Goal: Cost Estimation


Phase(s):

Planning, Design, and Construction
Planning, Schematic Design and Design Development

Objective(s):

[_] Rough order of magnitude estimates for planning, schematic and design development stages
[_] Feedback on the construction cost change as the design progresses and changes
[_] Accessible and documented building cost information along with the proposed budget
[_] Preliminary quantities and cost estimates of building materials, elements and system components

Description:

A process in which a BIM model can offer a reasonable accurate quantity take-off and cost estimate early in the design process and provide cost effects of additions and modifications with potential to save time and money and avoid budget overruns. This process also allow designers to see the cost effects of their changes in a timely manner which can help curb excessive budget overruns due to project modifications. If the BIM model is shared with contractor, time for detail estimate can decrease dramatically and precision can go up.

Potential Benefits:

[_] Precisely estimate material quantities and generate quick revisions if needed (BIM and Cost Estimating, 2007)
[_] Stay within budget constraints with frequent preliminary cost estimates while the design progresses
[_] Better visual representation of project and construction elements that need to be estimated: taken off and priced (Khemlani, 2006)
[_] Provide cost information to the owner during the early decision making phase of design (Lee et al., 2008)
[_] Focus on more value adding activities in estimating like identifying construction assemblies, generating pricing and factoring risks then quantity take-off, which are essencial for high quality estimates (BIM and Cost Estimating, 2007)
[_] Exploring different design options and concepts within the owner’s budget
[_] Saving estimator’s time and allowing to focus on more important issues in an estimate since take-offs can be automatically provided
[_] Quickly be able to determine costs of specific objects (Khemlani, 2006)

Levels of Detail Considerations:

  • Type of estimate: rough order of magnitude, assembly, etc decides required level of detail in the model. It depends on the desired outcome: if only quantities are wanted, not much detail is needed, but if you want a very detailed estimate, a more detailed model would be required.

Team Competencies Required:

  • Knowledge of estimating in general and BIM authoring and estimating software.
  • Knowledge of potential misalignments between BIM Model Design and QTO program
  • General construction knowledge of how buildings come together.

Potential Modeling Methods:

Very dependent on the software used to do the estimate:
  • Quantities can be extracted and sent to the contractor or cost estimator (BIM and Cost Estimating, 2007),
  • BIM modeling application can read cost data from an object library based on assemblies (Lee et al., 2008 and Khemlani, 2006), or
  • BIM model can be opened within costing software which will match cost with construction elements (BIM and Cost Estimating, 2007).

Potential Outputs:

  • Rough order of magnitude estimates, material quantity take-offs, cost estimates of various assemblies done for comparison and possible value engineering in a fraction of time needed for traditional estimate.

Legal / Commitment Considerations:

  • It will be difficult to eliminate detailed cost estimates prepared by contractors for proposals and bids. Additionally, contractors will still need to create a schedule of values for payment which may be able to be based off a spreadsheet output from a BIM model. Preliminary cost estimates can be used to form the budgets and for early decision making and value engineering. (Dean and McClendon, 2007)
  • Designers must be very cautious when entering any type of cost information into a BIM model. If a contractor uses the designer's cost information to create a bid and some of the information is incorrect, the designer could be held legally responsible. It is important for the designer to consider this both during BIM model creation as well as during the creation of the contract.
  • A quantity take-off could be inaccurate due to how the BIM was created. For example, if a thickened slab around a column is modeled as the slab plus another concrete element instead of only one element (as it would be built), a quantity take-off of the concrete surface finishing area would be incorrect because it would double count the surface of the slab that was thickened. For this reason, it is important for a designer to be careful while creating a BIM model. It is also critical that the designer states the extent of the BIM's accuracy and for what purposes the BIM model may be used.

Additional Resources:

Case Study Examples:

  • Medical Research Lab and Expeditionary Hospital Facility (Manning and Messner, 2008)
  • Various Case Studies (Buckley, 2008)

Software Applications:

  • DProfiler
  • Innovaya
  • Sage Timberline Office Estimating
  • Autodesk Revit
  • Digital Project
  • U.S. Cost Success Design Exchange (Success Estimator)

References:

(1) Manning R., Messner J. (2008). Case studies in BIM implementation for programming of healthcare facilities, ITcon Vol. 13, Special Issue Case studies of BIM use, Pg. 246-257, http://www.itcon.org/2008/18
(2) Lee, H., Lee, Y., Kim, J. (2008). A cost-based interior design decision support system for large-scale housing projects, ITcon Vol. 13, Pg. 20-38, http://www.itcon.org/2008/2
(3) AutoDesk Revit. "BIM and Cost Estimating." Press release. Jan. 2007. AutoDesk. 11 Sept. 2008. http://images.autodesk.com/adsk/files/bim_cost_estimating_jan07_1_.pdf
(4) Dean, R. P., and McClendon, S. (2007). "Specifying and Cost Estimating with BIM." ARCHI TECH. Apr. 2007. ARCHI TECH. 13 Sept. 2008. http://www.architechmag.com/articles/detail.aspx?contentid=3624
(5) Khemlani, L. (2006). "Visual Estimating: Extending BIM to Construction." AEC Bytes. 21 Mar. 2006. 13 Sept. 2008. http://www.aecbytes.com/buildingthefuture/2006/visualestimating.html
(6) Buckley, B. (2008). "BIM Cost Management." California Construction. June 2008. 13 Sept. 2008. http://california.construction.com/features/archive/0806_feature1.asp