What characterizes re-entrant corner irregularity in plan configurations?

In analyzing re-entrant corner irregularity in plan configurations, the defining characteristic is that projections in the building's plan exceed 15% of the plan dimension of the structure. This indicates that the design includes corners that are recessed or indented, creating a complex geometry that can affect structural performance, especially in terms of handling lateral forces during events like earthquakes. Regular corner configurations typically help to distribute loads evenly; however, when projections extend beyond the customary limit, this can lead to local stress concentrations and complicate the design and analysis.

The other options do not accurately depict the characteristic of re-entrant corners. For instance, simply exceeding a 10% threshold or having all corners orthogonal does not necessarily imply irregularity, as it may describe various forms of layout that are still structurally sound. Additionally, having no projections beyond the corners would indicate a straightforward plan shape without the irregularity associated with re-entrant corners. Hence, a proper understanding of the implications of corner projections is essential for structural design and safety assessments.