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)?

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:

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