Structural Foundations

1. Purpose and Function of Foundations

At its core, a structural foundation transfers the loads from a building or structure to the ground. Its primary functions include:

• Supporting structural loads (dead loads, live loads, environmental loads).
• Spreading these loads to prevent excessive bearing pressure on the soil.
• Preventing differential settlement, which can cause structural damage.
• Providing anchorage against uplift or lateral forces (especially in seismic and wind zones).

2. Types of Foundations

Engineering exams often test your knowledge of foundation types and when to use each.

A. Shallow Foundations

Shallow foundations are used when suitable bearing strata are relatively close to the ground surface.

• Spread Footings: Most common for buildings. Can be isolated, combined, or continuous under walls. Design based on allowable bearing capacity of soil.
• Mat (Raft) Foundations: Used when loads are heavy or soils are weak. Spread the entire load over a large area. Reduces contact pressure and differential settlement.

B. Deep Foundations

Used when suitable bearing strata are deep, or when surface soils are weak or compressible.

• Driven Piles: Precast concrete, steel, or timber piles. Load transfer via skin friction and/or end bearing.
• Drilled Shafts (Caissons): Large-diameter bored holes filled with reinforced concrete. More versatile than driven piles.
• Micropiles and Helical Piers: Used in retrofitting or restricted-access sites. Installable with minimal vibration.

3. Soil-Structure Interaction

A. Bearing Capacity

The ultimate bearing capacity is the maximum load the soil can sustain. Engineers apply a factor of safety to ensure the allowable bearing capacity is not exceeded.

Common methods:

• Terzaghi’s Bearing Capacity Equation
• Empirical charts and tables (e.g., NAVFAC, AASHTO)

B. Settlement Analysis

Foundations must be designed to limit both total and differential settlement:

• Immediate Settlement: Occurs right after load application.
• Consolidation Settlement: Happens over time due to water expulsion from clayey soils.

4. Foundation Design Concepts

A. Footing Design

• Flexural strength (moment capacity)
• Shear strength, especially punching shear
• Development length and reinforcement detailing
• Minimum cover and anchorage

B. Pile Design

• Axial capacity = skin friction + end bearing
• Lateral load design: P-y curves, soil springs
• Consider buckling, group effects, corrosion

C. Load Combinations

As per ASCE 7, foundations must be checked under various combinations:

• Strength Design (LRFD)
• Service Load Design

5. Common Failure Modes

Be prepared to identify and prevent these foundation failures:

• Bearing failure of soil
• Punching shear in footings
• Lateral instability of piles
• Scour or erosion
• Frost heave
• Liquefaction-induced settlement

6. Code References and Standards

• ACI 318 – Concrete design
• AISC Steel Manual – Pile caps and steel framing
• ASCE 7 – Load combinations
• IBC / IRC – Building codes
• FHWA and AASHTO – Transportation and bridge foundations

Tip: Know how to navigate these documents quickly.

7. Exam Strategy and Problem Types

Expect problems involving:

• Bearing capacity calculations
• Reinforcement sizing
• Pile capacity analysis
• Settlement estimation
• Load combination checks

Strategic Tips:

• Sketch load paths and soil profiles
• Memorize key equations and load factors
• Practice unit conversions
• Manage time—foundation problems are detailed
• Review exam reference materials in advance

Want More Help?

Check out our PE Exam Prep Courses or our Practice Problem Sets (Crash Courses). These resources are designed to help you drill the most test-relevant content in a clear, structured way.

You’ve got this. One problem at a time, one foundation at a time.