Combined footing detailing — when the computed shear stress in a combined footing exceeds 5 kg/cm², what configuration of nominal stirrups should be provided for adequate shear resistance and confinement?

Introduction / Context:
Combined footings (supporting two or more columns) must be detailed to safely resist shear stresses from column loads and soil reactions. When shear demand rises beyond nominal levels (here, exceeding 5 kg/cm²), the transverse reinforcement (stirrups) must provide both shear capacity and confinement across the wide footing beam portion near columns.

Given Data / Assumptions:

• Type: reinforced concrete combined footing.
• Shear stress demand > 5 kg/cm² is indicated.
• Objective: select a reasonable standard stirrup configuration.

Concept / Approach:

In wide footing beams and near-column zones, multi-legged closed stirrups are preferred to enclose all longitudinal bars, improve shear resistance, and confine the concrete core. An 8-legged stirrup arrangement is a commonly adopted detailing step-up beyond 6-legged ties when nominal shear is exceeded but before very heavy cage requirements arise.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Check shear design against concrete shear capacity and contribution of shear reinforcement (Vus). Ensure spacing, anchorage, and bar cover satisfy code requirements. If shear is still high, increase leg count or bar diameter and reduce spacing accordingly.

Why Other Options Are Wrong:

6-legged may be insufficient at higher τv; 10- or 12-legged may be overly conservative for the threshold stated; “none of these” ignores common practice.

Common Pitfalls (misconceptions, mistakes):

Using open stirrups; not enclosing all longitudinal bars; excessive spacing; ignoring punching shear near columns.

Final Answer:

8 legged