Benefits of heritage building information modelling
The Building Information Modelling (BIM) methodology applied to building heritage has generated a significant catalogue of experiences classified under the title Heritage Building Information Modelling (HBIM) . Some researchers have already highlighted the fact that parametric modelling, database formulation and structured information management through BIM could offer many benefits to managing a built historic environment in the operation and maintenance phases, and particularly in Conservation, Repair and Maintenance (CRM) programming [2,3,4]. HBIM could also play an important role in defining the best retrofit solutions among a set of alternatives proposed by research and design participants as well as other stakeholders . Furthermore, HBIM could help integrate other criteria, often neglected or applied in a non-structured way, such as compatibility with restoration and conservation guidelines, economic aspects through the whole lifetime, including the energy efficiency of the end solution, as well as environmental aspects of the long-term heritage management plan.
Despite advantages, communicating information through the BIM process for historical architecture and thus creating HBIM models has proven to be problematic [3, 6]. One of the challenges in HBIM is to provide appropriate geometrical accuracy. The creation of a HBIM model for conservation projects requires the collection of available information, interpretation, and the final modelling of the different structural elements . The HBIM can include the following BIM dimensions: (1) analytical survey (3D), including establishing the geometrical model; (2) historical evolution (4D), including an overall evaluation of the cultural significance of the asset; (3) diagnosis (5D), including characterization of the state of knowledge, protection, possible conservation and dissemination; (4) cultural context (6D), including environment and territorial infrastructure; and (5) related assets, conservation and intervention (7D), including the programme for future research, protection and conservation activities .
It is not unusual to capture the different historical stages of built assets in HBIM with several simultaneous models that correspond to each phase so as to understand and represent the development of the object under study [8,9,10]. Furthermore, dating periods as well as historic information on building materials and historical changes to the building, such as demolitions and renovations, can be integrated into the model for a better understanding and managing of the asset. Lastly, a damage assessment for conservation practices using HBIM is often performed too. This can be a part of the overall analysis and generation of knowledge regarding the building itself. Usually, a considerable amount of such data is condensed into a conservation plan prepared by experts in ethnology and history [4, 11,12,13,14]. A major problem within HBIM is the interoperability of heritage data models. An early call for a unified HBIM library was made by ; however, to date there is no such library. Attempts to support the HBIM ontology through various standards have been made, such as the CIDOC CRM by . While such a system has major advantages over using BIM or CIDOC CRM in isolation, the approach does not natively integrate the heritage information with the BIM, but keeps the two paradigms as separate entities that link together. The ideal approach would harmonise the heritage information with the BIM concept.
Industry foundation classes (IFC)
Interdisciplinary data sharing and the exchange of information between a diverse set of software tools are accepted standards in the Architecture, Engineering, Construction/Facility Management (AEC/FM) industry . Thus, the interoperability of HBIM is of great importance, since interdisciplinary weighted decisions are made based on the model created and shared using BIM tools. Hull and Ewart  acknowledged that common data environments (CDEs), structured datasets, and the use of naming conventions would offer a significant benefit to heritage-asset management. Projects that involve the creation and management of built environments are increasing in complexity; the needs of data sharing and exchange are expanding; and thus BIM technology has been adopted to address the creation, storage and management of information throughout a building’s lifecycle in an integrated way. However, the data interoperability between diverse software tools involved in managing built environments is traditionally limited to point-to-point, direct conversions, which turned out to be a limiting factor in collaborations .
When it comes to software, public authorities need to be vendor-impartial . With heritage, public authorities play a major role in the process, as the conservation plan is required by the cultural heritage institute. Vendor lock-in by proprietary software vendors can cause accessibility problems in BIM. This is even more so in fields where BIM is not one of the core businesses, as is the case with heritage. The exchange of information between different actors is complicated, because not all of them have the resources to use the same software. In these cases, there is a need for open formats. The price of proprietary software packages that deal with BIM models can be a barrier to purchasing . Alternative systems to commercial packages give problems if proprietary data formats are used. These are often only compatible with one distributor. OpenBIM copes with these accessibility issues, as a vendor-neutral and open concept. The IFC schema is a manifestation of the openBIM concept.
The IFC schema was initiated in 1994 as an open-data-model standard. It is governed by the buildingSMART International (bSI) organisation and standardized by the International Organization for Standardization (ISO). The most up-to-date version IFC4 ADD2 TC1 is defined in ISO 16739-1:2018 . The data model is intended to provide software-agnostic data interoperability in the AEC/FM industry . It is very common to drop data into CDE using the IFC schema, which supports data sharing and exchange between heterogeneous software tools. bSI  reports that over 200 software tools can import or export IFC files. OpenBIM is widely recognised in the area of heritage as a valuable asset [22,23,24], although setbacks exist when it is combined with heritage data: the IFC language is not familiar to people working in these field. Diara and Rinaudo  attempt to overcome this by opting for an extension of the IFC schema itself in the form of data-extraction methods and not IFC classes, which do not yet exist. However, as IFC is a standardised schema, such an approach can be counterproductive and lead to reduced interoperability. A better approach would stay within the boundaries of the IFC schema and use existing methods to include heritage information, for example, property sets.
Information delivery manual (IDM) and model view definition (MVD)
The IFC schema is implemented in BIM environments through MVDs, proposed by bSI. As described by, e.g., Jiang et al. , this “handshake” protocol MVD defines a subset of the IFC schema, concept templates and properties that are needed to satisfy the data-exchange requirements. The IFC is always exported according to one MVD. To exchange the data, the exporting and importing software need to understand the same MVDs on the export and import sides, respectively. This is also the basis for software implementation and certification.
However, there are different challenges to face when using MVDs: interoperability between MVDs, the low level of the understanding of the difference between IFC and MVD on the user’s side, the implementations of MVDs in the software tools and the challenges related to releasing versions of the IFC, for example, updating each software release for new IFC versions [27, 28]. The IFC2 × 3 schema has four final MVDs: (1) Coordination View (CV); (2) Space Boundary Addon View; (3) Basic FM Handover View; and (4) Structural Analysis View . The two common MVDs based on the IFC4 ADD2 TC1 schema are the Reference View (RV) and the Design Transfer View (DTV). Apart from RV and DTV, there are four other official MVDs within the IFC4 ADD2 TC1 schema: (1) Quantity Takeoff View; (2) IFC4Precast; Energy Analysis View; and (3) Product Library View [27, 29,30,31].
The idea of MVD as a handshake protocol is only partly implemented. While it is possible for an organisation to develop a new MVD, it is not possible for existing software implementations to export according to these MVDs without the developers implementing it first. This takes time and the responsibility lies with the software vendors. Exports are currently achieved using the base MVDs, such as the CV in IFC2 × 3 and the RV/DTV in IFC4 . However, it is possible to validate the BIM models exported as IFC files using any valid MVD .
The development process of an MVD can be described by three core steps: establishing an IDM, establishing an MVD, and implementation . The specification relating to what business processes and information need to be included in each MVD is standardised in ISO 29481-1:2016 . The standard was developed by bSI to have a methodology that can capture and specify processes and information flow during the lifecycle of a facility. The IDM documents the business processes and information requirements. It also defines for one or more use cases what data are needed, who will provide the data and who is requesting it, at which stage of a project this happens and why it is needed. The Exchange Requirements are the part of the IDM that show what data are needed for the use case. Another part is the process map, a visual representation of the process. It shows how actors exchange information during the relevant phases [32, 35].
While the IDM forms a human-readable predecessor of the MVD, mvdXML represents the MVD as a computer-interpretable format. MvdXML has multiple purposes, one of which is the support of automatic IFC file validation [32, 36]. It can be used to define the correct nomenclature, as well as data types, must-haves, may-haves and similar rules to prepare a specific BIM project for easy data mapping and implementation. Several researchers have attempted to define valid and functional mvdXML documents using a variety of software, such as xPPM and IfcDoc [27, 32]. No matter which path prevails in the future, HBIM-specific interoperability challenges will need to be addressed. In previous research we have seen how important the quality of metadata is for heritage science [37, 38]. Therefore, it is not only necessary to advance the geometrical accuracy of HBIM models, but also to agree on the semantic classification of cultural heritage peculiarities in HBIM models. Thus, previous research proposed an experimental IFC classification implemented within HBIM open-source software (FreeCAD), whereby the limitations of IFC standards can be overcome thanks to the freedom of access to libraries and codes .
The conservation plan
The Register Kulturne Dediščine (RKD, eng. Cultural Heritage Registry) in Slovenia lists 16,207 heritage buildings that have some form of protection or restrictions . In addition, based on national datasets on buildings and protected areas, we can deduce that 135,105 buildings are under some form of cultural protection, for example, protected city views [41, 42]. While there is no official data available for Europe on this matter, we can safely say that there are a vast number of buildings under cultural protection. In the case of renovation, acquiring approval from a cultural heritage authority is necessary to start the process.
In Slovenia, a conservation plan is a legal document based on the Cultural Heritage Protection Act . It is produced by conservators to provide guidelines on the renovation or restoration of monuments and heritage buildings. The Slovenian conservation plan draws its roots from the Australian James Semple Kerr’s ‘The Conservation Plan’ (first edition 1982) . This book is still widely used by heritage professionals and had a great influence on conservation research field in Europe . This can be seen in other authors taking similar approaches to conservation documentation, such as [13, 23, 46]. The Slovenian protection act defines the duties of the conservators, including research oversight and restoration interventions in heritage, advising owners on protection procedures, participation in the preparation of cultural protection conditions and consents for interventions on immovable heritage and in the preparation of management plans . A conservation plan typically consists of images, 2D floor plans and descriptions of the elements that have some particular importance or that are vulnerable.
The plan can be divided into three sections: (1) a basic document dealing with the conservation; (2) a specific document in the case that the heritage is more complex (the heritage survey); and, if extensive changes are planned; and (3) a document focusing on the execution of the conservation/restoration (the conservation/restoration project). The conservation plan deals with understanding the object, assessing the significance of the heritage itself, and the elements inside. It assesses the vulnerabilities and threatened objects and declares policies on how to deal with the heritage .
The conservation plan is a comprehensive document, containing mostly textual descriptions of objects . While objects are typically grouped by element type (floors, windows, doors etc.), the spatial context around the object is not always self-evident. In the best approach to the renovation of heritage buildings and restoration we would integrate the guidelines and the design into a single common environment. However, with the current approach, the conservation plan is only read by a small group of people . This hinders an understanding of the importance of heritage conservation in buildings. The entire chain of problems boils down to one common factor: limited communication.
The inclusion of HBIM in the process reduces these problems. A spatial context is provided, and data can be added as semantic information to objects or the complete project. Conservators are, however, usually neither BIM nor IFC specialists. So far, there is no MVD that limits the scope of the IFC for conservators. In addition, to prevent a loss of information that is added to HBIM models during the process, a data model must be formalised. This could add to the interoperability between software when dealing with HBIM models.
Previous research on a similar topic to the conservation plan includes data models for damage assessment [13, 46] or other structural or physical properties . Moreover the use of HBIM for stratigraphic analyses has been researched [9, 51]. Acierno et al. looked at the interactions between actors and HBIM data for the purpose of heritage conservation . The connection between standardised heritage ontology and BIM was described in . The authors expressed the need for a methodology that links the IFC and the heritage data model. This research is not the first with regard to cultural heritage proposals for the IFC schema: Diara and Rinaudo  proposed informal additions to the schema using FreeCAD. Instead of including heritage elements in the ISO standard, they proposed the use of data-extraction methods to exchange the heritage information between different BIM software.
The use of MVD instead of data-extraction methods for data exchange was not yet investigated. It has the capability to be more standardised, automated and to ensure a higher-quality model. While the focus of Diara and Rinaudo  was on the classification of elements into ‘roles’ fit for heritage (such as Cladding Wall or Retaining Wall for the IfcWall element), the focus of this study is the standardisation of the information in the conservation plan into the IFC schema.
Aim of the research
The interoperability and accessibility of HBIM data should be addressed to increase usability. A formalisation of the needs for heritage is required to ensure the transparency and traceability of the decisions and actions. Even given the many advantages of HBIM (see [2,3,4,5, 23, 50]), it is understandable that conservators are reluctant to move to a BIM-enabled workflow: the existing processes are well documented and clear, whereas there is no framework for dealing with Heritage BIM that ensures a quality of data such that the output is fit for the replacement of a conservation plan [23, 39].
Therefore, this study describes the concept of a new bSI domain: the heritage domain. In this research the process of MVD creation was followed to open discussion about the domain. An IDM is developed to describe a part of the conservation-and-restoration process. Consequently, an MVD is extracted from the IDM. Concepts not available in modern buildings are introduced and evaluated based on the state-of-the-art data-exchange specification format mvdXML. Additionally, a completely new data model is proposed and mapped to IFC to facilitate data exchanges in HBIM. The MVD in the context of this research is by no means meant as a comprehensive definition for cultural heritage, but rather a showcase about what is possible and the beginning of a larger process of IDM/MVD development within heritage domain. This process should lead to a better quality of data quality in HBIM.