×

Building Design

1-2-3 Revit: BIM and Cost Estimating, Part 1

6 Aug, 2006 By: AIA ,Rick Rundell

---

How BIM can support cost estimating.

A purpose-built BIM (building information modeling) solution such as Autodesk Revit features a computable BIM: a design model that can be understood by a computer as a building. A wall for example, knows what it is and how to react to the rest of the building. As such, it can be scheduled or quantified as a wall and that it's an assembly made of real materials.

Computable BIMs enable firms to create, manage and share design information more effectively. Some previous articles featured examples of the uses of computable building information, including: structural analysis, MEP system modeling, building energy analysis and specification management.

This month I begin a two-part series exploring how reliable data held within a computable BIM can be used to support yet another aspect of the building process -- cost estimating.

Model-based Estimating
Designing a building is the responsibility of architects. Assessing the cost to build it is the domain of estimators. In general, the architect's scope of work doesn't extend to material takeoffs or cost information. That's left to the estimator.

When preparing cost estimates, estimators typically begin by digitizing the architect's paper drawings, importing CAD drawings into a cost estimating package or doing manual takeoffs from their drawings. All of these methods introduce the potential for human error and propagate any inaccuracies there may be in the original drawings.

By using a BIM instead of drawings, users can generate the takeoffs, counts and measurements directly from the underlying model. Therefore the information is always consistent with the design. When a change is made in the design, 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.

The time spent by the estimator on quantification varies by project, but around 50% to 80% of the time needed to create a cost estimate is spent on quantification. Given those numbers, you can instantly appreciate the huge advantage of using a BIM for cost estimating -- the elimination of manual takeoffs saves time, cost and reduces potential for human error. In fact a common complaint from estimating firms is how much they hate paying estimators to simply count or quantify when they bring so much more expertise and experience to the table.

By automating the tedious task of quantifying, BIM allows estimators to use that time instead to focus on higher value project-specific factors -- identifying construction assemblies, generating pricing, factoring risks and so forth -- that are essential for high-quality estimates. For example, consider a commercial project slated for construction in northern Minnesota in the winter. The estimator knows that winter heating and dewatering is needed for a portion of the concrete substructure. This is the sort of specialized knowledge only professional estimators can factor into the cost estimate accurately. This construction wisdom, not counting, is the real value they bring to the cost estimating process.

Many Paths from BIM to Costing
There are a variety of ways to get quantities and material definitions out of a BIM into a cost estimating system. There's no right or wrong approach -- each path is based on the estimating workflow used by a specific firm, the costing solutions they have in place, the pricing databases they use and so on.

Broad categories of integration approaches include:

• API (application programming interface) to commercially available estimating programs from vendors such as U.S. COST or Innovaya (which integrates with Sage Timberline Office Estimating). . This approach uses a direct link between the costing system and Revit. From within Revit, a user exports the building model using the costing program's data format and sends it to the estimator, who then opens it with the costing solution to begin the costing process.

• ODBC connection to estimating programs, such as CostX or ITALSOFT -- popular Australian and Italian cost estimating solutions, respectively. ODBC is a tried and true standard, useful for integrating data-centric applications such as specification management and cost estimating with BIM. These approaches typically use the ODBC database to access the attribute information in the building model, and then use exported 2D or 3D CAD files to access the dimensional data. Part of the integration includes a reconstitution of the building data within the costing solution -- linking cost geometry, attributes and pricing.

• Output to Excel. In comparison to the approaches outlined above, quantity takeoffs done within Revit and output to Excel may seem lackluster, but the simplicity and control is perfectly suited to some costing workflows. For instance, many firms just create material takeoffs in Revit, output the data to a spreadsheet, and then hand it off to the cost estimator.

Better Numbers
Ryan Companies is a national commercial real estate firm offering integrated design-build and development as well as asset, property and facilities management services to its customers. With offices throughout the United States, Ryan uses BIM to accurately coordinate building information within the documentation set and provide its estimators and construction team with solid, up-to-date building information.

As a design/build firm, Ryan was an excellent candidate for adopting a model-based approach to estimating because design and costing are done under one organizational roof, minimizing scope of work issues related to adding information to the building model and takeoffs. Established Revit users, it has just started to use Innovaya's Visual BIM and Quantification products.

Dick Bates, Ryan's CAD manager, uses this example to explain the difficulties associated with quantification from a design model. "For any given wall, we need to calculate three numbers to get a price: the gross area of one side of the wall to get material quantities totaled for each wall type, and then the double gross and net areas to quantify finishes." The programmatic link between the two solutions means that Innovaya's solution can count and calculate materials directly from the Revit BIM. The quantity extraction methods are easily customized (for instance, calculating the three wall areas cited in the example above from a single design element) and are reusable for multiple projects. Quantities are extracted based on building element types and their dimensions. Once the quantification is complete, Ryan's estimator outputs the data from Innovaya to an Excel spreadsheet for manual entry into its in-house pricing program.

With the API link, the quantities are automatically associated with the Revit building, so they remember where they came from. This means that the Innovaya software can easily detect design changes from the Revit model, allowing the estimator to validate previous quantities against the new design and then automatically update those quantities. As Bates explains, "In the past, our estimators had to manually check design revisions for cost implications. Now we just re-export the Revit model to Innovaya, and let the costing software figure out what has changed."

According to Bates, "We're very excited about the integration between Revit and Innovaya. All the data we need for high-quality costing is in the Revit model. With this integration, our estimators are getting better numbers from the building model, and we can continue to use each program for its intended purpose: Revit for building design and Innovaya for construction estimating."

Coming Attractions
Using a BIM for quantification leads to faster, more accurate cost estimates and updates. Next month I'll detail some other examples of BIM-to-costing workflows, featuring other Revit customers, integration approaches and partner products.