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Chapter-3-Types-of-Fnds - Chapter 3 TYPES OF FOUNDATIONS...

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Chapter 3 TYPES OF FOUNDATIONS, FORCES AND DESIGN CRITERIA A thorough geotechnical investigation will usually suggest the ideal type of foundation to be used at a site. This implies functionality, safety and cost effectiveness. Usually, the final report will suggest the best two alternatives. Typically, a building is primarily designed for function. After that criteria is satisfied, then the two subsequent criterions considered by its designer are its aesthetics and its structural requirements. Finally, after all these are satisfied, foundations are accommodated around the above constraints. Rarely are foundations considered at the outset. Exceptions are in seismic severe areas, and in predominantly civil works, such as earth dams, deep excavations, etc. This has not always been so. The influence of foundations on architecture has existed since prehistoric times, and become notable in chaldean and egyptian public works. In contrast, most greek monuments, so influential to modern architecture, ignored the fine foundation experience developed by older builders, primarily because the Greeks benefited from their peninsula's surficial rocks. The structural solution of a building must consider the behavior of the soil over time. For example, a reinforced concrete building behaves poorly in consolidating clay, when compared to a more flexible steel structure. Foundations must therefore be visualized as a natural connection between the structure and the soil, minimizing poor connectivity by adding flexibility to the structure. For example, the use of basements should be viewed both from a geotechnical and a space requirement points of view. The designer should answer these questions: 1. Is the rock shallow?; does it require blasting? 2. Is the bearing stratum within 4 meters from the surface? These 4 meters are easily reachable if you had planned for a basement, but expensive otherwise. Also, a basement may wipe out a good surficial stratum, forcing the use of deep foundations. 3. In very soft ground, use a floating or compensated mat foundation. The economics of a compensated mat
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suggests the use of one basement per each six stories of building height. However, two or three levels below the water table may require the use of expensive cut-off walls. TYPE OF BUILDING The average cost of a foundation for a multistory building ranges from 3 to 8% of the construction cost. Special structures, however, or difficult soils may raise the cost to 20% or higher. Light structures, such as 2 or 3 story buildings can be supported on inexpensive shallow foundations. Taller buildings might require more expensive solutions, such as mats or deep shafts to minimize differential settlements between columns. Very tall buildings (skyscrapers) experience large horizontal forces from wind and seismic activity, and require greater constraints, such as: a) Reduce soil deformations to minimal magnitudes, if necessary, via extensive soil improvement (for example, consolidation and grouting); b) Take the building load to much deeper levels, in order
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