AEC

Modeling Technology for Building Engineers (AEC Insight Column)

1 Aug, 2008 By: Jerry Laiserin

A specialized BIM workflow suits structure and building services.


Much of the initial attention paid to building information modeling (BIM) technology has focused on architectural design and the architect's workflow. It can be traced back to the earliest published work on the tools and methods that we now know as BIM, such as Chuck Eastman's 1975 architectural journal article titled "Designing with Computers Instead of Drawings." The concept then, as now, entails architects designing, documenting, and delivering their projects through internally consistent, multidimensional computer models rather than potentially inconsistent 2D paper drawings (or their digital/CAD equivalents).

As the architecture profession struggles to adapt to the radically different design workflows and business processes required by computer modeling, the engineers who serve architects and architectural projects face a different set of challenges. Whether engaged in structural design or building services design (mechanical, electrical, and plumbing, or MEP), building engineers' design and documentation workflow traditionally has differed from that of their architect colleagues.

Much of building engineering work starts with information imported from architects' design files, whether in the form of column grids for structural design or reflected ceiling plans and the like for MEP design. Building engineers then use the architectural information (primarily building geometry) as input for analysis programs (whether of structural loads, heating and cooling loads, or the like). The calculated results of these analysis programs typically are applied to the sizing of engineered components, whether structural members, heating and cooling systems, or other building equipment. Engineers then may aggregate loads and size connections such as structural joints and MEP distribution systems, then detail the actual elements in which the discipline-specific design is implemented, such as structural framing elements, ductwork, piping, and so on.

As practiced in the United States, several steps in this process might be performed by engineers working with or for construction subcontractors or fabricators.

Multiple Tools, Multiple Models

The notion of a single BIM modeling tool serving the needs of a building engineer, whatever discipline, simply isn't applicable in the same way that a single model-authoring tool can serve an architect's BIM needs. In most instances, especially in the Autodesk and Bentley BIM model-authoring suites, the structural modeler and the MEP modeler or modelers are software tools that serve more as containers or carriers of architecture-derived information used in engineering analysis. Although the transport mechanism of, say, Autodesk Revit Structure, may represent some degree of innovation over the traditional transmission of column grids by 2D drawings, the subsequent workflow surrounding analysis, sizing of members, and so on often is unchanged or only slightly changed from traditional practice. Ironically, many engineering analysis tools were model-based long before architects began to adopt model-based design for their own processes.

In fact, one of the key roles played by programs such as Revit Structure and Revit MEP is the reformatting of building information from the architectural model-authoring tool (Revit Architecture) into a an arrangement suitable for input to pre-BIM and/or BIM-adjacent engineering analysis tools.
In an architectural BIM model, for example, a column typically appears as a spatial object composed of the actual structural element (steel, concrete, or timber) surrounded by appropriate fireproofing and/or blast-proofing and its finish materials. In a structural analysis model, however, that same column most often is represented by its centerline as the basis for the application of loads and forces. Similarly, the representation of concrete in an architectural BIM model-authoring tool typically omits reinforcing steel within concrete elements and treats an entire concrete structure as if it were a single, monolithic element, rather than a composite assembly made of numerous lifts and pours.

Round-Trip Transportation

On the MEP side, architectural BIM models rarely reflect the kind of partitioning into spatial volumes and surfaces that is required as input to whole-building energy analysis packages such as the U.S. Department of Energy's EnergyPlus energy simulation software. This repartitioning of architectural models into an arrangement suitable for analysis is one of the key functions of software such as Revit MEP, but the actual engineering analysis likely is performed in a program such as IES Virtual Environment (VE, figure 1) — a tool that contains its own modeling capabilities and can perform very nicely and completely without input from Revit (or any other architectural or MEP model-authoring tool).

 Figure 1. MEP analysis techniques such as computational fluid dynamics can be applied to highly sophisticated tasks, such as calculating airflows across a building atrium, within a BIM workflow or independently by tools such as IES Virtual Environment. (Courtesy of IES)
Figure 1. MEP analysis techniques such as computational fluid dynamics can be applied to highly sophisticated tasks, such as calculating airflows across a building atrium, within a BIM workflow or independently by tools such as IES Virtual Environment. (Courtesy of IES)

Interestingly, for both structural and MEP analysis, the reverse workflow doesn't hold true. In other words, an MEP engineer can accomplish most if not all the necessary design and documentation workflow in tools such as Design Master or IES VE, absent Revit MEP, but cannot accomplish that complete workflow in Revit MEP absent an analysis and documentation package. What the building engineering flavors of BIM model-authoring tools really provide is the ability to round-trip building information from architectural models to engineering analysis tools and back again with engineering-sized components to reintegrate and coordinate with the architectural model.

Some tools now offer extended capabilities to integrate certain aspects of engineering analysis directly into the architectural design process for more interactive feedback to architects about the engineering effects of their design decisions. This approach was pioneered by Ecotect and by Green Building Studio (both now owned by Autodesk), as well as the various toolkits offered by IES as plug-ins to Revit MEP or Revit Architecture.

Structural Strictures

On the structural side, where many analysis packages are available, a trend for CAD/BIM vendors has been to snap up previously independent software programs. In recent years, Bentley has acquired the likes of RAM and STAAD, and Autodesk picked up Robobat. Because engineers may favor one analysis package for their internal workflow, yet have a model-authoring suite dictated by the project, these acquisitions have led to some interesting mix-and-match possibilities. In fact, facilitating cross-platform workflows was among the initial motivating factors for efforts in interoperability such as the Industry Foundation Classes (IFC) standard, now popularly known by a catchier name: buildingSMART.

Tekla makes one of the most interoperable structural solutions that addresses detailing and fabrication more specifically than analysis and modeling, per se. In fact, Tekla often is an integral part of the standard workflow from design engineer to detail engineer, fabricator, and erector — independent of other analysis and modeling tools — whether in precast concrete, cast-in-place concrete, or steel structures (the latter in accordance with the CIS-2 standard for steel design and detailing workflow).

BIM engineering modeling tools also provide the ability to reintegrate engineer-designed building content with the architectural BIM model, for purposes of clash detection and the like. Of course, many projects neither need nor can justify modeling or remodeling of all engineered content in a multidiscipline, coordinated BIM model. For example, flexible ducting, piping smaller than a certain diameter, and most electrical distribution systems (figure 2) typically are not modeled as full 3D components.

Overall, the extent to which building engineers integrate BIM automation tools into their workflow depends at least as much on practical choices about round-trip information exchanges with architects using BIM model-authoring tools as on any inherent benefits to themselves.

Figure 2. After electrical loads have been calculated as part of a BIM workflow, electrical distribution systems still are documented with traditional one-line diagrams. Here, Design Master Electrical integrates AutoCAD drafting data directly with engineering calculations. (Courtesy of Design Master Software)
Figure 2. After electrical loads have been calculated as part of a BIM workflow, electrical distribution systems still are documented with traditional one-line diagrams. Here, Design Master Electrical integrates AutoCAD drafting data directly with engineering calculations. (Courtesy of Design Master Software)


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