Model-Based Estimating and Quantity Takeoff using Navisworks

Model-Based Estimating and Quantity Takeoff using Navisworks

Overview

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.

Model-Based Estimating

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 constructable.
  • 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.

Learning Objectives

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.

Tutorials

Creating and Comparing Conceptual Estimates

In this exercise, students will learn how to:

  • Create conceptual mass families to quickly model conceptual design alternatives.
  • Add parametric controls to dynamically flex the mass size and shape.
  • Use mass floors to divide a mass form into floor levels and quantify key metrics (area, perimeter, and volume) for each level.
  • Schedule and total mass floor areas.
  • Add parameters and simple formulas to compute conceptual cost estimates.
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Figure 7.3.1. Using parameters to control the mass size and shape

Video Tutorial
Student Exercise
  • Create a conceptual cost estimate for the conceptual mass model contained in the dataset.
    • Divide the conceptual mass into mass floors with a 12-foot floor-to-floor height.
    • Create a schedule of the mass floors and calculate the total area available.
    • Add a calculated value parameter to compute a cost estimate for each floor and the total conceptual cost for the entire building based on the areas enclosed.
  • Duplicate the previous estimate and adjust the copy to allow you to refine the estimate on a floor-by-floor basis:
    • Add a shared parameter to assign a function to each of the mass floors.
    • Update the schedule to use different costs per square foot based on the function assigned to each mass floor.
    • Add a calculated value to the schedule to compute a conceptual cost for each mass floor based on the function specified.
  • Add parametric control to the conceptual mass to enable you to quickly test different configurations and see the impact on the conceptual estimate.
    • Edit the conceptual mass family and add parameters to quickly change the length, width, and height of the mass.
    • Reload the mass family into your project and flex the new parameters to see the impact on the conceptual cost estimate.
    • Set up design options to enable you to test and display conceptual estimates for three different length/width/height configurations.
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Figure 7.3.2. Sheet view showing design options and their corresponding conceptual estimates

Using Preliminary Cost Estimates to Inform Design

In this exercise, you will learn how to:

  • Convert the surfaces of a conceptual mass model into wall, floor, and roof building elements.
  • Create schedules and material takeoffs to tabulate key quantities of building elements.
  • Add parameters and conditional formulas to compute preliminary cost estimates.
  • Using preliminary estimates to provide feedback for continued design.
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Figure 7.3.3. Converting surfaces of a conceptual mass into building elements

Video Tutorial
Student Exercise
  • Create schedules of the wall, floor, and roof elements in the building model contained in the dataset.
    • Include fields showing the type and key dimensions of the elements (length, height, and area – as applicable).
    • Group the items in each schedule by type and add subtotals for the Area parameters in the group footers.
  • Add new parameters to these schedules to convert the model quantities into preliminary cost estimates.
    • Add a shared parameter for recording a preliminary unit cost (per square foot) and associate this shared parameter with the wall, floor, and roof elements as a type parameter.
    • Add this preliminary unit cost parameter to each of the schedules.
    • Add another new parameter – a calculated value multiplying the preliminary unit cost by the area of each element – to compute a preliminary cost estimate for each element.
    • Look up preliminary cost data ($ per square foot) for each of the wall, floor, and roof types in the building model.
    • Enter these values quickly by selecting the appropriate cells in the schedules, and entering the values there. Since the preliminary unit cost is a type parameter, the value entered will be applied to all elements of that type.
    • Add subtotals for the preliminary cost estimate parameter to the group footers to report a subtotal by each type and a grand total for all the wall, floor, and roof elements.
  • Create design options and use schedules to quickly compare the preliminary cost impacts as you vary the size, shape, and types of the model elements in each option.
    • Create duplicate of the preliminary cost estimate schedules for each design option and use the visibility graphics overrides to display the appropriate data for each option.
    • Place the preliminary cost estimate schedules for several options side-by-side on a sheet to easily compare the estimates for the different alternatives.
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Figure 7.3.4. Schedule view showing part materials takeoff

Creating a Detailed Quantity Takeoff

In this exercise, you will learn how to:

  • Transfer a Revit project model to Autodesk Quantity Takeoff using the DWF file format.
  • Choose which model elements to takeoff and quantify.
  • Extract quantities from 2D views and the 3D project model.
  • Build a catalog and create formulas for estimating the cost of different types of elements (each, LF, SF, and CF).
  • Apply cost data to takeoff items.
  • Summarize takeoff quantities in Autodesk Quantity Takeoff and by exporting to spreadsheets or cost estimation software.
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Figure 7.3.5. Extracting quantities from a Revit project model

Video Tutorial
Student Exercise
  • Use Autodesk Quantity Takeoff to tabulate key quantities for the building model contained in the dataset including:
    • Walls and curtain walls
    • Floors
    • Roofs
    • Doors and windows
    • Structural framing
    • Stairs and railings
  • Create a report summarizing these key quantities and export it for analysis in spreadsheet or cost estimating software.
  • Create an estimate within Autodesk Quantity Takeoff:
    • Add formulas appropriate for estimating each of these types of building elements to the workbook.
    • Enter cost data from online or printed cost estimating sourcebooks for each of the key items listed above.
    • Create a report summarizing the cost estimate.
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Figure 7.3.6. Autodesk Quantity Takeoff workbook with computed estimates for key cost items

Assessment

  • What is the advantage of using design options in Revit for comparing and analyzing conceptual designs?

By using design options, you can easily apply any special views or schedules that you define to analyze you design to evaluate each of the alternatives. Using this approach, you create master views that can be applied to all design options, then update and enhance them in one place. To apply these views to specific design alternatives, duplicate the view and adjust the visibility graphics overrides for that view to display the desired design option.  You can also display the views and schedules for several design options side-by-side on a sheet for each comparison and presentation.

  • Why is using conceptual masses a better approach for conceptual design?

Conceptual masses enable you to quickly model and explore different building shapes, massing, and placement—some of the most important decisions to be made during conceptual design—without getting distracted by the details of modeling and editing individual building elements. You can easily convert the faces of the conceptual mass into building elements after deciding upon the desired shape.

  • Why are the advantages of using conceptual mass families versus in-place masses?

Conceptual mass families offers several advantages: 1) they are repeatable and can be used in many projects; and more importantly, 2) they enable you to add parameters that control the shape and critical dimensions of the mass, so you can quickly test and evaluate different many alternatives. Using these parameters, you can continue to resize and reshape the conceptual mass and update any building elements that based on its faces.

  • What is the advantage of using parts when creating material takeoffs?

Creating parts from multi-layers building elements (for example, floors, walls, roofs, and ceilings) gives more accurate quantities for each of the different material layers. Without parts, all of the material layers are reported as the same surface area. Using parts, each of the layers is reported with an accurate area that reflects the corner wrapping and joining conditions, which can be a large difference when quantifying the innermost out outermost layers of a thick wall type.

  • When should you use a schedule of building elements versus a material takeoff?

Schedules of building elements and material takeoffs are alike in some ways, but have a different focus. So, both are useful, depending upon the answer you need. Schedules of elements display and summarize information about specific categories of elements—for example, walls, floor, curtain panels, and fixtures. To build an estimate, you choose the categories of elements that are most important and create a schedule for each of those categories, reporting the information that is relevant for that type of element. Material takeoffs aggregate information from all of the elements (across all categories) that are assigned to use a specific material. You can filter and group the takeoff to show and summarize specific categories of interest. Which one to use depends on the answer you seek. If the answer is based on tallying pieces, schedules of elements will likely serve your need. If it is based on material quantities, then a material takeoff is likely the better tool to use.

  • What types of building elements are typically quantified... Using counts? Using length? Using area? Using volume?

Counts (each) are typically used to quantity components that are installed as individual units—for example, doors, windows, skylights, fixtures, furniture, lighting, structural framing elements, and so on. Length (LF) is used to quantify elements where the cost will depend on the length installed—for example, railing, piping, fascia and so on. Area (SF) is used to describe element with a common thickness, where the cost will depend on the area installed—for example, wall assemblies, roof assemblies, material surfaces, and so on. Volume (CF) is typically used to quantify elements whose shape and volume will determined by external constraints—for example, cast-in-place concrete or expanding foam insulation.

  • What are the best sources for cost data?

The best source for cost data is a firm's historical records. The actual data from past projects takes into account all of the specific features of a firm's techniques, construction strategy, and management style, so it is by far the most accurate predictor of future costs. For this reason, it is vitally important for firms to keep accurate cost accounting records for current projects. The data affects not only the current project, but also impacts the reliability of future predictions. When historical data is not available, estimators typically rely on external cost sourcebooks and online databases for cost information. When external cost sources are used, it is critical to adapt and scale the numbers provided to accurately reflect the specific project conditions and differences from the norms. For example, it is common to apply adjustments that consider the effects of project scale, project location, local labor costs differences, materials cost differences, and appropriate inflation factors.

  • What factors should be considered when comparing projects to establish conceptual cost metrics?

When using comparable building projects to establish cost metrics to be used in preparing a conceptual estimate, it is critical to adapt and scale the data to accurately the new project conditions and differences from the projects used as the basis for comparison. For example, it is common to apply adjustments that consider the effects of differences in project scale, project location, uses and functions, and appropriate inflation factors. Conceptual estimating is both an art and a science. But, experienced estimators can create amazing accurate conceptual estimates to confirm the feasibility of a proposed design and provide a target value for continuing design work.

Key Terms

Key Term
Definition
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.
Conceptual Estimate
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 Estimate
A rough estimate made in an early stage of the design work, prior to the completion of detailed design or the receipt of firm bids. 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.
Detailed Estimate
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.