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.
- 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.
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 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.
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 [email protected].
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/.
This is exactly what CostOS BIM Estimating does have a look at the following link:
http://www.nomitech.eu/cms/en/c/bimestimating.html
Posted by: George Hatg | 06/26/2010 at 09:39 AM