## 1 Introduction
The main objective of neutral formats is to ensure long-term access to data without uncertainty regarding legal rights or technical specifications. The most frequently used open neutral format in BIM is the Industry Foundation Classes (IFC), developed by buildingSMART. It is the de facto standard format that describes objects, their characteristics, and their relationships. It defines the standard for the technical exchange of files with model data and it is certified (ISO 16739-1:2018). IFC can be labelled the core of a collaborative-lifecycle BIM and aims to ensure the interoperability of BIM software, independently from the CAD/CAE software [12]. The most relevant advantages of an IFC-based BIM approach are its ability to: (i) combine the functionalities of different software applications, (ii) compare results between different analysis runs, and (iii) share project materials across different disciplines and phases [10]. IFC-based horizontal interoperability is widespread and, in principle, a BIM-based CAD software can adequately manage the exchanged information of an architectural model. In reality, several issues arise from the BIM interoperability, involving the exchange between CAD/CAE software. In the transition from a geometrical/architectural model to an analytical/structural model for structural analysis, vertical interoperability shows all its shortcomings, highlighting how, despite the fact that the IFC format can deal with structural information, the software's implementation of all its functionalities is still limited. A recent critical review related to BIM CAD-to-CAE interoperability was reported by [PERSON] and [PERSON] [13], showing that criticalities in the connection between structural elements exist independently from the tested software. In the present study, the level of interoperability reached by several very common commercial CAD and CAE BIM-based software implementing the IFC format was investigated. The assessment was designed to provide to the technical community and to the authorities in charge of the management of complex civil engineering works a qualitative and quantitative analysis of the issues in data exchange processes using NL procedures. Attention was paid to the way in which the software packages dealt with the geometries and additional information exported and imported in the CAD-CAD and CAD-CAE bidirectional paths, adopting both the most common (IFC 2x3) and the latest (IFC 4) formats. We also focused on CAD-CAE bidirectional interoperability, which is a key aspect of BIM interoperability. The shared building information model was expected to be re-editable by the receiving application, and the end user should have been able to access any information within the model and edit it when and where needed. Moreover, the results of self-tests (from CAD and CAE to IFC, then back to the same CAD and CAE) [14] are reported to offer a better understanding of the way a software environment can handle the IFC entities. Useful and significant outcomes that delve deeper into the translation and mapping mechanisms implemented into the software are also proposed. The present study covers a three-year period during which the authors tested different releases of the same software packages, and so the elements that changed over the years are also reported. The assessment was intended as a reference and a guide to the most common dissimilarities in the data treatment that can cause issues or prevent the data exchange from being successful; obviously, it was not designed to rank software packages and their performance in the AEC process. The paper is divided into eight sections. Section 2 shows some important aspects of the IFC standard related to versions 2x3 and 4 and provides schemas to describe the data exchange for a specific use or workflow. It also addresses the choice made in the benchmark study. Section 3 reports several useful contributions to data interoperability via IFC released in the past ten years.