Chapter 18 Foundations

Objectives After reading the chapter and reviewing the materials presented the students will be able to: Describe the construction of basic types of foundations. Describe the purpose of reinforcing steel in foundations. Name the parts of concrete wall forms. Explain how construction walls can be made waterproof.

Introduction A structure consists of a substructure and a superstructure. The substructure is all structural work below the ground level. The superstructure is all structural work above ground level. Foundation: is a base that transfers the load created by a building to the earth supporting it. Bearing surface: is the point of contact between the substructure and the earth. Footings are designed to spread the weight of the structure over a greater area.

Types of Foundations Spread Foundations: Most small buildings have spread foundations (fig 18-2, page 307). The flat concrete footings are wider than the walls are thick. The footings are placed below the frost line. The foundation is built on top of the footing. Poured concrete or concrete blocks are most often used for the walls.

Types of Foundations Floating Foundations: Light buildings built on soft soil use floating foundations. A single reinforced concrete slab supports the building. Rebar and welded wire reinforcing are used to strengthen the concrete. The slab needs extra strength under walls and columns. More concrete and steel are added at these locations (fig 18-3, page 307). Water and sewer pipes, electrical conduit, and any other utilities that are required are roughed in below the slab.

Types of Foundations Friction Pile Foundation: Friction piles adhere to the soil they are driven through, creating friction between the pile and the soil, and providing stability. A cluster of piles are used under columns (fig 18-4, page 308). They are capped with a mat of steel reinforced concrete. This pile cap spreads the weight evenly over all the piles and serves as a footing for walls and columns.

Types of Foundations Bearing Pile Foundation: Bearing piles transmit the weight of the structure to the bearing surface (bedrock). Bearing piles are longer than friction piles (fig 18-5, page 308). Like friction piles, bearing piles are used in clusters. A steel reinforced concrete cap is used for each cluster of piles and serves as the footing for foundation walls and columns.

Types of Foundations Pier Foundation: Piers are supports made by drilling or digging holes that are then filled with steel reinforced concrete. During excavation, liners called caissons are used to keep the water out. Caissions are large steel pipes that are inserted as the hole is being drilled.

Building a Spread Foundation The trench for the concrete footing must extend below the frost line. The sides of the excavation must be protected against cave-ins. Concrete Footing: Footings are constructed before foundation walls are constructed. Water in the trench must be pumped out before work can begin. Form boards are placed on the outside and inside of the footings. The forms are held in place with stakes. Horizontal rebar is placed next. Concrete is then placed into the forms. If the foundation wall will be made from concrete, a keyway and short lengths of rebar are inserted in the liquid concrete (fig 18-11, page 311). A keyway is a groove molded down the center of the footing, that strengthens the joint between the footing and the foundation wall. Plastic sheets can be used to cover the concrete to keep it moist. Insulating blankets or straw is used to prevent freezing during cold weather.

Building a Spread Foundation Concrete Foundation Walls: Foundation walls are usually made of poured concrete or concrete blocks. Poured walls require forms to create their shape (fig 18-12, page 313). Form oil allows easy removal of the forms after the concrete hardens. The poured concrete is allowed to cure for at least one day before the forms are removed. Any defects on the surface are repaired as soon as the forms are removed.

Waterproofing Foundation Walls Foundation drains are plastic pipes that contain many small holes in its wall to remove water from the soil around the foundation (fig 18-17, page 316). Gravel placed over the pipe prevents the soil from clogging the pipe. Open cell insulation placed on the outside of the foundation wall prevents water from reaching the wall. Parging is a layer of mortar put on the outside of concrete block foundation walls to increase water resistance (fig 18-19, page 316). A traditional waterproof coating is asphalt, a sticky black petroleum product.

Summary Foundation: is a base that transfers the load created by a building to the earth supporting it. Spread Foundations: Most small buildings have spread foundations. Floating Foundations: Light buildings built on soft soil use floating foundations. Friction Pile Foundation: Friction piles adhere to the soil they are driven through, creating friction between the pile and the soil, and providing stability. Bearing Pile Foundation: Bearing piles transmit the weight of the structure to the bearing surface (bedrock). Pier Foundation: Piers are supports made by drilling or digging holes that are then filled with steel reinforced concrete. The trench for the concrete footing must extend below the frost line. A keyway is a groove molded down the center of the footing, that strengthens the joint between the footing and the foundation wall. Foundation walls are usually made of poured concrete or concrete blocks. Foundation drains are plastic pipes that contain many small holes in its wall to remove water from the soil around the foundation. A traditional waterproof coating is asphalt, a sticky black petroleum product.

Home Work 1. Why do we need a foundation for a building? 2. Where are footing placed? 3. Why are foundation drains used?