## 3 Beacon-Based Hazard Alarm System Design
### Detailed Design of the Components
#### 3.3.1 Beacon-Based Distance Monitoring
Alarm fatigue has been identified as a major issue in many sensor-based danger alarm systems in construction and other industries, such as health care [56; 57]. If a worker keeps receiving alarm notifications that are not relevant to their task or workspace, redundant, or repetitive, it is likely that they will ignore the alarms or even disable the alarm device. Therefore, how widely the hazard alarm notifications should be sent is not a trivial issue when designing a location-based hazard alarm system. This research took an experimental approach to address these questions related to the beacon sensor deployment problem in the context of a hazard alarm system. A beacon

Figure 4: Application interfaces ((a) when a hazard is notified and (b) when the user has confirmed the recognition of the hazard). signal-sending device was put at 4 different locations within a construction workspace (e.g., an unfinished room under construction). Each of these 4 different locations represented (1) at the entrance of the room, (2) behind the front side wall, (3) on the internal wall facing the access point, and (4) on the internal wall not facing the access point. Then, many trials of approaching the workspace on various paths were performed, and the location where the user received the first hazard alarm notification was marked in each trial. Then, all the locations of the first hazard alarm notification from all trials were visualized as a heat map, which could provide insights into how the beacon sensors would need to be deployed for the system to notify of hazards most effectively while avoiding alarm fatigue in construction workers. ### Experiment Setting

An education building project that was under construction, located in Xi'an, China, was selected for this experiment. The total construction area of the project was 77,535.52 m\({}^{2}\), including a 3-story basement, a 25-story building, and a 12-story building. The experiment was conducted on a floor of this building for which the structure construction was just completed, but the finishing work did not begin yet. Specifically, a semi-enclosed room with two concrete walls and two open access points on the second floor of the building was selected as the experiment site. The simulated scenario was that the room had a particular hazard that required notifying all the affected workers. Figure 5 shows the floor plan where the experiment took place and the 5 paths followed by the experimenter to approach the workspace. Since the alarm system's operation is based on the relation between the RSSI value and distance, the parameters included in Equation (1) needed to be calibrated based on initial measurements collected in the same environment. For this purpose, a preliminary test was conducted for a known distance at the experiment site, and the parameters in the

Figure 5: The floor plan of the experiment site. RSSI equation were determined. The hazard alarming distance was set to be 5 m, which means the alarm system would be triggered when the distance between the beacon receiver and sender sensors calculated based on the RSSI value was less than 5 m.

### Procedure

Beacon sensors were placed at 4 different locations--at the entrance of the room and 3 corners of the room--as marked with small, numbered circles in Figure 5. With the beacon sensor located at 1 of the 4 locations in the room, the user approached the room with one beacon receiver and the mobile phone equipped with the hazard alarm application starting from 5 different positions in the wider area of the floor, as marked with larger, numbered circles in Figure 5. The lines with an arrow in Figure 5 show the directions of the routes. To mark the locations on the building floor, a 2D coordination system was established. For each route, the experimenter moved toward the room at a regular walking speed and recorded the point at which the hazard alarm was received for the first time. This trial was repeated 20 times for each route.