Minimum number of piles for a column load transfer For a typical axially loaded building column supported on pile foundations, what is the minimum practical number of piles usually provided beneath a single column to ensure stability against rotation and for constructability?

Introduction / Context:
Pile groups distribute column loads to deeper strata. While a single pile can carry load, practical design typically uses groups that limit rotation, provide redundancy, and allow for tie beams or pile caps with proper geometry.

Given Data / Assumptions:

• Conventional building column with vertical load (no exceptional lateral system considered).
• Pile cap required for force transfer and detailing.
• Symmetry and rotational stability are desired.

Concept / Approach:

A three-pile group is the smallest arrangement that can prevent rocking about an axis through the cap centroid without relying on a single line of action. It also enables a triangular pile cap, which is efficient and stable. Two piles in a line can permit rocking; one pile gives no redundancy.

Step-by-Step Solution:

Verification / Alternative check:

Design handbooks often illustrate 3-pile triangular caps as the smallest recommended group for columns, barring special single-pile columns or micro-pile retrofits.

Why Other Options Are Wrong:

1 pile lacks redundancy; 2 piles can allow tipping; 4 or 5 piles are feasible but not the minimum.

Common Pitfalls:

Assuming “more piles” always better; ignoring eccentricities and constructability.

Final Answer:

3 piles