Concrete bed for foundations — factors governing required depth The depth (thickness) of a plain concrete bed beneath a footing depends primarily on which of the following design and construction factors?

Introduction / Context:
A plain cement concrete (PCC) bed below a footing provides a level, clean, and slightly stiffened base to spread construction loads and protect reinforcement from soil contamination. Determining its depth requires an understanding of load transfer, edge stresses across its projection, and the compressive and shear capacity of the concrete.

Given Data / Assumptions:

• PCC bed projects beyond the edges of the structural footing.
• Bearing pressure from soil acts upward on the underside of the bed.
• Concrete class (mix) influences compressive and shear strength.

Concept / Approach:
The bed acts as a load-spreading slab. Larger projections increase bending and shear demand near edges, necessitating greater thickness. Higher soil pressures raise the internal stresses in the bed. Stronger mixes permit thinner beds, while weaker mixes require more thickness to satisfy stress and minimum thickness criteria for constructability and durability.

Step-by-Step Solution:

Verification / Alternative check:
Design charts or simple slab-on-elastic-foundation checks confirm the relationship between projection, pressure, and thickness, consistent with common handbooks.

Why Other Options Are Wrong:

• Picking any single factor ignores the multivariate nature of bed design.
• none of these: incorrect because all listed factors apply.

Common Pitfalls:
Using arbitrary bed thickness without checking projection and soil pressure; disregarding local specifications for minimum thickness and cover.

Final Answer:
all the above