# Digital Twin Framework for Road Infrastructure Management
## 4 Digital Twin Framework for Road Infrastructure Management
### Analysis of Existing DT Frameworks for Road Infrastructure Management
#### 4.1.2 Road Bridges  
Over the past decade, the traditional bridge management paradigm has evolved with the emergence of the DT concept [39]. DT-based bridge management integrates multisource data, including real-time sensor inputs and historical maintenance records, to facilitate the visualization, simulation, and prediction of bridge performance, thereby enhancing maintenance effectiveness [40]. [PERSON] et al. [4] proposed a detailed workflow for developing a DT for existing assets in the built environment, focusing on bridge infrastructure. The framework consists of five layers: data and need acquisition, digital modeling, sensor data transmission, data/model integration, and operation. The data and need acquisition layer involves extracting relevant information from the physical structure to support DT development. This information is incorporated into operational processes to manage the structure, define the DT's objectives, identify the decisions it is intended to support, and establish feedback mechanisms between the digital and physical twins. The digital modeling layer includes a mathematical model implemented through software to represent the structure and its condition. This layer facilitates data management, functional integration, scenario testing, and simulation. It enables the creation of a digital model capable of simulating, visualizing, and controlling the system, as well as modeling the structure's behavior. The sensor data transmission layer collects, transfers, and stores data, along with associated metadata, using open interfaces. The quality of the data, such as their resolution, depends on the specific requirements of the application, which, in turn, influence the selection of appropriate sensing and communication technologies. The data/model integration layer establishes the connection between physical and virtual entities through an interoperability architecture. The core value of a DT lies in its integrated framework, which enables data exchange and processing, parameter tuning, and iterative validation. The operation layer facilitates effective and efficient asset management in line with the intended purpose of the DT. It supports operational decision-making by providing access to DT-generated outputs for evaluation and implementation within the decision-making process. [PERSON] et al. [40] developed a mixed reality (MR)-based DT model for bridge maintenance systems. The model comprised six layers: data acquisition, digital three-dimensional (3D) model, data integration, transmission, deployment, and service. The data acquisition layer is responsible for collecting inspection data. The digital 3D model, built on BIM, consists of two primary components: the model inventory system and bridge element models. These components enable the DT to integrate the inspection process with the 3D BIM framework, allowing for accurate virtual representations of physical bridge structures. The bridge DT incorporates both static and dynamic data models and is developed in a MR environment visualized through game engines such as Unity. In the service layer, MR facilitates the projection of a 3D digital bridge model into the real-world environment, enabling stakeholders to interact with it directly. Users can query real-time inspection and maintenance data, as well as monitor structural health and condition status in near real time. [PERSON] et al. [41] designed a cloud-based bridge DT architecture comprising three main interaction layers: bridge-machine interaction, HMI, and human-bridge interaction. The bridge-machine interaction layer incorporates various sensing and robotic technologies and consists of four core components: data acquisition and preprocessing, communication, cloud servers, and HMI. Data acquisition and preprocessing involve on-site inspection and monitoring using tools such as drones and sensors. The communication component ensures bidirectional data transmission through middleware and communication protocols.