Final answer:
While horizontal lifelines may support multiple workers, vertical lifelines and self-retracting lifelines are typically designed to support only one worker due to safety concerns regarding the equipment's breaking strength and the dynamics of fall arrest.
Step-by-step explanation:
The student's question pertains to safety measures in construction, specifically about the use of Personal Fall Arrest Systems (PFAS) by Devon and Michael while constructing an elevator shaft. The query involves determining under which conditions Devon and Michael can be attached to the same lifeline, given they are working 6 feet above the foundation.
It's important to note that while certain types of lifelines can support multiple workers, such as a horizontal lifeline, vertical lifelines typically support only one worker. This is primarily due to the dynamic and static forces involved and the potential for severe injury if the system fails. Therefore, for vertical lifelines, each one typically has a minimum breaking strength to safely support one worker, often estimated at around 5000 pounds to account for both the worker's weight and the forces involved during a fall. Attaching more than one worker to a single vertical lifeline could exceed the breaking strength and is generally not recommended without a specialized system designed for such use.
Using a self-retracting lifeline that limits free fall distance can be very effective for individual workers, as it can reduce the forces experienced during a fall arrest. These devices are typically designed for one person and provide a quick activation to stop the fall, but they are not intended for multiple workers to be attached simultaneously.