# Developing a BIM-Based MUVR Treadmill System for Architectural Design Review and Collaboration
## 1 Introduction  
The advanced building information modeling (BIM) technology has completely changed how architects approach building design. The potent BIM tools energize architects' innovation to create exceptional designs that could not be made by most traditional methods [1]. However, it is challenging for architects to present complicated and sophisticated designs if conventional 2D or 3D representations are adopted. Similarly, the ineffective design presentation might result in communication errors among team members [2]; the consequence is often costly design variations in the construction stage [3]. Thus, virtual reality (VR) has much potential for architects and designers, as this revolutionary technology transports users into an immersive virtual environment, allowing them to explore a simulated representation of a particular room, floor, or building design as a whole with their partners. VR offers practical solutions with computer-generated artificial realities, and it is now entering mainstream use in various fields, such as entertainment, education, design, engineering, and health services [4]. VR popularity in the architecture, engineering, and construction (AEC) industries is increasing as it becomes more affordable for end-users [5, 6]. Using VR as a platform for content delivery allows better user engagement than other traditional methods, as VR head-sets remove external distractions by shutting out the physical world, creating a complete immersion experience. Most AEC researches mainly leverage the visualization function offered by BIM-based VR applications to support design and construction activities [7, 8, 9, 10]. For instance, BIM design can be monitored and compared through VR applications for decision-making and simulated for pre-construction planning. On the other side, VR is also helpful in dealing with misunderstanding, spatial sense issues, and clash-detection problems in the design review process [11]. Thus, it offers considerable potentialities for improving the inefficiencies in current 2D and 3D design review and coordination practice. In addition, interactive multiuser VR (MUVR) has been long established in construction researches, such as safety training. MUVR has been proven beneficial to the overall site safety performance [12]. However, current BIM-based VR applications in architectural design are still limited to single-user usages and lack of interactions among users. Collaborative VR allows multiple users to attend virtual design or presentation sessions in the 3D environment, and they can work collaboratively in the same room or remotely from different locations [5]. There is also a common problem seldom addressed by researchers or practitioners when using VR devices in architectural applications. VR users are usually facing locomotion limitations due to the designated space constraint [13]. The limits of the real environment are identified by a virtual grid fence that avoids VR users bump into actual physical walls or objects in the perceived environment. This constraint affects the navigation efficiency of 3D models for buildings with considerable floor areas (e.g., airports, exhibition halls, and shopping arcades) and leads to the risk of two users bump each other when using MUVR applications. Although artificial VR locomotion (e.g., teleportation) is one of the solutions for users to navigate the virtual environment by moving themselves to different locations with the use of hand-held controllers, this method tends to cause VR sickness due to the discrepancy between expected and perceived sensory information [14]. As such, omnidirectional treadmills are the cutting-edge solutions for walking simulations in the virtual environment. The treadmills enable users to move freely, without the concern of navigational space limitations. Thus, they enhance the illusion of unconstrained human movements and make the overall VR experience more realistic and immersive [13].