In building construction, what rise is commonly adopted for a Jack arch (flat brick arch) expressed as a fraction of its clear span?

Introduction / Context:
A Jack arch is a nearly flat masonry arch used between joists or steel beams. Selecting an appropriate rise (camber) ensures adequate arch action, economy of materials, and acceptable headroom.

Given Data / Assumptions:

• Jack arch spanning between supports (e.g., steel joists).
• Conventional masonry and service load levels.
• Rise expressed as a fraction of span.

Concept / Approach:
Jack arches are designed with a relatively small rise to remain functionally flat yet develop thrust lines within the masonry. Traditional practice adopts a rise in the range 1/8 to 1/12 of span to balance structural performance and architectural constraints.

Step-by-Step Solution:

Verification / Alternative check:
Trial thrust-line checks with the selected rise typically keep compression within the masonry section without requiring excessive thickness, corroborating the guideline.

Why Other Options Are Wrong:
Higher rises like 1/2–1/3 or 1/3–1/4 are inappropriate and uneconomical for a Jack arch; 1/4–1/8 is still large for a nearly flat arch; extremely small rises (e.g., 1/16–1/20) may be insufficient to keep a safe thrust line.

Common Pitfalls:
Confusing Jack arches with semicircular or segmental arches; ignoring lateral thrusts and tie requirements; neglecting the effect of live loads on the line of thrust.

Final Answer:
1/8 to 1/12 of the span