# A Systematic Review of the Trends and Advances in IFC Schema Extensions for BIM Interoperability
## 2 Research Objective and Methodology  
This study conducted a comprehensive review of prior research cases for IFC schema extensions with the goal of determining the extension needs and trends for different domains, identifying the extension approaches used based on the type of requirements, and categorizing the implementation and validation approaches. Specifically, this study aimed to address the following research questions:  
* What are the limitations of the existing standard IFC schema that the extension studies attempted to address? Extension studies typically focus on representing products, processes, and their concepts for a specific domain and sector that were not available in the IFC schema at the time of their research. Thus, categorizing and quantifying these studies with respect to their domain and purpose (i.e., product versus process) would show relative deficiencies of the schema and also highlight the needs of the industry. Conducting a timeline analysis would also provide insights into the temporal trends of how these needs were addressed throughout the period. * What are the main approaches used to extend the IFC schema with respect to their respective purposes? The cases also use different extension approaches, e.g., by creating entirely new entities or extending the properties of existing ones. The cases were thus evaluated to determine an appropriate category for the extension approaches to be deployed and subsequently cross-referenced with the purpose of their representations. This would allow generalizations, if any, to be formalized. For example, when representing new products, it is most likely to require new entities, whereas processes may only require adding property sets. * What are the main approaches used to implement and validate the IFC extensions? The corpus of cases was evaluated to determine appropriate categories for their implementation and validation approaches. For example, some cases only provided conceptual diagrams of their extensions, while others presented full software implementations. Some cases were limited to demonstrating data interoperability, while others went so far as to show the utility of the achieved exchanges. The distinctions provide insights as to the necessary levels of implementation and validations for acceptance. * What are the research gaps, insights, and their implications for future research? Addressing these questions allows to determine which domain and purposes still need developing, and which extension approach should be used for these extensions. It also provides the basis for recommending valid implementation and validation approaches in future extensions. To address these questions, this study collected publications from online academic publication databases, namely Web of Science and Scopus. The databases include the major journals in construction engineering and management, including ASCE's _Journal of Construction Engineering and Management_, ASCE's _Journal of Computing in Civil Engineering_, and KSCE's _Journal of Civil Engineering_, as well as journals specific to BIM, including _Automation in Construction_, _Advanced Engineering Informatics_, and the _Journal of Information Technology in Construction_. Additionally, conference papers from forums such as CIB W78 and the International Conference on Computing in Civil and Building Engineering were also explored to review more cases. The search period was set from 2001 to 2022, and the search terms 'IFC schema' and 'IFC schema extension' were used in the keywords and titles to identify relevant papers. This initial search resulted in a total of 84 papers. Of these, 20 papers were excluded, as these cases were not deemed qualified to be specific extension studies. Specifically, these were cases that discussed the limitations of the existing IFC schema and suggested recommendations generally without specificizing concrete new extensions. In addition, these were cases in which the same authors had published the extension, implementation, and validation processes in multiple papers.