Grillage foundation in structural engineering: identify the correct comprehensive statement about its purpose, construction, depth, and economy.

Introduction / Context:
Grillage foundations distribute heavy concentrated loads—commonly from steel stanchions—over a larger soil area where the allowable bearing pressure is relatively low. This is achieved without deep excavation by arranging steel sections in tiers encased in concrete to spread the load effectively.

Given Data / Assumptions:

• Heavy loads from steel columns (stanchions).
• Shallow depth preferred due to soil or construction constraints.
• Use of rolled steel joists (RSJs) in one or two tiers.

Concept / Approach:
The grillage transfers load laterally through steel beams embedded in concrete, increasing the effective bearing area. It is a classic solution where isolated footing size becomes very large yet deep excavation is undesirable. Encasing RSJs in concrete protects steel from corrosion and ensures composite action with the surrounding mass.

Step-by-Step Solution:
Identify purpose: spread heavy loads on low-capacity soil—true.Construction: RSJs in one or two layers within concrete—true.Depth: large area achieved at shallow depth—true.Economy: avoids deep, expensive excavation and heavy concrete blocks—true.

Verification / Alternative check:
Design guides illustrate grillage footing sizing via allowable bearing pressure and spacing/layout of RSJs to control pressures and settlement.

Why Other Options Are Wrong:
Each single statement is correct but incomplete alone; “All the above” captures the full picture.

Common Pitfalls:
Treating grillage as a deep foundation; omitting concrete encasement around RSJs.

Final Answer:
All the above