Lecture09-Design-of-Shallow-Foundations

Lecture09-Design-of-Shallow-Foundations - CEG CEG-4012...

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Unformatted text preview: CEG CEG-4012 Foundation Engineering 4012 Foundation Engineering Lecture # Lecture #09 09 The Design of Spread Footings- Design Criteria- Design Procedure- Example Problem for a Square Footing- Example Problem for a Rectangular Footing- Example Problem for a Continuous Footing- Complex Cases: Large Billboard Sign Luis Prieto-Portar 2009 The Design Procedure. 1. Determine the structural loads and member sizes at the foundation level; 2. Collect all the geotechnical data; set the proposed footings on the geotechnical profile; 3. Determine the depth and location of all foundation elements, 4. Determine the bearing capacity, 5. Determine possible total and differential settlements; check effects at 2B depths; 6. Select the concrete strength (and possibly the mix), 7. Select the steel grade, 8. Determine the required footing dimensions, 9. Determine the footing thickness, T (or D in some textbooks), 0. etermine the size, number and spacing of the reinforcing bars, 10. Determine the size, number and spacing of the reinforcing bars, 11. Design the connection between the superstructure and the foundation, and 12. Check uplift and stability against sliding and overturning of the structure-soil system. The first studies performed on foundation structural failures were done by Professor Talbot at the University of Illinois in 1913. Advances in the next 50 years include Prof. F.E. Richarts tests at the University of Michigan. His results were synthesized into the methodology used today by a committee sponsored by the ACI and ASCE and published in 1962. Spread footings is still the most popular foundation around the world because they are more economical than piles, adding weight to them does not affect any other member, and their performance has been excellent. Selection of materials. Spread footings are usually designed to use 3 ksi < f' c < 4 ksi, whereas modern structural members frequently use concrete between the range of 4 ksi < f' c < 8 ksi. A higher concrete strength helps reduction the members size. However, the footings design is govern by the bearing capacity and settlement. That means that the strength of the soil might be limiting factor, and a higher concrete strength would not be relevant. Where a footing must carry a load greater than 500 kips, an f' c = 5 ksi might be justified. Since flexural stresses are usually small, a grade 40 steel would usually be adequate, although it is currently unavailable in the US. The most common grade used for construction is Grade 60 steel, which is almost universally used in the world today. The typical details of a spread footing, as sketched for drafting. The standard thicknesses T are given in English Units as multiples of 3": 12", 15", 18"..., etc. A high precision in specifying the depth of excavation D f is unnecessary. ACI code specifies that at least 3 inches of concrete cover must be included from ground contact, which takes into consideration irregularities in the excavation and corrosion factors. The same footing as built. Note that forming is required in this site due to the presence of a clean gravelly sand, that would not stand vertically at the sides of the excavation. Design Criteria.Design Criteria....
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Lecture09-Design-of-Shallow-Foundations - CEG CEG-4012...

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