important for early-stage quantity estimation compared to detailed quantity estimation. For example, the accuracy will be of more importance for detailed quantity estimation than for early-stage quantity estimation, because the bandwidth of early-stage cost estimations is larger. The researcher normalized the importance ratings to weight factors. This study used interviews with cost engineers, a historic cost estimation of a bridge project and literature regarding decomposition of bridges to identify the cost determining objects of a bridge project that has been used as case study. This study used cost determining objects, because early-stage designs do not contain enough information to estimate the costs of all construction objects. A workshop was performed using this case study of a bridge project located in Zwolle. During this workshop, six cost engineers estimated the quantities of the cost determining objects of the case study using the three quantity estimation methods. The researcher recorded the time the cost engineers needed to estimate the quantities and determined the deviation of the estimated quantities to compare the quantity estimation methods. The traceability of the quantity estimation methods was assessed by the cost engineers that participated in the workshop. This study required a multi-criteria decision analysis (MCDA) method, because the performances of the three criteria used different units, namely: minutes, percentage and assessment score. To operationalize the performance per criterium, a MCDA method was required. This study used the Simple Multi-Attribute Rating Technique (SMART) with utility functions that range from the lowest score to the highest score for every criterium. SMART used utility functions to operationalize the performances of the criteria to similar units named utility scores. The utility scores multiplied by the weights resulted in the overall performance of the quantity estimation methods. To validate the overall performance, all six cost engineers that participated in the workshop were asked for their preferred quantity estimation method. For verification purposes, this study performed a sensitivity analysis of the weights and this study used a different MCDA method. The Multi-Attribute Utility Theory (MAUT) was used instead of SMART to verify if the result, i.e. quantity estimation with the highest overall performance, changed. Literature regarding decomposition structures of bridge projects resulted in the conclusion that there is not one single best decomposition structure. Literature uses different decomposition structures. Therefore, this study used the decomposition structure of Arcadis. During the workshop, the six cost engineers estimated the quantities of nine cost determining objects. The six cost engineers that participated in the workshop determined the weights of the criteria. The weights of the criteria efficiency, accuracy and traceability are respectively: 0.29, 0.38, and 0.33. Therefore, the cost engineers considered accuracy the most importance criterium, followed by traceability. The result of the workshop was that BIM-based QTO and the BIM-viewer are equally accurate. This was expected, because BIM-based QTO and the BIM-viewer use the same 3D model. Manual quantity estimation was significantly less accurate than the quantity estimation methods that used BIM. Regarding efficiency, BIM-based QTO performed best, followed by the BIM-viewer. This was expected, because BIM-based QTO automatically estimates the quantities. On the other hand, for the BIM-viewer the cost engineer must select the objects of which the quantities are required. All cost engineers considered the quantities estimated using the BIM-viewer most traceable, because the objects to which the quantities belong are directly visualized and highlighted. The quantities estimated using BIM-based QTO are least traceable, because the six cost engineers consider it difficult to understand to what object the quantity belongs. The BIM-viewer had the highest overall performance, mainly because the cost engineers assessed the BIM-viewer more traceable than BIM-based QTO. The result was validated by the cost engineers. All six cost engineers that participated in the workshop preferred early-stage quantity estimation using the BIM-viewer over the alternative quantity estimation methods. The result of the verification was that the quantity estimation method with the highest overall performance did not change. Therefore, the result has been validated and verified. This study shows that for early-stage quantity estimation, using a BIM-viewer has a higher overall performance compared to fully automatic quantity estimation using BIM-based QTO and manual quantity estimation. Literature currently often proposed BIM-based QTO using detailed designs. However, this study shows that for early-stage quantity estimation, the BIM-viewer has a higher overall performance. Therefore, using the BIM-viewer for early-stage quantity estimation results in improved cost estimations. Improved early-stage cost estimations result in enhanced decision-making during the early-stage design phase and contributes to decreasing the high rate of failure of contractors. Previous scientific literature that focused on estimating quantities using BIM did not use traceability as criteria for determining the overall performance. This study shows that cost engineers consider traceability of high importance for quantity- and cost estimation. Using traceability as comparison criteria for quantity estimation methods results in enhanced comparisons. This study concludes by mentioning that for early-stage quantity estimation of infrastructure projects the expertise of cost engineers is always required. Expertise is required, because the additional costs of phasing of a project and surroundings are not possible to automatically estimate, however, do determine a large part of the total costs of a project.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Construction Management and Engineering MSc (60337)
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