When does weight irregularity occur in a building structure?

Weight irregularity in a building structure occurs when there is a significant disparity in the mass distribution between adjacent storeys. Specifically, the scenario described in the correct answer involves any storey having more than 150% of the effective mass compared to an adjacent storey. This substantial difference can lead to various structural issues, as it affects the building's stability under loads, particularly seismic forces.

A structure where one storey is significantly heavier than those above or below may experience differential movement during an event such as an earthquake. This can cause stress concentrations and possibly lead to structural failure or excessive deformation. The 150% threshold is a common guideline in engineering practices to identify potential weight irregularities that could compromise the integrity and performance of the building in resilience against loads.

In contrast, scenarios where adjacent storeys have equal mass, the buildings are of uniform height, or where weight is distributed evenly typically indicate a regular mass distribution, which is more favorable for structural stability. Thus, they do not experience the same issues associated with weight irregularities.