Why Buildings Need Foundations

Summary

In this episode of Practical Engineering, Grady explores the critical role of foundations in construction, highlighting their necessity for stability and the challenges they face from environmental factors. He examines various types of foundations, from shallow to deep, and explains their purpose in distributing weight, resisting horizontal and uplift forces, and preventing degradation. The video delves into the complexities of designing cost-effective foundations that balance structural integrity with practicality.

Takeaways

• 🏠 The narrator's house had a 75-year-old foundation that needed replacement, highlighting the importance of foundation stability in homes.
• 🏗️ Foundations are crucial for all structures as they provide stability and distribute weight evenly, preventing differential movement and stress.
• 📚 The primary rule for engineers designing foundations is that structures placed on the ground should remain stable and not move.
• 🛠️ There are multiple types of foundations, including pier and beam, strip footing, and raft foundations, each designed to address specific soil conditions and structural needs.
• 🌪️ Foundations must resist not only the weight of structures but also external forces like wind and earthquakes, which can cause uplift and horizontal pressure.
• 🐛 Foundations need to protect against long-term degradation from environmental factors such as moisture, bugs, and fungi that can weaken building materials.
• 🌡️ The ground is subject to changes due to freezing and thawing cycles and soil expansion from moisture, which foundations must account for to prevent structural damage.
• 💧 Deep foundations, such as piles, are used for larger structures or when surface soils are not strong enough, transferring loads to deeper, more stable soil layers or bedrock.
• 🔨 The installation process of piles, such as driving them into the ground, can differ significantly from the static loads they will bear in service, requiring special testing methods.
• 💰 Foundations must be cost-effective, balancing the need for structural stability with economic considerations, as over-engineering can lead to unnecessary expenses.
• 🔬 The script emphasizes the complexity and importance of foundation engineering, which involves understanding soil mechanics, structural loads, and environmental factors.

Q & A

Why did the narrator decide to replace the piers of their house?

The narrator decided to replace the piers because they were 75 years old and nearing the end of their useful life, which was essential to maintain the structural integrity of the house.

What is the primary rule for engineers designing foundations?

The primary rule for engineers designing foundations is that when something is put on the ground, it should not move, to prevent stress and potential structural damage.

What is differential movement in the context of structures?

Differential movement refers to the phenomenon where one part of a structure moves relative to the other parts, which can introduce stress and potentially cause the structure to pull apart.

What is bearing failure and how does it occur?

Bearing failure occurs when the forces exerted on the soil are high enough to shear through soil particles, causing the soil directly below the load to be forced downward and the surrounding soil to bulge up at the edges.

What is the main function of a foundation in relation to the weight of a structure?

The main function of a foundation is to evenly distribute the downward force of a structure over a large enough area to reduce bearing pressure and avoid shear failures or excessive settlement.

How do wind and earthquakes affect the loads on structures?

Wind and earthquakes create additional loads on structures by exerting horizontal pressure, uplift, and rapid fluctuations, which the foundation must resist to prevent lifting or sliding.

What are the challenges that a foundation must overcome in relation to the ground?

A foundation must overcome challenges such as resisting the effects of long-term degradation and decay from biological factors like termites and moisture, which can lead to mold and rot.

Why is it important for a foundation to reach a deep enough layer in the soil?

Reaching a deep enough layer in the soil is important to avoid problems caused by water in the subgrade, such as freeze-thaw cycles and expansive clay soil, which can cause a structure to shift or settle unevenly.

What is the difference between deep and shallow foundations?

Deep foundations rely on piles driven or drilled deep into the earth to stronger soil layers or bedrock, while shallow foundations transfer the structure's weight to the surface of the earth or just below it.

What is the purpose of a basement in the context of a building's foundation?

A basement serves as a building's foundation by providing additional support for the structure and can also be used for storage or living space. It is usually constructed on a raft or strip footings.

How do driven piles support a structure and what is their advantage?

Driven piles support a structure by transferring loads at the bottom (end bearing) and along their length through skin friction. Their advantage is that they are installed in the same way they will be loaded, allowing for efficiency in installation.

What is the statnamic testing of piles and how does it differ from traditional static tests?

Statnamic testing of piles is a method where a mass is accelerated upward using explosives to create an equal and opposite force on the pile. Unlike traditional static tests, which require heavy weights or reaction piers, statnamic testing simulates a static force for a longer duration, making it more representative of actual service conditions without the need for massive infrastructure.