Shear Reinforcement – Maximum Spacing Rule The maximum spacing of vertical shear reinforcement (stirrups) should not exceed 0.75 d, subject to what absolute limit (whichever is less)?
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A300 mm
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B350 mm
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C400 mm
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D450 mm
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E500 mm
Answer
Correct Answer: 300 mm
Explanation
Introduction / Context:Stirrups resist diagonal tension cracking and confine the concrete core in beams. Codes provide a dual limit on the maximum spacing of shear reinforcement: a depth-related fraction and an absolute millimetre cap, with the governing value being the lesser of the two.
Given Data / Assumptions:
- Member: reinforced concrete beam with vertical stirrups.
- Symbol: d is the effective depth.
- Objective: determine the absolute spacing cap paired with 0.75 d.
Concept / Approach:
Crack control and shear transfer mechanisms require sufficient stirrup frequency near potential failure planes. The absolute cap ensures a minimum confinement even in deep beams where 0.75 d could otherwise be quite large.
Step-by-Step Solution:
1) Identify the dual limit: s_max ≤ 0.75 d and s_max ≤ s_cap.2) Standard practice sets s_cap = 300 mm.3) Apply the governing rule: use the lesser of 0.75 d and 300 mm.Verification / Alternative check:
QA/QC checklists, bar bending schedules, and site inspections commonly reference “0.75 d or 300 mm, whichever is less” as the maximum stirrup spacing for beams without special seismic requirements.
Why Other Options Are Wrong:
- 350, 400, 450, 500 mm: Too large; may allow wide crack spacing and inadequate shear restraint.
Common Pitfalls:
Forgetting that closer spacing is required near supports where shear demand is higher; confusing maximum spacing with minimum spacing or with confinement requirements in seismic zones.
Final Answer:
300 mm