Which of the following elements is NOT a requirement for fire-rated door assemblies?

Fire-rated door assemblies are designed to resist the spread of fire and smoke, and certain elements are essential for their effectiveness. Among the choices, signage indicating fire safety, while important for general awareness and compliance, is not a structural or functional requirement of fire-rated door assemblies.

Self-closing devices are crucial because they ensure that the door closes automatically after being opened, maintaining the barrier against fire and smoke when not in use. Glass panels may be included in fire-rated door designs, subject to specific ratings and standards, particularly when they are evaluated and tested for fire performance. Heavy metal construction is another key requirement, as the materials used must meet certain strength and resistance levels to provide adequate fire protection.

In summary, D is not a requirement because while signage plays a role in safety communication, it does not contribute to the physical effectiveness of the door in preventing fire movement, which is the primary focus of fire-rated assemblies.