Today, let’s discuss the commonly used BIM system information exchange standards available. To ensure complete and efficient information exchange between different BIM systems, a standardized information exchange protocol is essential. If the BIM system represents the body, then this information exchange standard is its soul. Such a standard must be capable of describing information across every stage of the building lifecycle and be supported by various BIM systems. Currently, the widely accepted standards among BIM system developers include STEP (Standard for the Exchange of Product Model Data), IFC (Industry Foundation Classes), and CIS/2 (CIMsteel Integration Standards Release 2).
1. STEP
STEP, officially ISO 10303, is a product information exchange standard developed by the International Organization for Standardization (ISO). Its goal is to provide a comprehensive mechanism to describe product information throughout the entire product lifecycle, facilitating seamless information exchange. STEP covers a broad range of industries including manufacturing, construction, electromechanical, shipbuilding, and aviation.
The ISO Technical Committee 184, Subcommittee 4 (ISO TC184/SC4) oversees the maintenance and periodic revision of STEP standards. STEP is divided into multiple parts, organized into nine groups as follows:
1. Description Methods
Parts 1-19 specify the STEP framework, including Part 11, which introduces the EXPRESS data modeling language. EXPRESS supports object-oriented concepts for information description, with syntax similar to the PASCAL programming language. It is independent of any specific programming language and serves as a pseudocode-like tool to describe data models. EXPRESS is also used to define IFC and CIS/2 specifications.
2. Implementation Methods
Parts 20-29 guide the implementation of STEP standards. Notably, Part 21 defines the STEP Physical File format, which represents the information model as a physical file. Both IFC and CIS/2 adopt this format to represent their models.
3. Conformity Testing Methodology and Framework
Parts 30-39 specify testing methods to validate data compliance with STEP standards after design.
4. Integrated Generic Resources
Parts 40-49 describe generic resources such as shapes, sizes, and units.
5. Integrated Application Resources
Parts 100-199 define domain-specific resources like beams, columns, and walls within construction.
6. Application Protocols
Parts 200-299 specify detailed requirements for applying the standard in specific domains.
7. Abstract Test Suites
Parts 300-399 are used to test conformity with application protocols.
8. Application Interpreted Constructs
Parts 500-599 assist in developing new data models.
9. Application Modules
Parts 1000 and beyond define small information models used for creating new application protocols.
2. IFC (Industry Foundation Classes)
IFC is an open information exchange standard tailored for the Architecture, Engineering, Construction, and Facility Management (AEC/FM) sector. It was proposed and is maintained by the International Alliance for Interoperability (IAI), now known as buildingSMART. IFC aims to enable software across the building lifecycle to represent building information consistently, improving data exchange and reuse. Since its initial version 1.5, IFC has evolved to the current version 2×3 Final, which is the version used in this discussion.
IFC Architecture
IFC is based on object-oriented principles, referencing the STEP standard and incorporating features inspired by C/C++ for implementation. The IFC architecture is structured into four layers, from bottom to top:
Resource Layer
This foundational layer defines generic entities, similar to STEP’s integrated general resources. These entities are usually referenced by higher layers. For example, a product references its geometric shape, which is only defined once the product exists. Exceptions include utilities and measurement resources, which can exist independently.
Core Layer
This layer defines the fundamental IFC entities and common interfaces that can be referenced or inherited by upper layers. It consists of two sublayers: the Kernel layer and the Product Extension layer. The Kernel layer defines basic entities applicable beyond AEC/FM, which only reference entities in the Resource layer. Examples include the “Product” entity with attributes like “Location” and “Shape Description.” The Product Extension layer builds on the Kernel layer with entities specific to the AEC/FM domain, such as building components.
Information Exchange Layer
This layer defines entities commonly exchanged within the AEC/FM domain, such as beams, columns, doors, windows, and spaces. It also allows professional disciplines to attach domain-specific data to these entities, including material properties, construction dates, or structural analysis results.
Professional Field Layer
This layer captures specialized entities for various professional fields within AEC/FM, including architecture, structural analysis, construction management, facility management, mechanical, electrical, plumbing, and HVAC systems. Entities here include structural analysis models, constraints, and construction management resources. This layer is still under development due to the complexity and diversity of disciplines involved.
IFC 2x follows the Gravity Principle, which dictates that entities can only reference entities at the same or lower architectural layers. For example, Resource layer entities cannot reference Core layer entities. This ensures a clear, hierarchical structure.
Many software vendors recognize the IFC standard and actively support IFC model import and export. Numerous applications have passed IAI’s IFC compatibility testing and are listed on the IAI Implementer Support Group (ISG) website. Although software support levels vary, IFC adoption continues to grow as the standard matures. Further details on IFC specifications and implementations will be explored in Chapter 3 of this study.
3. CIS/2 (CIMsteel Integration Standards Release 2)
CIS/2, developed by the National Institute of Standards and Technology (NIST), is a standard designed specifically for steel structure building information exchange. It supports the entire steel lifecycle, including analysis, design, detailed design, and manufacturing. CIS/2, like IFC, uses the EXPRESS language for its specifications.
CIS/2 is composed of three main models:
Analysis Model
Contains numerous nodes and elements to support various static and dynamic structural analyses.
Design Model
Includes design assemblies, which can be broken down into design parts and joint systems.
Manufacturing Model
Comprises manufacturing assemblies used for detailed design, planning, and fabrication.
Many software applications support CIS/2. NIST has also developed a “CIS/2 to VRML and IFC Translator” tool, enabling conversion of CIS/2 files into IFC or VRML (Virtual Reality Modeling Language) formats.
That concludes our overview of commonly used BIM system information exchange standards. I hope this article helps you gain a clearer understanding of these essential standards!
Must log in before commenting!
Sign Up