Which of the following best identifies indicators that lightweight construction may fail under fire conditions?

• A. Sagging roof/ceiling; premature failure of roof decking; visible twisting or bowing of structural members.
• B. Only sagging roof.
• C. No indicators present.
• D. Only visible twisting.

Answer: (C)

Lightweight construction is especially vulnerable to heat because its members can lose strength and stiffness quickly when exposed to fire. Indicators that such a structure may fail include a combination of signs that show the frame yielding under load: a sagging roof or ceiling as the upper chords and supporting members lose support, premature failure of roof decking as the decking cannot transfer loads effectively, and visible twisting or bowing of structural members as the members deform under heat and load redistribution. Seeing these signs together is a strong warning that the framing is no longer capable of carrying the loads it was designed to, and collapse can follow.

Single signs spoken in isolation, such as only sagging or only twisting, do not provide as complete a warning, and asserting no indicators at all would ignore the progressive damage that fire can cause.