# Performing Fatigue State Characterization in Railway Steel Bridges Using Digital Twin Models
###### Abstract
In contrast, the second involves using data alone (from sensors and other sources) to create a model, generally relying on statistical models and/or machine learning algorithms that analyze the given data to identify patterns and correlations to determine the structural condition and performance. However, there are more direct data-driven systems that do not involve Artificial Intelligence and are more focused on creating 3D geometric models using parametric modeling algorithms (so-called data-driven design). For instance, in the context of BIM, to create a DT to automate a maintenance system of pre-stressed concrete bridges, the researchers of [19; 20] applied the concept of parametric modeling to create the 3D model (using an open-source application programming interface) in which through algorithms, isolated concrete objects were created (including their position, orientation, or restrictions), identified by an ID, and then combined to form a single 3D model (Figure 1 [20]). This BIM-based digital bridge model was then overlapped on another Bridge Reality Model/Inverted 3D Surface Model, as the authors refer to it (created through a 3D scanning  
Figure 1: Example of a parametric flowchart for parametric modeling of pre-stressed concrete (PSC) bridge. procedure, reflecting the current state of the bridge), forming a federated model (which is also referred to as the \"initial model\" and represent the current actual state model), and through the interoperability of the BIM solution, a Mechanical Model was derived, allowing analysis based on the finite element method (FEM), taking into account the actual state of the bridge. BIM has been explored as a starting point for creating DT of bridges and other infrastructures, either via parametric modeling or direct modeling with pre-existing objects in software for creating 3D geometric models (such as the procedure used in this work). BIM not only serves to create 3D models, but it is also an excellent platform for integration, storage, and sharing of information covering the entire life cycle of a project [21]. The aid of BIM for Bridge DT creation for various purposes of bridge lifecycle management, resilience, and climate change was further reported in the literature [10; 22; 23]. In the particular case of fatigue in bridges, taking into account the complexity of its analysis and considering that DTs in the bridges sector and architecture, engineering, and construction (AEC) generally are still in their \"embryonic\" stages, works aimed at the implementation of these models in this field are still scarce. Regarding some results in the literature, we can refer to [24], which studies the \"orthotropic steel deck of a cable-stayed bridge\" in which, while admitting that the welding residual stress is an essential cause of fatigue cracking, the author attempts to synthesize the monitoring data and the finite element model (FEM) through a DT to reconstruct the un-monitoring structure responses. The operationalization of the model proposed by the author relies fundamentally on updating the FEM based on regular measurements of the bridge and identifying fatigue cracks at strategic locations that may be inaccessible, thus being able to simulate loads to predict the response of the structure. However, the proposed model is unclear regarding the automation process and visualization, which are essential characteristics of a DT. It should be noted that a DT representing fatigue in steel bridges consists of two complex parts: the fatigue analysis part, which is generally complex and little understood, and the difficulties in the technology for the construction of the DT [25]. Inspired by the different research works previously reported, this paper proposes an integrated model of a Fatigue Analysis System (FAS) based on a FEM and the BIM model of the bridge in the process of continuous automatic updating of the two parts to form a DT model (Figure 2).