What feature is absent in Pre-1933 Unreinforced Masonry (URM) buildings that contributes to their potential for early collapse?

In Pre-1933 Unreinforced Masonry (URM) buildings, the absence of fire cut beams is a significant factor contributing to their potential for early collapse during a fire. Fire cut beams are designed to prevent catastrophic failures by allowing beams to drop safely if they lose their structural integrity due to intense heat. When exposed to fire, the connections of these beams can fail, causing the beams to fall without compromising the rest of the structure.

In contrast, without fire cut beams, the beams in pre-1933 URM buildings can remain rigidly connected to load-bearing walls. This rigidity can lead to a transfer of force and increase the risk of collapse when the masonry loses its strength under heat. The absence of such a strategic design feature means that URM buildings constructed before 1933 are more vulnerable to progressive collapse as the structural integrity diminishes during fire exposure.

The options of rebar, steel beams, and concrete blocks do not directly relate to this specific aspect of structural failure under fire conditions, as they either represent reinforcement that was not typical in these older buildings or materials that do not contribute to the same collapse mitigations as fire cut beams.