In building construction, what rise is commonly adopted for a Jack arch (flat brick arch) expressed as a fraction of its clear span?
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A1/2 to 1/3 of the span
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B1/3 to 1/4 of the span
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C1/4 to 1/8 of the span
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D1/8 to 1/12 of the span
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E1/16 to 1/20 of the span
Answer
Correct Answer: 1/8 to 1/12 of the span
Explanation
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:
Identify the structural form: shallow brick arch needing modest camber for compressive thrust.Choose a standard range that maintains compressive line within the depth of the arch.Adopt rise ≈ 1/8 to 1/12 of the span as a widely accepted guideline.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