Lecture08-Types-of-Shallow-Foundations

Lecture08-Types-of-Shallow-Foundations - Foundation...

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Unformatted text preview: Foundation Engineering Foundation Engineering Lecture #08 Lecture #08 Types of Shallow Foundations 1. Spread Footings . Combined Footings 2. Combined Footings 3. Continuous Footing 4. Mat Foundations 5. Repairing Shallow Foundations L. Prieto-Portar 2009 Footings for an old wooden house. Foundation design is one of the required parameters in designing a structure. Part of the foundation design is the design of footings. Footings and other foundation units transfer the loads from the structure to the soil or rock supporting the structure. Because the soil is generally weaker than the concrete columns and walls that must be supported, the contact area between the soil and the footing is much larger than that of the supported member. Concrete is the material most commonly used for footings because of its compressive strength, durability and economy. They are the lowest cost foundation solution, are the most widely used, and usually require no special equipment to build. Shallow foundation types can be classified as to their (1) function , or their (2) shape . Footings are designed to resist the full dead load delivered by the column. A footing carrying a single column is called a spread footing , since its function is to spread the column load laterally to the soil. This action will reduce the stress intensity to a value that the soil can safely carry. Spread footings are sometimes called single or isolated footings . They are square or rectangular pads which spread a column load over an area of soil that is large enough to support the column load. The soil pressure causes footings to deflect upward causing tension in two directions at the bottom. As a result, reinforcement is placed in both horizontal directions at the bottom. Spread footings with tension reinforcing may be called a two-way or one-way depending on whether the steel used for bending runs both ways or in one direction (as for example, in wall footings). Single footings may be of constant thickness, or stepped or tapered (sloped). Stepped or slopped footings are most commonly used to reduce the quantity of concrete away from the column where the bending moments are small and when the footing is not reinforced. However, it is usually more economical to use constant-thickness reinforced footings when labor cost are high relative to material (such as here in the US). The pressure distribution beneath most footings is indeterminate, because of the interaction of the footing rigidity with the soil type and the time response to stress. Because of the complication of the distribution, a linear pressure distribution is assumed beneath spread footings. Therefore, the resultant vertical soil reaction is collinear with the resultant downward structural load. The few field measurements reported indicate that this assumption is adequate....
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Lecture08-Types-of-Shallow-Foundations - Foundation...

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