Model-Based Estimating and Quantity Takeoff using Navisworks

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 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:

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 (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:

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.

After completing this lesson, students will be able to:

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.

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.

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.

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.

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.

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.

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.

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.