What restriction is placed on corbeling in masonry chimneys?

Corbeling in masonry chimneys is designed to provide structural support while minimizing potential weaknesses. The guideline that limits corbeling to no more than one-half of the chimney's wall thickness is based on structural integrity and stability. Exceeding this limit could compromise the strength of the masonry, leading to possible failure or structural issues over time.

This specific restriction helps ensure that the load-bearing capabilities of the chimney are maintained while allowing for the practical use of corbeling in design. The half-thickness limit is a balance between functional architectural design and safe engineering practices, promoting durability and safety in the chimney's construction.

Other options either present restrictions that are set too low (such as one-quarter of the thickness) or imply that corbeling is completely prohibited, which is not accurate, as corbeling can be an essential feature when done within specified guidelines. Additionally, suggesting that corbeling must not extend beyond the foundation is not directly relevant to the structural parameters governing corbeling itself.