This article compares cost estimating in building information modeling (BIM) to the traditional process of cost estimating. Professional estimators know there is more to cost estimating in BIM than simple automation of estimating from objects to spreadsheets.
Cost estimators also understand the challenges and obstacles beyond the technology that must be overcome if cost estimating is to become a viable dimension of BIM. For purposes of this article, there are two basic aspects of BIM relevant to this topic:
- BIM is intended to improve industry efficiency and productivity with accurate and complete information.
- A BIM should support the entire lifecycle of a facility, and therefore information contained in the model should facilitate the work of all stakeholders.
Cost estimating, 5D in BIM is emerging, and the industry associations of professional estimators are working together with the BuildingSmart Alliance (BSA) to define and develop processes in BIM that bridge the gap between traditional and BIM processes. These associations include the Association for the Advancement of Cost Engineering, the American Society of Professional Estimators, and the Royal Institution of Chartered Surveyors. Their group responds to the increasing demand for cost estimating, 5D in BIM. The following is a brief description of the traditional estimating process and some changes required in the process to produce valid and accurate estimates using BIM.
Traditional and BIM - Similarities and Differences for Cost Estimating
One convention employed by estimators in the traditional process is in identifying the expected accuracy range of an estimate based on the level of project definition. In the traditional process, the project plans and specifications were the primary means by which this was determined, and as such, there was a direct correlation between the project's level of definition and the expected accuracy of an estimate. It is reasonable to expect a similar convention exists in BIM, and that as a BIM contains more project definition, it also impacts the potential accuracy of an estimate. The difference in BIM, though, is in how a designer creates the objects for project 'plans,' and specifications now have an impact on the estimate.
Figure 1.
The method or sequence by which a designer created plans and specs in the traditional formats did not impact the estimate because the information relevant to an estimate was an overlay by the estimator and external to the graphical representation. In the traditional process, the estimator managed the information from these documents and extracted, organized, and used the information as best suited to accomplish the task of estimating. However, with BIM the point of organizing information shifts as more of it begins in the design model phase. To date, there is no industry standard to bridge the gap between the design model and the estimator. Consequently, this has an effect on the estimator's confidence about the information in a BIM relative to its use for cost estimating.
This is a challenge as estimators are now faced with the accuracy level of estimates dependent on the validity and reliability of information in the model. Model objects are rich with the information estimators need to create a cost estimate, and if this information is to be used by estimators, then there is a point where the estimator's process should filter into the information management during design.
The development of a model includes the graphical representation of data-rich objects. The primary purpose of the design model is to convey design intent. However, each of the objects inserted are available now for future extraction by other stakeholders. The difference in BIM is, that from an estimator's perspective, the development of a model is about the information associated with the objects and the input process for this information. This aspect of BIM is a significant shift in paradigm from which the estimator previously worked. It is crucial that the estimator has confidence that the information is a valid representation of the object beyond the model to physical reality. This is new in the world of estimating and is challenging estimators as they work within this new paradigm.
The estimator's responsibility, as sole individual responsible for organizing information in a way useful for an estimate, is different in BIM compared to the traditional process. The traditional responsibility of the estimator managing the information for an estimate is distributed now to earlier phases in a project simply by the nature of BIM. If estimators are not involved at earlier phases, it leads to redundancy and inefficient work processes. Currently, estimators working in BIM often invest valuable time in validating or revising the model to facilitate its use as a tool for legitimate output in 5D. Not only is rework inefficient, it also increases the potential opportunity for errors. More importantly, this activity adds no value to the project and mitigates the efficiencies intended to be associated with BIM. Reworking a BIM by the estimator goes against one of the basic principles of BIM listed above.
As estimating in BIM continues to emerge, it is important to keep in mind that traditional cost estimating goes beyond material quantity takeoffs and price loading. It includes the ‘modeling' of project construction with conditions and constraints that impact the construction process. As such, cost estimators 'build' the facility using quantity takeoffs of specified materials, then adding professional knowledge of means and methods, sequencing and phasing, conditions, and constraints. This is the cost-estimating process, and inputs from other stakeholders are embedded in this process. The previous methods used for developing plans to convey design did not impact the estimator. The estimator's ability to complete an accurate estimate using BIM is a challenge at this time. A major obstacle is the lack of a standard that establishes how a BIM is created so that it contains valid and reliable information to meet the needs of all stakeholders across the lifecycle of a facility.
By definition, BIM is an intelligent model, and it is logical to expect that the information within the model goes beyond the needs of the creator that inserted the information to facilitate other stakeholders' tasks and work processes. A BIM should be developed for the lifecycle of a facility and with information input from multiple stakeholders for extraction and use by multiple stakeholders. It is important to think beyond the usefulness of information for one's own use to how the information will be used by others.
Figure 2.
Cost-estimating process has always relied on the inputs from the design process, and this remains unchanged. The difference in BIM is that the method and organization of the inputs by designers have an impact beyond the design process. All inputs are rich with information and available for other stakeholders, as such the coordination of information at all points in a BIM is important. As 5D gains momentum and more projects require estimating in BIM, professional estimators are working to define a new process and capitalize on the opportunities available for improving cost estimating in BIM.
Tammy McCuen, Contributor
Construction Project Controls and BIM Report
Tammy McCuen is an assistant professor of construction science at the University of Oklahoma, College of Architecture, with a teaching and research emphasis in BIM. She is currently working on a joint project between the Association for the Advancement of Cost Engineering International (AACEI) and the National BIM Standard (NBIMS) Committee, which are working to define the detailed quantity take-off and cost estimating process in BIM. Prior to joining the OU faculty, McCuen spent 14 years as a cost estimator, superintendent, project manager, and an owner of a construction management company. She is a member of the buildingSMART alliance and the National Institute of Building Sciences. Please feel free to comment to Tammy below, or e-mail her at tammymccuen@ou.edu.
References
National Institute of Building Sciences (2007), National Building Information Modeling Standard, Version 1- Part 1: Overview, Principles, and Methodologies. Originally retrieved December 18, 2007, from the National Institute of Building Sciences website: http://www.nibs.org/. Available at: http://www.buildingsmartalliance.org/index.php/nbims/.
National Institute of Standards and Technology, NISTIR 7417, (2007, August). "General Buildings Information Handover Guide: Principles, Methodology and Case Studies." Retrieved September 18, 2007, from the FIATECH website: http://www.fiatech.org/pdfs/ax.php/nbims/rticles/NISTIR7417
