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AEC

BIM and Digital Fabrication (1-2-3 Revit Tutorial)

31 Jan, 2008 By: AIA ,Rick Rundell

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BIM enables a digital design-to-fabrication workflow.

According to data compiled by Stanford University's Center for Integrated Facility Engineering (CIFE) from the U.S. Bureau of Labor Statistics and Department of Commerce, the building construction industry continues to experience falling productivity rates that seem to be getting worse -- not better (Paul Teicholz, "Labor Productivity Declines in the Construction Industry: Causes and Remedies," AECbytes.com, April 14, 2004). This data points to fundamental productivity problems facing the building industry and is one of the major causes of unpredictable building outcomes.

Using building information modeling (BIM) to describe the design of a building can facilitate a variety of related building activities, including building construction and digital fabrication of building components. BIM is enabling digital design-to-fabrication workflows for all the building disciplines. This month's column examines this trend by focusing on how structural BIM is being used for the digital fabrication of structural steel and the role of BIM in that digital supply chain.

Model-Based Design and Manufacturing
For decades the manufacturing industry has used mechanical CAD systems to create digital models. These models not only describe the product design, they also are used for other applications such as stress analysis, field support, and, of course, manufacturing. CAD models are used to generate files that control computer numerical control (CNC) machines, thus increasing the automation of mechanical manufacturing processes.

Similar approaches also can be used by the building industry to automate building construction processes. Although a building isn't manufactured in its entirety and then shipped to the owner (as is a car, for example), many integral parts of a building are manufactured remotely and then shipped to the building construction site and assembled in the context of the overall building. Some common examples include building components such as doors and windows, precast concrete structures, and structural steel.

Steel Fabrication 101
To understand how BIM can be used to automate structural steel fabrication, it's important to understand how the structural steel components that make up a building's frame are created. First a steel mill typically uses a hot-rolling manufacturing process to create stock structural steel members. This stock material is purchased by steel fabricators who cut and prepare the stock structural beams and columns for building construction based on shop drawings -- instructions that describe exactly how to fabricate each individual piece of a structure. After they are fabricated, the steel members are shipped to the building site and put in place by steel erectors.

Where do shop drawings come from? The role of the structural engineer is to design, analyze, and certify a building's structural frame and then create construction drawings that document the structural design. The structural drawings contain only general requirements for steel fabrication -- instructions for typical steel connections. A steel detailer then takes those construction drawings and applies those general connection instructions to the specific structural components and the specific geometry of the building as represented in the construction drawings -- creating shop drawings that instruct the steel fabricator exactly how to fabricate each piece of steel in the building. Shop drawings include detailed information pertaining to material specifications, sizes, dimensions, welding, bolting, surface preparation, painting requirements, etc.

Shop drawings are either created with drafting software or with special steel detailing software such as Autodesk's Robobat RCAD -- a solution that uses a digital fabrication model to create steel details.

It's interesting to note that the number of shop drawings for a building project far exceeds the number of construction drawings. For example, documenting the steel frame of a 1,000-ton building (1,000 tons of steel) requires approximately 70 to 80 construction drawings versus 1,000 shop drawings. Prior to the release of the shop drawings for fabrication, the structural engineer checks each one to verify that the information matches the structural design.

Steel fabricators generally use CNC beam line machines that automatically cut and drill a beam. Some fabricators hand program the CNC machine based on the information from the shop drawing. Other fabricators use the digital fabrication models mentioned above (which are used to create the shop drawings) to automatically program the machine.

Extending BIM to Fabrication
How does BIM fit into all of this? Just as CAD-based models in manufacturing can feed the manufacturing process, a purpose-built building information model such as Revit Structure can feed the structural fabrication process. All the geometry regarding the structural steel is already in the Revit Structure design model. This design information can be exported to a CIS/2 file (an industry standard data format for exchanging steel information) for reuse in a steel detailing application.

The use of BIM for steel detailing and fabrication enables a fully digital, design-to-manufacturing process. Reusing the design model in this fashion is inherently more efficient (time normally spent creating a fabrication model is eliminated) and produces higher quality results (discrepancies between the design and fabrication models are eliminated). In addition, the source of the information used in the steel detailing and fabrication software is digital design data based on a highly accurate, coordinated, consistent BIM -- data worth sharing for related building activities.

After the steel details are complete, the fabrication model can be used by the design team or the contractor for the purposes of 4D modeling as well as clash detection with other building disciplines and models (MEP, architectural, etc.). The fabrication model doesn't necessarily represent as-built conditions; changes can occur during the steel erection process. But it does contain more detail than the structural model, and therefore it can be useful for interference checking, especially for building types or application where space is extremely tight.

Another advantage of using BIM in the structural supply chain has to do with the overall cost of the structural frame. In the past that cost has, in general, been equally distributed between raw material, fabrication, and erection. But in recent years, the proportionate cost is rising for fabricated steel and steel erection -- a trend that could be halted by considering simplicity of fabrication during the design process (another parallel to the manufacturing world and its emphasis on manufacturability). Using the design model directly for fabrication will create a natural feedback loop between fabricators and designers -- and bring fabrication considerations forward into the building design process. Sharing the design model with fabricators for bidding will shorten the bid cycle and lead to more uniform bids based on consistent steel tonnages. And the coordination between the fabricated steel and the other building components will reduce the on-site issues and drive down the rising cost of steel erection.

An important ingredient to a digital design-to-fabrication process is some form of collaboration between the structural engineer, the steel detailer, and the steel fabricator. In most cases, those three parties represent three different companies. The link between design and fabrication is therefore facilitated by alternate project delivery approaches where cross-functional project teams representing owners, builders, engineers, and contractors collaborate through all phases of design, fabrication, and construction. Steps that were once sequential (design, detail, and fabricate) can be done more concurrently. Thus the design model and shop drawings can be created in tandem. With the shop drawings done sooner, the mill order can be submitted sooner, the fabrication can start sooner, and the steel can be erected sooner.

Digital Fabrication in Action
One company at the vanguard of using BIM for digital fabrication is Rutherford and Chekene (R&C). Founded in 1960 and headquartered in San Francisco, this multidisciplinary firm provides structural and geotechnical engineering services for a broad range of buildings including hospitals, sports facilities, museums, historic buildings, and even aquariums. A Revit Structure user since 2005, R&C currently uses BIM for design, analysis (in conjunction with RAM and ETABS), and now for digital fabrication as well.

One of their most recent Revit projects is the Sutter General Hospital Sacramento, a 425,000-square foot, 11-story replacement facility for the existing hospital. The building represents more than 5,000 tons of steel, which will necessitate roughly 5,000 shop drawings. On this project, R&C has a partnering relationship with both the steel detailer (Dowco Consultants) and the fabricator (Herrick Steel) and is working closely with construction company (Turner Construction) as well.

"The relative expense of hospitals -- roughly $600 to $800 per square foot compared to $150 to $250 for commercial building -- makes them a good candidate for alternate project delivery models and relationships," explained David Bleiman, SE, principal at R&C. "We're currently in the planning stages of how we'll all work together on this project, but our goal is to keep all the data digital for as long as possible and use digital data exchanges whenever possible."

Construction for the entire site started early in 2007 and will be complete in 2012. The work is phased over time, as replacement facilities come on line allowing demolitions to occur that provide space for more new facilities. As a result, collaboration methods and workflows among the various stakeholders on this project are also being phased in.

"Phase one is comparing the rough fabrication model (in this case created using Tekla software) to our Revit Structure model," Bleiman reported. The fabrication model was already underway before R&C and the steel detailer started their collaboration efforts, so the two models have to be manually coordinated. "We have to walk before we run, but the end result will be the same: a fabrication model that matches the design model and everyone working together during structural design, fabrication, and erection to reduce costs and improve delivery schedules," Bleiman explained. Any questions that stem from original design -- dimensions, sizes, locations, etc. -- will be rectified earlier in the design process and result in fewer RFIs and change orders.

After the steel detailing gets underway, the second phase of the plan will begin: A completely digital review of the structural steel shop drawings. Shop drawing review -- normally done by hand and entailing printing, shipping, and checking multiple copies of each fabricated piece drawing -- will be done electronically within the steel detailing software used to originate the shop drawings, eliminating the paperwork that would total approximately 30,000 sheets of drawings for a project this size. In addition R&C will use Autodesk NavisWorks to compare and coordinate the completed fabrication model and the original design model. Turner also will be using Autodesk NavisWorks to combine the structural fabrication model with models of other building disciplines for the purposes of clash detection.

Summary
The productivity currently enjoyed by the manufacturing sector was made possible in part by the use of digital data models to automate manufacturing methods. BIM and digital fabrication has the potential to do the same for the building industry.

Like integrated project delivery, digital fabrication is just starting to gain traction within the industry. "We all realize that there are challenges that need to be addressed," Bleiman concluded. "But from my viewpoint, BIM solutions are definitely paving the way towards leaner building construction processes and more controlled project outcomes."