AEC from the Ground Up-CAD Data Standards for AEC

Collaboration in AEC depends on mandated or de facto standards.

We are now in the third decade of digital AEC design and documentation, enough time for the industry to create or adopt both actual and de facto standards. Among these are standards for raster or bit-mapped images, document compression schemes such as Adobe PDF and Autodesk DWF and vector information used in construction documents.

In the digital world, the appropriateness of any particular standard is mostly a function of what you want to do with it. As an example, ASCII is perfectly appropriate for representing the English alphabet, but is useless for representing Kanji, Katakana, Cyrillic and other writing systems. A standard called Unicode has been developed to deal with them.

Figure 1. Adobe Acrobat 3D software lets engineering and technical professionals publish and share 3D design information from major CAD applications.

Standards such as JPEG, TIFF and IFC (Industry Foundation Classes) are standards that were created by committees and adopted by industry organizations. A de facto standard such as DWG or PDF becomes a standard not through the blessing of a standards organization but by broad grassroots adoption and recognition throughout an industry.

Some standards are unique to AEC: layering standards such as NCS (National CAD Standard) and steel detailing and integration standards such as CIS/2. We even have a de facto green building standard called gbXML. gbXML became the draft schema for the International Alliance for Interoperability's Building Performance and Analysis working group.

In this article

This article is not a defense of any standard, but rather an overview of the common data formats in the AEC world.

Autodesk and DWG

DWG is the native format for Autodesk's AutoCAD-based products, which, according to Autodesk, have more than seven million users worldwide. Autodesk has never documented the DWG format, but it does provide a software development kit, RealDWG, that third-party developers can use to add read/write support for DWG to their applications. It's used internally by Autodesk to provide DWG support in non-AutoCAD products such as VIZ, Revit and Inventor.

Open Design Alliance and DWG

The Open Design Alliance develops software libraries to enable import and export of DWG files. These libraries are used by many of Autodesk's customers and competitors. Formerly known as the OpenDWG Alliance, the Open Design Alliance was founded as a nonprofit membership-based consortium by software developers, including Visio, ESRI, Bentley and SolidWorks.

Figure 2. Autodesk Design Review, formerly known as Autodesk DWF Composer, has many new features and functions added after requests from building industry customers.

Open Design Alliance president Evan Yares is very vocal about the DWG format:

The most important container format in the AEC CAD industry is DWG. It may seem odd that I'd refer to it as a container format, but DWG files can contain text, various forms of images, 2D linework, 3D models and more. While we tend to talk about DWG as a single monolithic format, the reality is that there are at least 30 variants of DWG out there. This situation presents an almost intractable problem when DWG is used as the primary conduit of design data in an extended project lifecycle. Downstream consumers of design data can never be sure if they'll actually be able to use the DWG files they're getting.

The Open Design Alliance doesn't have any kind of perfect solution to this problem. The best we can offer is a pragmatic solution: we have 15 developers who work full time to create libraries to accurately read and write DWG files.

Figure 3. The intent of the IFC is to provide a means of passing complete, accurate building data model from the computer application used by one participant to another without any loss of information.

Yares believes there is a need for an archivable version of the DWG, one that assures readability over a long time period.

The Open Design Alliance's DWGdirect libraries, based on OpenDWG (its documented version of DWG), are supported by hundreds of software vendors and used by millions of engineers, designers and architects. Within the next few weeks, the alliance will deliver updated libraries to support the 2007 version of DWG. The alliance also offers DGNdirect libraries that support import and export of MicroStation's DGN format.

Compressed Document Formats

These formats foster design collaboration by reducing file size for more efficient document exchange and enabling those without the originating CAD application to view an accurate version of the original drawing or model.

Adobe's PDF is more widely accepted for document transfer, but Autodesk's DWF format creates much smaller files for quicker transfer of DWGs.

Adobe PDF (portable document format). PDF is the open file format developed by Adobe Systems (www.adobe.com). It can describe documents containing any combination of text, graphics and images in a device- and resolution-independent format. A growing number of federal and state agencies, institutions and associations, including the FDA and U.S. courts, have mandated PDF-based workflows.

PDF has evolved variants such as PDF/A for archiving, PDF/X for high-end print workflows and PDF/E, a standard under development to enable the easy exchange of engineering information. PDF/E is expected to be ratified by ISO (International Organization for Standardization) next year.

PDF is broadly recognized and accessible. Adobe reports 34.5 million downloads of Adobe Reader 7 in its first four months of availability. Relatively new to the format is support for 3D models, specifically the U3D format. (For those interested, that's a lightweight 3D format developed by Intel and backed by a group called the 3D Industry Forum.) Initially, those who wanted to embed 3D models into PDF files needed a CAD application with U3D export capability. Adobe addressed this limitation with its Acrobat 3D product, which incorporates translation capabilities for many CAD formats.

DWF (design Web format) is Autodesk's open format for sharing complex designs. It supports engineering and architectural design information, including layers, while maintaining its integrity with files that are often one-tenth the size of other file formats. All Autodesk products export to DWF, and a free DWF Writer is available for use in other applications. To view and print DWF files, recipients need the free DWF viewer. For more extensive file markup and measurement, Autodesk offers Design Review (see for review).

National CAD Standards

Version 3.1 of NCS was published in January 2005, and work has begun on v4. Copies can be purchased from (NIBS) National Institute of Building Sciences at www.nationalcadstandard.org. Members pay $250, nonmembers pay $350 and academics pay $175. The standard is also available from Retrieve (www.retrievemedia.com) as a vBook, an online version where content is presented via a combination of traditional text, audio narration and screen-capture videos. Pricing is the same as for the hard copy, but covers 18 months use or 1,000 views, whichever comes first.

IFCs and the National BIM Standard

The IFC system is a data-representation standard for defining architectural and constructional CAD graphic data as 3D real-world objects, mainly so that architectural CAD users can transfer design data between different software programs (figure 3).

NIBS, through its Facility Information Council, is developing a national BIM standard using IFC as the transfer format. According to NIBS, the standard at minimum will cover:

• 1. BIM scope
• 2. Coverage of version
• 3. Reference standards
• 4. Business processes
• 5. Business rules
• 6. Data structures and models
• 7. Implementation guidance
• 8. Maturity model

The committee defines a building information model as a digital representation of physical and functional characteristics of a facility. Key concerns are the ability for all stakeholders to share and collaborate on the information contained in the model, hence the need for open standards.

H. Edward Goldberg, AIA, NCARB, is a practicing licensed architect and AEC industry analyst. Ed's full-length book, Autodesk Architectural Desktop 2006: A Comprehensive Tutorial (Prentice Hall, www.prenhall.com) is now available, and a 2007 version will come out this fall. Visit www.hegra.org or e-mail ed.goldberg@cadalyst.com

Check out Cadalyst's new Tech Forum: AEC (www.cadalyst.com/AECForum/), moderated by Ed Goldberg. The forum discusses software and subjects related to BIM for architects, engineers and contractors.