What is the primary concern of fire spread through a multi-story Type III building?

• A. Vertical extension (including the open stairwells)
• B. Horizontal spread through corridors
• C. Roof collapse
• D. Ground-floor ignition

Answer: (A)

In a multi-story Type III building, the main danger is vertical extension up through the structure, especially via open stairwells and other vertical voids. Once a fire takes hold on one floor, these stairwells and shafts act like chimneys, allowing flames and smoke to rise quickly to upper levels. That rapid vertical spread can overwhelm upper-floor compartments, compromise evacuation routes, and challenge firefighting efforts, often more so than spread within a single floor.

Horizontal spread across a corridor matters, but it generally stays contained within a floor's envelope if proper compartmentation and doors are in place. The reason vertical pathways are the primary concern here is that the building’s interior is frequently timber or other combustible materials, and the noncombustible exterior walls don’t stop flames from moving upward through the interior voids. Mitigation comes from protecting and maintaining stairwells, using rated doors, and ensuring effective smoke control to slow or block that vertical ascent.