Jack arch floors (masonry arches between joists): for proper action and economy, the rise of a jack arch is commonly kept as which fraction of the clear span?

Introduction / Context:
Jack arch floors consist of shallow brick or concrete arches springing between steel joists (or RCC ribs). The arch rise influences thrust, material usage, and headroom. Traditional practice adopts shallow rises for economy while maintaining adequate arch action.

Given Data / Assumptions:

• Short-span jack arches between joists with lime/cement mortar.
• Shallow arch action is intended (not full semicircular arches).
• Rise-to-span ratio is chosen for balance between thrust and headroom.

Concept / Approach:
A commonly recommended rise for jack arches is about 1/12 of the span. This provides sufficient arching to reduce bending in the units and control horizontal thrusts in the joists while keeping construction depth reasonable.

Step-by-Step Solution:
Recall typical ratios: 1/8 to 1/12 are used, with 1/12 being a standard exam value.Select the conventional figure: 1/12 of span.Therefore, answer is 1/12 of the span.

Verification / Alternative check:
Old building practice manuals cite 1/12 as a good compromise for jack arches under modest spans and loads.

Why Other Options Are Wrong:

• 1/6 or 1/8: larger rises increase thrust and reduce headroom.
• 1/10 or 1/15: may be used in specific cases, but 1/12 is the typical standard value taught and widely accepted.

Common Pitfalls:
Choosing too small a rise leading to near-flat action and higher bending; ignoring the need for adequate haunch filling and good abutment through joists.

Final Answer:
1/12 of the span