Lesson 3: Model-Based Estimating and Quantity Takeoff
In this lesson, students explore how to use a model-based estimating workflow to provide useful cost feedback to the project team and inform decisions during all phases of design, from inception through construction.
Students will learn how to use Revit features, including project and shared parameters, schedules and material takeoffs, and formulas to create conceptual estimates and compare the cost of proposed alternatives, and prepare preliminary estimates based on quantities of key building elements to confirm cost feasibility and evaluate proposed design changes. They will also learn how to transfer Revit project models to Autodesk Quantity Takeoff software to extract quantities from the BIM model and build detailed cost estimates.
BIM models can be used to accurate generate quantity takeoffs and assist in the creation of cost estimates throughout the lifecycle of a project. Using BIM models in this manner enables the project team to see the cost effects of their design decisions and proposed changes during all phases of the project, and this feedback supports better design decision-making and can help curb excessive budget overruns due to project modifications.
Quantity takeoffs from a BIM model enable project teams to quickly generate cost estimates to assist in decision-making and provide cost information about alternatives to owners early in the design phase and throughout the project lifecycle. The BIM model is integrated with cost information from an estimating database, and this approach has proven to be quicker (some leading firms report a savings of nearly 80 percent of the time compared to traditional estimating) and reduces the possibility for errors and omissions. It can also reduce quantity takeoff time and allow estimators to focus on higher value activities, such as identifying construction assemblies, generating pricing, and factoring risks.
The advantages of setting up a model-based estimating workflow far outweigh the upfront time and effort required to enable the process. It is leaner and smarter approach, because it automates the time-consuming task of quantity takeoff, quickly provides cost feedback, and allows project teams to focus on critical design and planning issues.
Cost Feedback at Every Project Stage
Cost estimates are valuable to project teams at every stage of the design process, and useful feedback can be provided using even the simplest project data.
Cost feedback is most beneficial in the early design stages of a project, when design decisions have the greatest impact on the eventual project cost. For this reason, cost estimates based on the area or volume of a proposed conceptual design and comparison to historical data for facilities with similar functions and programming are especially valuable for confirming that the design is feasible and in alignment with the owner’s proposed project budget.
As the design is being developed, material takeoffs that quantity key building elements (for example, floor areas of different types or surface areas of building envelope elements) can be used to compute real-time preliminary cost estimates to confirm that the evolving design is staying on-track and evaluate the cost impact of proposed design enhancements.
As the building design matures and construction details are accurately modeled, precise quantity takeoffs can be extracted from the BIM model to compute very detailed cost estimates that factor in the planned construction process and consider the labor, materials, equipment, and subcontractor costs for all building elements.
As the level of detail for each element in the model increases, the level of costing detail can grow with it. So, as the design evolves and more details are specified, our cost estimates grow more and more precise.
Having this cost feedback available at every state of design enables project teams to:
- Compare cost plans to the original project budget at any point in time. Stakeholders can easily see what has changed and decide when and to adjust scope as necessary.
- Understand and focus on design decisions that have the largest impact on the project cost.
- Evaluate the cost impact of proposed design enhancements and what-if scenarios.
- Compare the impact of using sustainable design strategies in terms of installation time, cost, and projected energy savings
Focusing Estimating Effort and Maximizing Value
Some elements of a building project have a much greater impact on the total project cost than others. So, rather than estimating all elements with an equal level of detail and effort, it is better practice to identify the key building elements that drive total cost and focus design and estimating attention on them.
The basic principle is simple: focus a high level of detail on estimating the parts of a project that are high risk (high value and high variability), and use a lower level of detail for parts that are low risk (low value or low variance—for example, elements that have been subcontracted at a fixed price).
Using this successive estimating approach, project teams always focus their efforts and attention on items that have the greatest potential cost impact.
Target Value Design
Target Value Design (TVD) is a lean construction strategy the reverses the typical relationship between design decisions and cost estimates—instead, using cost estimates to drive the design. The guiding principle of TVD is that the target cost for a project should never be exceeded. In most traditional project delivery approaches, cost follows design, but on projects where TVD is used, cost dictates what gets designed to ensure that the target cost is not exceeded.
Some key features of this strategy include:
- Rather than estimate based on a detailed design, design based on a detailed estimate.
- Rather than evaluate the constructability of a design, design for what is constructible.
- Rather than narrow choices to proceed with design, carry potential solution sets far into the design process.
Extracting quantities from BIM models and using model-based estimates techniques to provide rapid cost evaluation and feedback is essential to successfully using this technique. Using model-based estimating, project teams are able to get the needed rapid cost feedback to achieve TVD objectives and deliver the best possible value to the owner.
- BIM Execution Planning Website
Computer Integrated Construction (CIC) Research Program, Pennsylvania State University
- Model Based Estimating to Inform Target Value Design
Tiwari, S., Odelson, J., Watt, A. and Khanzode, A., DPR Construction, AECbytes (August 2009)
- A Cost-Based Interior Design Decision Support System for Large-Scale Housing Projects
Lee, H., Lee, Y.S., and Kim, J., ITcon Vol. 13, p. 20-38, (2008)
- BIM and Cost Estimating
Autodesk Whitepaper (2007)
- Specifying and Cost Estimating with BIM
Dean, R. P., and McClendon, S. , ARCHI TECH. (April 2007)
- Challenges in Cost Estimating with Building Information Modeling
Sabol, L., Design + Construction Strategies, LLC. (2008)
- 1-2-3 Revit: BIM and Cost Estimating
Rundell, Rick, cadalyst (August 2006)
- Visual Estimating: Extending BIM to Construction
Khemlani, L., . AECBytes (March 2006)
- Cost Estimating in BIM: The Fifth Dimension
McCuen, T., Construction Advisor Today (November 2009)
- Eastman, C., Teicholz, P., Sacks, R., and Liston, K.(2008)
BIM Handbook: A Guide to Building Information Modeling for Owners, Managers, Designers, Engineers and Contractors
- Hardin, Brad (2009)
BIM and Construction Management: Proven Tools, Methods, and Workflows
- Buckley, B. (2008)
BIM Cost Management
California Construction, June 2008
- Shen Z, and Issa, R R A (2010)
Quantitative Evaluation of the BIM-Assisted Construction Detailed Cost Estimates
Journal of Information Technology in Construction (ITcon), Vol. 15, p. 234-257.
- Shared Parameters
- Project Parameters
- Conceptual Design Analysis
- Mass Floors
- Material Takeoff Schedules
After completing this lesson, students will be able to:
- Appreciate the value of using conceptual mass families to quickly model and compare design alternatives.
- Utilize and leverage mass floors, parameters, and simple formulas to create conceptual estimates.
- Create schedules and material takeoffs to tabulate key quantities of building elements and compute preliminary estimates.
- Use preliminary cost feedback to inform design.
- Create a detailed quantity takeoff of the elements in a project model.
- Choose strategies and define formulas to convert key quantities into cost estimates
|Quantity Takeoff||A process used to create an itemized and quantified list of construction materials from project drawings, specifications, and the project BIM model.|
|Cost Estimate||A prediction of quantities, cost, and/or price of resources required to deliver a project. Cost estimates are used for budgeting, design decision making, project planning, and cost control. Cost estimates are subjective and rely upon the experience of the estimators to accurately the future cost of resources, methods, and management within a scheduled time frame.|
A very rough estimate made during the conceptual design phase of a project to help quantity the predicted cost of a design option.
Conceptual estimates are often based on very simple metrics, for example, the gross floor area a building (or subregion) and data about historical costs for constructing buildings with similar uses.
Preliminary estimates are often based on easily quantified metrics, for example, the surface area of major building elements like the exterior walls, floors, and roofs.
A precise estimate made at the later stages of the design work, after much of the detailed design work has been completed or firm bids have been received for some work items.
Detailed estimates are based on the actual quantities of the building elements as measured from the project drawings or directly from the project model. Estimator use these quantities to predict the cost of the project by considering the cost of the resources, the construction methods that will be used, and the cost of the equipment and management required through the duration of the project.
|Model Based Estimating||
Using the elements in a building information model (rather than drawings) to create quantity takeoffs, counts, and measurements that can be generated directly from the underlying model.
Using this approach, the estimate is always consistent with the design. When a change is made in to the design in the BIM model, the change automatically ripples to all related construction documentation and schedules, as well as all the takeoffs, counts, and measurements that are used by the estimator.
|Target Value Design||
A management practice that drives design to deliver customer values, and develops design within project constraints.
The guiding principle of Target Value Design is that the target cost of a facility can never be exceeded.