In the 1940s era commercial Type III medium building, what is the described fire spread pattern?

• A. Vertical spread (open stairways)
• B. Horizontal spread (open corridors)
• C. Vertical spread (open elevator shafts)
• D. Diagonal spread (through ducts)

Answer: (A)

Open vertical voids like stairwells act as a chimney, carrying heat, flames, and smoke upward from floor to floor. In a 1940s era Type III medium building, the interior often lacked tight compartmentalization and fire doors, so a fire on one floor can easily use the stairwell to propagate to upper levels. That vertical pathway is the fastest and most direct route for a fire to spread through multiple floors, which is why the described pattern is vertical spread through open stairways.

Horizontal spread through open corridors can occur, but it tends to stay on the same floor longer and isn’t as characteristic of how fires move between floors in these structures. Ducts and elevator shafts can contribute, but open stairwells were the primary vertical conduits in many mid-20th-century Type III buildings.