Minimum edge thickness of reinforced concrete pile caps / footings on piles (as per classic IS-style working stress practice): According to traditional guidance aligned with IS:456-era practice, the thickness of RCC footing on piles at its edges should not be less than which value?

Introduction / Context:
Pile caps and footings on piles require a minimum edge thickness to ensure adequate cover, shear capacity, and constructability. Historical working-stress-oriented guidance provides clear minimums for preliminary detailing.

Given Data / Assumptions:

• RCC footing supported on piles.
• Edge thickness requirement in centimeters.
• Classic IS:456-era working-stress guidance context.

Concept / Approach:
The minimum edge thickness prevents spalling, provides room for reinforcement and cover, and aids in distributing concentrated pile reactions. A value of not less than 150 mm (15 cm) is conventionally adopted in many textbooks and practice notes for preliminary sizing.

Step-by-Step Solution:
Identify minimum edge thickness requirement for pile-supported footing: ≥ 15 cm.Select the correct option corresponding to 15 cm.Note that actual depth may be governed by punching shear and strut-and-tie action near piles.

Verification / Alternative check:
Checking punching shear around pile heads or groups and ensuring development length for bars may lead to greater practical thicknesses; however, the minimum edge thickness baseline remains about 15 cm for preliminary checks.

Why Other Options Are Wrong:

• 5 cm or 10 cm: Too thin for cover, shear, and reinforcement detailing.
• 20 cm or 25 cm: Acceptable but not the minimum; these may be required by strength checks, not by the basic minimum edge thickness rule.

Common Pitfalls:
Using the minimum blindly without verifying punching shear and reinforcement anchorage; ignoring aggressive exposure that could demand larger covers and hence greater thickness.

Final Answer:
15 cm